transfer of energy science experiment

Enjoy fun science experiments for kids that feature awesome hands-on projects and activities that help bring the exciting world of science to life.

Energy Transfer through Balls

Energy is constantly changing forms and transferring between objects, try seeing for yourself how this works. Use two balls to transfer kinetic energy from the the big ball to the smaller one and see what happens.

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Elementary Heat Transfer Experiments

Thermal Energy Science Experiments for Kids

Teaching children how to understand the basics of heat transfer can be rather difficult. Since many students do not fair well learning strictly through textbooks, elementary experiments can be crucial for teaching how heat energy can be transferred. A variety of heat transfer experiments can be conducted quickly and without the need for expensive materials.

Coin Conduction Experiment

A simple experiment that utilizes coins can be used to teach heat conduction. Place six pennies on a flat surface, which will represent atoms. Fling a "shooter" penny towards the group of coins, which represents an atom with excess kinetic energy. Observe the reaction of the other coins, which represents a transfer of kinetic energy; the same principle that can be found in heat conduction.

Sunlight Conduction Experiment

Sunlight conduction experiments are incredibly easy to set up and can effectively teach children how sunlight can be absorbed in water to create energy. Simply fill a container with ice-cold water and place outside of the classroom in a very sunny area. Ensure that each child feels the temperature of the water, and allow the water to sit outside for at least two hours. Take the children outside and ask each to feel the water's new temperature, which will be warm or hot as a result of its absorption of sunlight.

Dark vs. Light Experiment

Expanding on the sunlight conduction experiment, you can take things one step further by teaching your students which type of container absorbs more heat energy; a black one, or a white one. Using black and white construction paper, wrap two jars in each color respectively and fill with water. Allow to sit outside for one hour and test the temperature of each jar. The black will almost always be warmer, since dark surfaces work as better conductors than light surfaces.

Radiation Experiment

Teaching children the basic principles of radiation can be done easily and safely. Take the class outside and stand in a shady location, asking them to decide whether they feel hot or cold in the current area. Ask them to move to a sunny location and repeat the analysis. The warmth of the sunny area represents radiation, which can be thought of as a series of waves emitted by the sun that warms the ground.

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About the Author

Based in Portland, Maine, Kurt Larsen began his writing career in 2008. As well as being proficient in constructing marketing and website content, he has been published in media outlets such as Buildipedia, an interactive community focusing on green and sustainable architecture. Larsen holds a Bachelor of Arts in sociology from the University of Vermont.

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Energy transfer.

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Energy is neither created nor destroyed. It does not disappear when we use it – it changes from one form of energy to another.

Let’s look at energy in two situations.

A pendulum as energy transfer

A pendulum is a simple example of energy transfer. Beginning at position A, the pendulum fob is not moving. It has some energy because of its height (h) – called gravitational potential energy. When it is allowed to swing, that energy is gradually converted to energy of motion – kinetic energy. When the pendulum is at position B, all of its potential energy has been converted to kinetic energy and it is moving at its maximum speed. When the pendulum reaches position C, it has regained half of its potential energy and lost half of its kinetic energy. It continues trading speed for height (kinetic to potential) until it reaches position D, at which point it not moving and has regained almost all of its original potential energy.

The ‘almost’ part is important. If the pendulum could regain all of its original height it would swing forever, but there are small energy losses along the way. If we measure very carefully, we would find that the bending of the string where it is tied at the top would have become slightly warmer because of the fibres of the string rubbing together. Also, because of its speed in swinging through the air, the air would be moving, and some of the kinetic energy of the pendulum would have been transferred to the air. These small energy losses will eventually stop the pendulum, and it will come to rest at position B.

A peanut as energy transfer

A peanut is a much more complex example of energy transfer. If we look at how a peanut grows, a peanut plant begins life as a planted peanut. When soil conditions are right, the tiny embryo (nub at the end of a peanut) begins to grow using the stored energy in the rest of the seed to begin plant development. The plant eventually tunnels its way up and out of the soil into the light and forms leaves so that it can begin to collect energy from the Sun and absorb nutrients from the soil. The plant eventually matures, blooms and produces more peanuts. The energy from the Sun is translated into chemical energy and stored inside the seeds for the next generation of peanuts.

A peanut could also transfer its energy to a person who eats it. How much energy could we get from one peanut? The average peanut weighs about 2.5 grams and contains about 60 kilojoules (14 calories) of energy. How much energy is that? If the body was 100% efficient, it would be enough energy for a 65 kg person to climb to the top of a 25-storey building! It would also be enough energy (if you burned the peanut) to raise the temperature of 1 litre of water by 14°C. The peanut, if converted into electrical energy, would power a smartphone (screen on) for about 3 hours.

Energy conversion

Whenever energy is converted from one form to another, it loses some energy along the way. When we eat a peanut, our body only converts about 25% of the energy in the peanut into usable energy for our body – the rest is lost along the way. If we tried to warm up water by burning a peanut, we would find that a portion of the energy would be wasted warming the air and the container. The energy content of food is found by carefully burning the food item in a device known as a calorimeter and measuring the amount of heat generated. Our automobiles are even less efficient than our bodies at converting energy. The average car is able to convert only about 20% of the energy in the fuel into useful motion.

When we switch on a light, we are converting electrical energy into light energy. Old-style incandescent light bulbs convert about 10% of the electrical energy into usable light energy, and the other 90–95% is lost to heat. Compact fluorescent bulbs are about twice as efficient, and the newer technology of LED bulbs are much more energy efficient, converting up to 40% of the electrical energy into visible light.

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Learn about energy transfer in the contexts of heat pumps , food and waves in the water .

Use a slinky to investigate waves and energy .

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What is Heat Transfer? Heat Transfer Experiments

Heat transfer projects are an exciting and engaging choice for your next STEM activity. Kids love the hands on nature of these projects. Plus they offer a lot of practical life skills. Like how to build a natural solar heater or how to slow heat loss, or how to make a slurpee with science !

Science Fair Heat Transfer Experiments

What you will discover in this article!

Heat Transfer Science and Definitions

Before jumping into a bunch of Heat Transfer Projects it’s a good idea to chat about the science behind these experiments.

Heat Energy is often called thermal energy. Thermal energy is present in the molecules of an object. When an object is hot the molecules have a lot of energy and move fast. When an object is cold, the molecules have little energy and move slowly.

One thing to keep in mind, is that the faster molecules are moving, the more space they take up. The Bottle Crush experiment below is a fantastic way to demonstrate this principle.

How is Heat Transferred?

The Second Law of Thermodynamics states that heat will always move from a hot object to a cooler one.  Heat transfer is the movement of thermal energy as it transfers from one object to another or between an object and it’s surroundings. Thermal energy will naturally work towards a state of balance or equilibrium. This is known as thermal equilibrium, where two objects or an object and it’s surroundings achieve the same level of heat energy (thermal energy).

Keep in mind the greater the difference in temperature the more rapid the transfer of heat. The Mpemba Effect is a great way to explore this principle in water.

What is the Difference Between Heat and Temperature?

It’s important not to confuse Heat and Temperature. Heat refers to the energy present in the molecules of an object (picture how fast those molecules are moving). Heat is affected by the speed of the particles, the number of particles (including their size or mass), and the type of particles. Temperature is a measure of the average kinetic energy of the molecules in an object and is not affected by the number or size of the molecules. Heat and temperature are directly related to each other, but not the same thing.

Picture a steaming mug of coffee, now picture a bathtub filled with the same steaming coffee. The temperature is the same, but the thermal energy is higher in the tub because there is more coffee.

In a nutshell, heat is energy. Temperature is a measurement of that energy.

So with these heat transfer projects we are exploring the transfer of energy, with temperature being a common method of measurement and quantification of the results.

Heat Transfer Projects and Experiments

Science fair worthy projects, greenhouse effect experiment – climate change in a jar.

In this climate change experiment students learn hands on about the power of greenhouse gases and how they capture and hold heat from the sun. A simple chemical reaction is all that is needed to replicate the carbon dioxide in the atmosphere and see the greenhouse effect in a jar .

Starlite Thermal Insulating Experiment

This project is absolutely fascinating and would make an amazing science fair project for middle grade. Our Starlite recipe uses ingredients that you probably already have and it provides incredible thermal protection from heat transfer. We tested it a number of different ways, and each was utterly fascinating!

Passive Solar Energy Project

This activity uses recycled materials to create a solar chimney . Using the energy from the sun, it is transferred to the air inside the chimney, heating the air.

Winter STEM – Exploring the Effect of Salt on Ice

A fun project that explores how salt impacts ice and the transfer of heat between the ice and adjacent objects and the surroundings.

Slurpee Science

Using the principles explored in the previous Winter STEM project, this heat transfer project has a tasty treat at the end as students make their own slurpees with science !

How to Make Ice Cream in a Bag

Want something other than a slurpee? Try making Ice Cream in a Bag using the principles of Heat Transfer and a little muscle power. We have recipes for regular and dairy free Ice Cream in a Bag. So yummy!

Why Does Water Rise?

This activity is like magic and a great example of how rapid changes in heat energy and temperature can create a vacuum.

More Fun Heat Transfer Projects

Color changing magic playdough recipe.

A wow worthy project making playdough that changes colour as you play, simply from the heat of your hands, or by using chilled or warmed objects. This Magic Playdough Recipe is so cool!

Heat Sensitive Color Changing Oobleck Recipe

Looking to add some non-Newtonian Fluid demonstrations to your heat transfer lessons? Try this fun Colour Changing Oobleck that changes colours from the warmth of your hands, especially as you work it to keep it in a solid state. But release it and watch as it turns to liquid, flowing from your hands and changes colour as it flows. A fantastic demonstration of heat transfer and non-Newtonian fluids.

Magic Moon Dough

This luxurious sensory activity is absolutely mesmerizing. As you play with the silky feeling magic moon dough it will change colour from your touch, just like magic! Takes only minutes to make and provides hours of play.

Bottle Crush

This activity was mentioned above. Bottle Crush is a very simple science project that kids of all ages will enjoy. It does a fantastic job of showing how high heat energy takes up more space and low heat energy takes up less space.

Mpemba Effect – Making Snow

The Mpemba Effect is about the peculiar property of water where it will freeze faster when it is hotter, rather than colder. The greater the difference in temperature, the faster the heat transfer and the more dramatic the results. And at -40 the results are breathtaking!

Convection Currents Experiment

A potentially messy but fun experiment that shows how heat transfers between liquids when they are mixed together.

Ocean Currents Experiment

Similar to the experiment above, this one also explores heat transfer in liquids and how liquids at extreme temperature differences react to each other.

5 Days of Smart STEM Ideas for Kids

Get started in STEM with easy, engaging activities.

Science in School

Conservation and transfer of energy: project-based learning with rube goldberg machines teach article.

Author(s): Sarah Ferguson, Francis Estacion, Nicole Del Russo, Becki Grimes

Silly or serious? Rube Goldberg machines are not only a lot of fun but can help students to understand the principles of conservation and transfer of energy.

What are Rube-Goldberg machines?

Reuben Garrett Lucius Goldberg (1883–1970) was an American cartoonist and inventor best known for illustrations of his contraptions, named Rube Goldberg machines, which solve simple tasks in the most complicated and funny ways possible. These contraptions live on in the form of pop culture and competitions. [ 1 ] Below is a comic demonstrating one of Rube Goldberg’s machines.

A comic of a self-operating napkin created by Rube Goldberg Artwork Copyright © and TM or ® marks as All Rights Reserved. RUBE GOLDBERG ® is a registered trademark of Rube Goldberg Inc. All materials used with permission. rubegoldberg.com

Project-based learning (PBL) builds on the notion that students will be more engaged in learning through a curriculum built on exploring real-world problems or designed challenges. [ 2 ]

Combining the idea of Rube Goldberg machines and PBL creates an engaging learning sequence designed to help students understand the conservation and transfer of energy. This activity is designed for physics students aged 15 to 18 and can be appropriately scaled up or down, depending on students’ learning needs and classroom dynamics. It provides an exploration of the conservation of mechanical energy. The main objective is to design and build a Rube Goldberg machine, while exploring the conservation and transfer of energy.

Through this project, students are expected to address the following performance objectives: [ 3 ]

• Demonstrate how energy in a closed system is conserved if no work is done on, by, or within the system.
• Use everyday life to illustrate that energy can be transformed from one form to another.
• Investigate conservation of energy in a mechanical system to verify whether any energy is lost outside the system.

The project concept map outlines the connectedness of all major and minor ideas that need to be explored. 

The major terms that the map is built upon, mechanical energy and law of conservation, serve as the focal points of the concept map. These two major topics are connected via energy, more specifically, kinetic and potential energy, but the mechanical energy idea also includes work and force. 

Implementation

The project is delivered in three class periods, spanning seven days to include a weekend, giving students a good amount of time to be creative with their Rube Goldberg machine designs and creations. These lessons are designed for a virtual experience, but they can also work well in an in-person classroom environment.

Lesson 1: Introductions and sketches

• Engagement video [ 4 ]
• Assignment worksheets
• Scratch paper
• Project website (optional – virtual learning)
• To begin the first day, the teacher should ask the class to list some chores that they must complete at home.
• The teacher can then ask students whether those listed chores could be accomplished by a machine. The students will excitedly discuss the chores that they dislike having to do and brainstorm if they think the task can be accomplished by a machine. 
• Once the students are fully engaged with the discussion, they should watch the engagement video , which is a highly entertaining music video containing a complicated Rube Goldberg machine. [ 4 ]
• After the video, the teacher should ask the driving question: What kind of machine could you build that would complete a chore or task for you? To engage the students into figuring out how to answer this question, the teacher can probe with further questions, such as “ what is a Rube Goldberg machine” and “how does the machine continue after only one action?”
• After a brief discussion, the teacher introduces the full project via a PowerPoint presentation. 
• Students are assigned a Project Worksheet to supplement their projects, which is due on the third and final day of the unit. The worksheet requires sketches as well as input–output energy equations, which students will work through to calculate potential, kinetic, and possibly rotational energy.

• After hearing about their assignment, students can begin to explore energy transfers and Rube Goldberg machines by participating in an online game . This game is broken into levels, each with their own challenges for students to work through. While exploring Rube Goldberg machines through the game, students are also instructed to pay attention to energy transfers and the initiating tasks that begin each machine. Students are encouraged to notate ideas from the game that they would like to incorporate into their own Rube Goldberg machine creations. The students are given ten minutes of class time to work through as many levels as possible for a little competition.
• After completing the game activity, the remaining lesson time is allotted for students to begin sketching their machines. 
• Based on the sketches, the students should have created a list of materials needed to build their machines. 
• Online teaching: there are many activities and learning sequences within this small unit of instruction, so a project website was created to house all supplemental materials and to provide easy access to directions, timelines, requirements, and project details for students. If using this, students can be shown the site at end of the lesson (where and how to upload their assignments) and be given their first exit ticket. The exit ticket requires the students to list two examples for each of the six different types of simple machines.

Lesson 2: Construction

• Engagement video [ 5 ]
• Video assessment worksheets
• Optional: exit tickets (available on the project website )

Below is a list of suggested materials to have in the classroom for students to use, but students are encouraged to bring materials from home.

• Paper clips
• Aluminium foil
• Ice lolly sticks
• The teacher introduces another engagement video to get students thinking about physics and their projects, [ 5 ] and the students are given the video assessment worksheet to supplement the video assigned. Afterwards, the worksheet problems are worked through as a class.
• The students are given the remaining class time to work on assembling their machines. In a virtual teaching environment, some students will build small machines near their computers and others will build larger machines elsewhere and check back in with the teacher periodically. The teacher assists students with building ideas or questions as needed.   
• At the end of the class, students are instructed to create a video recording of their machines before the next class. 
• The students are assigned their second exit ticket, which requires them to list three things they learned about simple machines, as well as two questions they still have about energy transfers. The teacher collects these questions, answers them, and distributes the questions and answers back to the students via email, prior to the next class. The questions posed by students are also listed, with answers, on the project website.

Lesson 3: Presentations

• PowerPoint slideshow of all Rube Goldberg machines
• Peer-review worksheet and rubric
• Voting ballots (strips of scratch paper)

The final component of a PBL lesson is presentation. In a virtual setting, it is best to have students create videos showcasing their Rube Goldberg machines, and class time on the third day will be dedicated to video sharing and peer review. The students will arrive at class excited to present their Rube Goldberg machines.

• The teacher explains how the presentations will be conducted and how the peer-review process functions.
• Students are given a peer-review worksheet to complete during the presentations. About half the class time is designated for the presentation showcase, so there is time for replaying video recordings multiple times as well as in slow motion. 
• After all student videos are presented and peer-review comments are submitted, the teacher will show the students a national Rube Goldberg competition and discuss their smaller machines in comparison to the elaborate machines showcased in the national competition.

This project can be modified in many ways. If situations permit, this can be a good project for groupwork or for a competition. Also, the number of energy transfers can be scaled up or down, depending on the students’ abilities. Additionally, if the project is conducted in person, then the driving question can be changed to be more applicable to tasks that can be completed in the classroom. Along with being in person, the presentations can either be live or completed through video recordings. An option could be for students to record their machines in the classroom, so they are able to record multiple takes if their machines malfunction.

Student learning and understanding regarding energy transformations increases after having the opportunity to create their own energy transformations in a fun and educational way. 

[1] An introduction to Rube Goldberg: https://www.rubegoldberg.com/rube-the-artist/

[2] Larmer J, Ross D, Mergendoller JR (2017) PBL Starter Kit . Buck Institute for Education, Novato CA. ISBN: 0974034320

[3] Board of Education (2018) Science standards of learning for Virginia public schools: https://www.doe.virginia.gov/testing/sol/standards_docs/science/2018/standards/stds_physics.pdf

[4] Demonstration of a Rube Goldberg machine from OK Go: https://www.youtube.com/watch?v=qybUFnY7Y8w

[5] A video from 3M demonstrating a Rube Goldberg machine: https://www.youtube.com/watch?v=GEzcO3nfjZk

• Find all the resources for this activity on the Rube Goldberg PBI site .
• Sign up for one of the fantastic Rube-Goldberg contests offered by rubegoldberg.com : The Rube Goldberg Machine Contest, The Rube Goldberg Crazy Contraption Cartoon Contest, or The Rube Goldberg/Minecraft Competition. These activities are free and open to all ages.
• Play a game on dynamic systems .
• Check out Joseph’s machines on YouTube for wonderful examples, such as passing the salt while maintaining social distancing.
• ESA (2021) Landing on the Moon – planning and designing a lunar lander . Science in School 51 .
• Florean C (2018) Adventures in creative recycling . Science in School 45 : 27-30.
• Toro S (2021) Biomimicry: linking form and function to evolutionary and ecological principles . Science in School 53 .

Dr Sarah Ferguson is a master teacher with the Old Dominion University’s MonarchTeach program. Experienced with problem-based instruction, she enjoys working with pre-service teachers to expand their understanding of problem-based instruction techniques.

Francis Estacion is a physics teacher with King’s Fork High School in Suffolk, Virginia.  He enjoys bringing hands-on experiences into his physics classroom and encouraging his students to explore their curiosity.

Nicole Del Russo and Becki Grimes are pre-service teachers majoring in physics. They enjoy working with students in hands-on lessons that encourage curiosity while making physics relatable and understandable.

If you are looking for a novel and exciting way to teach classic physics concepts to older students, this article may be what you are looking for!

Energy transfers are investigated through a project-based learning activity intertwining science and popular culture to create a Rube Golberg contraption. This activity can be used as part of a seven day guided physics project or even be adapted into a fun, cross-curricular competition using skills from other subjects such as design and technology. Designed to be delivered online as well as in person, this activity offers a creative and easily adaptable physics option for today’s changing classroom.

Koulla Andronicou, Head of Science, Med High Private English School, Cypress

Supporting materials

Assignment worksheet

Peer-review worksheet

Student project worksheet

Video assessment worksheet

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Energy Transfers and Transformations

Energy cannot be created or destroyed, but it can be transferred and transformed. There are a number of different ways energy can be changed, such as when potential energy becomes kinetic energy or when one object moves another object.

Earth Science, Physics

Water Boiling Pot

There are three types of thermal energy transfer: conduction, radiation, and convection. Convection is a cyclical process that only occurs in fluids.

Photograph by Liu Kuanxi

Energy cannot be created or destroyed, meaning that the total amount of energy in the universe has always been and will always be constant. However, this does not mean energy is unchangeable. It can change form and even transfer between objects. A common example of energy transfer is the transfer of kinetic energy —the energy associated with motion—from one moving object to a stationary object via work. In physics, work is a measure of energy transfer . It refers to the force applied by an object over a distance. When a golf club is swung and hits a golf ball, some of the club's kinetic energy transfers to the ball as the club does "work" on the ball. In this type of energy transfer , energy moves from one object to another but stays in the same form. A kinetic energy transfer is easy to observe and understand, but other important transfers are not as easy to visualize. Thermal energy has to do with the internal energy of a system from its temperature. When a substance is heated, its temperature rises because its molecules move faster and gain thermal energy through heat transfer. Temperature measures the "hotness" or "coldness" of an object, and the term heat is used to refer to thermal energy being transferred from a hotter system to a cooler one. Thermal energy transfers occur in three ways: through conduction , convection , and radiation . When thermal energy is transferred between molecules that are in contact with one another, this is called conduction . If a metal spoon is placed in a pot of boiling water, even the end not touching the water gets very hot. This happens because metal is an efficient conductor . That means that heat travels through the material with ease. The vibrations of molecules at the end of the spoon touching the water spread up the spoon, until all the molecules are vibrating faster (i.e., the whole spoon gets hot). Some materials, such as wood and plastic, are not good conductors . That is, heat does not easily travel through the material. They are known as insulators . Convection and Radiation Convection only occurs in fluids, such as liquids and gases. When water is boiled on a stove, the water molecules at the bottom of the pot are closest to the heat source and gain thermal energy first. They move faster and spread out, creating a lower density of molecules, or quantity of molecules in that volume, at the bottom of the pot. These molecules rise to the top of the pot. They are replaced at the bottom by cooler, denser water. The process repeats, creating a current of molecules sinking, heating up, rising, cooling down, and sinking again. The third type of heat transfer— radiation —is critical to life on Earth. With radiation , a heat source does not have to touch the object being heated. Radiation can transfer heat even through the vacuum of space. Nearly all thermal energy on Earth comes from the sun and radiates to the surface of our planet, traveling in the form of electromagnetic waves. Electromagnetic waves, such as visible light, are waves of energy . Materials on Earth absorb these waves to be used for energy or reflect them back into space. In an energy transformation , energy changes form. A ball sitting at the top of a hill has gravitational potential energy , which is an object's potential to do work due to its position in a gravitational field. Generally speaking, the higher on the hill this ball is, the more gravitational potential energy it has. When a force pushes it down the hill, that potential energy transforms into kinetic energy . The ball continues losing potential energy and gaining kinetic energy until it reaches the bottom of the hill. In a frictionless universe, the ball would continue rolling forever, since it would have only kinetic energy . On Earth, however, the ball stops at the bottom of the hill due to the kinetic energy being transformed into heat by the opposing force of friction. Just as with energy transfers , energy is conserved in transformations. Energy Transfer on a Sand Dune In nature, energy transfers and transformations happen constantly, such as in a coastal dune environment. When thermal energy radiates from the sun, it heats both the land and ocean. However, water has a high specific heat capacity, so it heats up slower than land. This temperature difference creates a convection current, which manifests as wind. This wind possesses kinetic energy , which it transfers to grains of sand on the beach by carrying them short distances. If moving sand hits an obstacle, it stops due to the friction created by the contact and its kinetic energy is then transformed into thermal energy , or heat. Once enough sand builds up, these collisions can create sand dunes, and possibly even an entire dune field. These newly formed sand dunes provide a unique environment for plants and animals. A plant may grow in these dunes by using light energy radiated from the sun to transform water and carbon dioxide into chemical energy , which is stored in sugar. When an animal eats the plant, it uses the energy stored in that sugar to heat its body and move around, transforming the chemical energy into kinetic and thermal energy . Though it may not always be obvious, energy transfers and transformations constantly happen all around us and are what enable life to exist.

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Heat Transfer – Conduction, Convection, Radiation

Heat transfer occurs when thermal energy moves from one place to another. Atoms and molecules inherently have kinetic and thermal energy, so all matter participates in heat transfer. There are three main types of heat transfer, plus other processes that move energy from high temperature to low temperature.

What Is Heat Transfer?

Heat transfer is the movement of heat due to a temperature difference between a system and its surroundings. The energy transfer is always from higher temperature to lower temperature, due to the second law of thermodynamics . The units of heat transfer are the joule (J), calorie (cal), and kilocalorie (kcal). The unit for the rate of heat transfer is the kilowatt (KW).

The Three Types of Heat Transfer With Examples

The three types of heat transfer differ according to the nature of the medium that transmits heat:

• Conduction requires contact.
• Convection requires fluid flow.
• Radiation does not require any medium.
• Conduction is heat transfer directly between neighboring atoms or molecules. Usually, it is heat transfer through a solid. For example, the metal handle of a pan on a stove becomes hot due to convection. Touching the hot pan conducts heat to your hand.
• Convection is heat transfer via the movement of a fluid, such as air or water. Heating water on a stove is a good example. The water at the top of the pot becomes hot because water near the heat source rises. Another example is the movement of air around a campfire. Hot air rises, transferring heat upward. Meanwhile, the partial vacuum left by this movement draws in cool outside air that feeds the fire with fresh oxygen.
• Radiation is the emission of electromagnetic radiation. While it occurs through a medium, it does not require one. For example, it’s warm outside on a sunny day because solar radiation crosses space and heats the atmosphere. The burner element of a stove also emits radiation. However, some heat from a burner comes from conduction between the hot element and a metal pan. Most real-life processes involve multiple forms of heat transfer.

Conduction requires that molecules touch each other, making it a slower process than convection or radiation. Atoms and molecules with a lot of energy have more kinetic energy and engage in more collisions with other matter. They are “hot.” When hot matter interacts with cold matter, some energy gets transferred during the collision. This drives conduction. Forms of matter that readily conduct heat are called thermal conductors .

Examples of Conduction

Conduction is a common process in everyday life. For example:

• Holding an ice cube immediately makes your hands feel cold. Meanwhile, the heat transferred from your skin to the ice melts it into liquid water.
• Walking barefoot on a hot road or sunny beach burns your feet because the solid material transmits heat into your foot.
• Iron clothes transfers heat from the iron to the fabric.
• The handle of a coffee cup filled with hot coffee becomes warm or even hot via conduction through the mug material.

Conduction Equation

One equation for conduction calculates heat transfer per unit of time from thermal conductivity, area, thickness of the material, and the temperature difference between two regions:

Q = [K ∙ A ∙ (T hot – T cold )] / d

• Q is heat transfer per unit time
• K is the coefficient of thermal conductivity of the substance
• A is the area of heat transfer
• T hot  is the temperature of the hot region
• T cold  is the temperature of the cold region
• d is the thickness of the body

Convection is the movement of fluid molecules from higher temperature to lower temperature regions. Changing the temperature of a fluid affects its density, producing convection currents. If the volume of a fluid increases, than its density decreases and it becomes buoyant.

Examples of Convection

Convection is a familiar process on Earth, primarily involving air or water. However, it applies to other fluids, such as refrigeration gases and magma. Examples of convection include:

• Boiling water undergoes convection as less dense hot molecules rise through higher density cooler molecules.
• Hot air rises and cooler air sinks and replaces it.
• Convection drives global circulation in the oceans between the equators and poles.
• A convection oven circulates hot air and cooks more evenly than one that only uses heating elements or a gas flame.

Convection Equation

The equation for the rate of convection relates area and the difference between the fluid temperature and surface temperature:

Q = h c  ∙ A ∙ (T s  – T f )

• Q is the heat transfer per unit time
• h c  is the coefficient of convective heat transfer
• T s  is the surface temperature
• T f  is the fluid temperature

Radiation is the release of electromagnetic energy. Another name for thermal radiation is radiant heat. Unlike conduction or convection, radiation requires no medium for heat transfer. So, radiation occurs both within a medium (solid, liquid, gas) or through a vacuum.

Examples of Radiation

There are many examples of radiation:

• A microwave oven emits microwave radiation, which increases the thermal energy in food
• The Sun emits light (including ultraviolet radiation) and heat
• Uranium-238 emits alpha radiation as it decays into thorium-234

Radiation Equation

The Stephan-Boltzmann law describes relationship between the power and temperature of thermal radiation:

P = e ∙ σ ∙ A· (Tr – Tc) 4

• P is the net power of radiation
• A is the area of radiation
• Tr is the radiator temperature
• Tc is the surrounding temperature
• e is emissivity
• σ is Stefan’s constant (σ = 5.67 × 10 -8 Wm -2 K -4 )

More Heat Transfer – Chemical Bonds and Phase Transitions

While conduction, convection, and radiation are the three modes of heat transfer, other processes absorb and release heat. For example, atoms release energy when chemical bonds break and absorb energy in order to form bonds. Releasing energy is an exergonic process, while absorbing energy is an endergonic process. Sometimes the energy is light or sound, but most of the time it’s heat, making these processes exothermic and endothermic .

Phase transitions between the states of matter also involve the absorption or release of energy. A great example of this is evaporative cooling, where the phase transition from a liquid into a vapor absorbs thermal energy from the environment.

• Faghri, Amir; Zhang, Yuwen; Howell, John (2010). Advanced Heat and Mass Transfer . Columbia, MO: Global Digital Press. ISBN 978-0-9842760-0-4.
• Geankoplis, Christie John (2003). Transport Processes and Separation Principles (4th ed.). Prentice Hall. ISBN 0-13-101367-X.
• Peng, Z.; Doroodchi, E.; Moghtaderi, B. (2020). “Heat transfer modelling in Discrete Element Method (DEM)-based simulations of thermal processes: Theory and model development”. Progress in Energy and Combustion Science . 79: 100847. doi: 10.1016/j.pecs.2020.100847
• Welty, James R.; Wicks, Charles E.; Wilson, Robert Elliott (1976). Fundamentals of Momentum, Heat, and Mass Transfer (2nd ed.). New York: Wiley. ISBN 978-0-471-93354-0.

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Energy Activities For Elementary Students: Ideas, Crafts, And Experiments For All Types Of Energy

December 20, 2023 //  by  Florence Florah

Are you studying the scientific ideas behind various forms of energy in your classes? Do you want to conduct hands-on activities with your kids to bring your energy lessons to life? Why not consider including some Energy Science Experiments in your lesson plan? Using experiments, you may genuinely involve your kids in understanding various types of energy. It allows learners to engage and participate in the course, adding an interactive component.

Potential and Elastic Energy

1. rubber band stretching.

Rubber bands are great illustrators of elastic energy because of their extensibility. Students participate in this exercise by stretching and releasing rubber bands to observe the correlation between the amount of strain and the subsequent distance traveled by the band.

Learn more: The University of British Columbia

2. Rubber Band Car

In this elementary grade level project, students construct a vehicle propelled by a rubber band's force. Winding the car's axle stretches the rubber band, storing potential energy. The car's potential energy turns into kinetic energy when the rubber band is released.

Learn more: Scientific American

3. Paper Airplane Launcher

Students will create a rubber band-powered launcher for paper airplanes that will use the elastic energy of a rubber band to send them soaring. The youngsters learn how using the hand and arm to launch an aircraft is different from using a rubber band launcher.

Learn more: My Baba

4. Catapult made on popsicle sticks

Elementary grade level kids construct a basic catapult in this exercise using recyclable materials, craft sticks, and rubber bands. When you push down on the launching stick, it stores up potential energy, much like an elastic band would do when you stretch it. The energy stored in the stick is transformed into kinetic energy when it is released.

Learn more: Little Bins for Little Hands

5. Chain Reaction of Popsicle Sticks

Learners gently weave wooden sticks together in this project, ensuring each piece flexes. The twisted sticks are maintained in position and store potential energy. The free stick snaps back to its usual shape when the first stick is released, converting elastic energy to kinetic energy.

Learn more: Clearway Community Solar

Gravitational Energy

6. acceleration and gravity.

Using cardboard tubes, students study the link between drop height and object speed in this assignment. Gravity increases an object's speed by 9.8 meters per second (m/s) when it is in free fall. Students test the effects of gravity by timing how far a marble slides down a cardboard tube in one second, two seconds, etc.

Learn more: Science Sparks

7. Gravity modeling

In this activity, students study how gravity functions in the solar system using a broadsheet, a pool ball, and marbles. Using a pool ball for the Sun and marbles for the planets, students test the gravitational force of the Sun's mass and attraction.

Learn more: Science Learning Hub

8. Maneuvers Using Gravity Assist

This lesson explores how a gravity assist or "slingshot" maneuver might help rockets reach faraway planets. Students study the elements contributing to a successful slingshot movement while simulating a planetary encounter using magnets and ball bearings.

Learn more: Science Learn

Chemical Energy

9. colors of fireworks.

In this chemical energy lesson, students test how fireworks colors relate to chemicals and metal salts. Because of the chemical energy they generate, various chemicals and metal salts burn with varying light hues.

Learn more: ThoughtCo.

Light Energy

10. reflecting light off a cd.

Ever wonder why CD light reflects a rainbow? Your kids probably have too. This project explains to kids why and how light energy works. It's a wonderful way to bring science outdoors.

Learn more: Twinkl

Nuclear Energy

11. observing nuclear energy in a cloud chamber.

This energy activity aims for students to construct and test a cloud chamber. A water- or alcohol-supersaturated vapor is present in a cloud chamber. Particles enter the cloud chamber as the atom's nucleus releases nuclear energy upon disintegration.

Learn more: Jefferson Lab

Kinetic Energy and Motion Energy

12. car safety during a crash.

Students explore techniques to prevent a toy automobile from crashing while studying Newton's law of conservation of energy. In order to design and construct an effective bumper, students must consider the toy car's speed and direction of motion energy just before impact.

Learn more: STEM Inventions

13. Creating a device for dropping eggs

This motion energy activity aims to have students create a mechanism to cushion the impact of an egg being dropped from various heights. Although the egg drop experiment may teach potential & kinetic types of energy, and the law of conservation of energy, this lesson focuses on preventing the egg from shattering.

Learn more: Get Smart about STEAM

Solar Energy

14. solar pizza box oven.

In this activity, kids use pizza boxes and plastic wrap to build a simple solar oven. By capturing the Sun's rays and transforming them into heat, a solar oven is able to prepare meals.

Learn more: Blendspace

15. Solar Updraft Tower

This project has students create a solar updraft tower out of paper and look into its potential for converting solar energy into motion. The top propeller will rotate when the device's air warms up.

Learn more: Walk with Easha!!

16. Do Different Colors Absorb Heat Better?

In this classic physics experiment, students investigate if the color of a substance impacts its thermal conductivity. White, yellow, red, and black paper boxes are used, and the order in which the ice cubes melt in the sun is predicted. In this way, they can determine the sequence of events that caused the ice cubes to melt.

Learn more: Teach Engineering

Heat Energy

17. homemade thermometer.

Students create basic liquid thermometers in this classic physics experiment to examine how a thermometer is made using the thermal expansion of liquids.

Learn more: Yuri Ostr

18. Heat-curling metal

Within the context of this activity, students investigate the relationship between temperature and the expansion of various metals. Students will see that strips produced from two materials behave differently when set over a lit candle.

Learn more: Science Buddies

19. Hot air in a balloon

This experiment is the best way to show how thermal energy affects air. A tiny glass bottle, a balloon, a big plastic beaker, and access to hot water are required for this. Pulling the balloon over the bottle's rim should be your first step. After inserting the bottle into the beaker, fill it with hot water so that it surrounds the bottle. The balloon begins to expand as the water gets hotter.

Learn more: Go Science Girls

20. Heat conduction experiment

Which substances are most effective in transferring thermal energy? In this experiment, you will compare how different materials can carry heat. You'll need a cup, butter, some sequins, a metal spoon, a wooden spoon, a plastic spoon, these materials, and access to boiling water to complete this experiment.

Learn more: STEM Little Explorers

Sound Energy

21. rubber band guitar.

In this lesson, students construct a basic guitar from a recyclable box and elastic bands and investigate how vibrations produce sound energy. When a rubber band string is pulled, it vibrates, causing air molecules to move. This generates sound energy, which is heard by the ear and recognized as sound by the brain.

Learn more: Wiki How

22. Dancing Sprinkles

Students learn in this lesson that sound energy may cause vibrations. Using a plastic-covered dish and candy sprinkles, students will hum and observe what happens to the sprinkles. After conducting this investigation, they can explain why sprinkles react to sound by jumping and bouncing.

23. Paper cup and string

Your kids should be accustomed to engaging in activities like this sound experiment. It's a great, entertaining, and straightforward scientific idea showing how sound waves may pass through things. You only need some twine and some paper cups.

Learn more: Global Call Forwarding

Electrical Energy

24. coin-powered battery.

Can a pile of coins generate electrical energy? Within the context of this activity, students make their own batteries using a few pennies, and vinegar. They get to study electrodes as well as the movement of charged particles from one metal to another through electrolytes.

Learn more: Generation Genius

25. Electric Play Dough

Students gain background knowledge on circuits in this lesson using conductive dough and insulating dough. Kids build basic "squishy" circuits using the two types of dough that light an LED so they can observe firsthand what occurs when a circuit is open or closed.

Learn more: The Dad Lab

26. Conductors and insulators

Your kids will love using this worksheet on conductors and insulators to explore how electrical energy may travel through various materials. The document includes a list of several materials, all of which you should be able to acquire quickly. Your pupils must guess whether each of these substances will be an insulator that doesn't carry an electric form of energy or a conductor of electricity.

Learn more: Science Notes

Potential and Kinetic Energy Combined

27. paper roller coaster.

In this lesson, students construct paper roller coasters and try out adding loops to see if they can. The marble in the roller coaster contains potential energy and kinetic energy at different locations, such as at the summit of a slope. The stone rolls down a slope with kinetic energy.

Learn more: Instructables

28. Bouncing a Basketball

Basketballs have potential energy when they are first dribbled, which is transformed into kinetic energy once the ball hits the ground. When the ball collides with anything, part of the kinetic energy is lost; as a result, when the ball bounces back up, it is unable to achieve the height it had reached before.

Learn more: Research Gate

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7 energy and momentum demos that will  engage your students.

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Expanding your energy and momentum demo toolbox.

Aside from the Newton’s Cradle and the Faith in Physics pendulum, there are not very many well-known energy and momentum demonstrations. In this video and article, I aim to resolve this. I have especially focused on energy because those demonstrations are usually very hard to find.

Energy as Water Analogy

Young children are often confused as to whether the amount of water is changed when it is poured from a short glass to a tall glass. But adults know better because we understand measurement. In this demo, I like to show how energy is often converted between kinetic and potential, but the total MECHANICAL energy (the sum of these) is a constant. Of course, spilling would change the amount of water that is kinetic or potential, but the total volume is still constant, although some is no longer MECHANICAL, it is now HEAT or some other unusable form such as sound and light. When I did this demonstration, I started with very little blue food dye and poured it into a second container that had a drop of red food dye in the bottom. This accounts for the first color change. I then used “movie magic” to reverse the color back – in this case digital compositing of two very similar scenes. If I was performing this demonstration live I would use an acid-base indicator, such as methyl red, for the second case and have a few drips of acid in the second container. Then sneak a few drops of ammonia into the first container when I switched back.

Figure 1: Pouring water back and forth illustrates conservation of energy. Spilling some or letting some evaporate can illustrate this still further. The water color change is optional.

The point is, the total volume of water is a constant, even if it evaporates and we can never get it back. Energy cannot be created or destroyed. Richard Feynman used a child’s wood blocks that keep getting lost in a messy room for his analogy. I think that water is a better choice however because we have the same intuitive challenges with mistakenly feeling that the water is “gone” when it evaporates.

Racing Marbles Lab

One of the best energy demos is the racing marbles lab . Two marbles are released at the same time and travel these different paths, but which one will reach the end first? The results of this experiment are usually quite a surprise. In the video I pause to have those watching explain to their neighbor their predictions. After the results, which lets the low marble win, we now must allow the students a second chance to explain what has happened. This technique is consistent with learning theory that explains that learning is a social process. Often, students are much more interested with what their neighbors think than with what their teacher actually thinks. The marble that was allowed to dip lower converted its potential energy into kinetic energy, which resulted in a higher velocity. The higher-ramp marble moved at pretty much a constant speed. By the end of the race, they both finish at the same speed, just not the same time.

Figure 2: The Racing Marbles Lab is one of the best energy demos. Be sure to stop and let the students thing about what might happen before you perform the solution.

This experiment can be made quantitative by the use of photogates , you can verify that the final speeds are pretty much the same, which makes sense because the change in potential energy is the same for both. However, if you wish to calculate the speed with which these marbles are rolling you must also consider rotational kinetic energy, rather than simply using potential energy lost becomes kinetic energy gained.

Galileo’s Pendulum

Galileo performed many experiments to investigate motion. At least he said he did. It is much more likely that he was only claiming many of these, such as his Leaning Tower of Pisa demonstration. But this one seems easy enough to do, so he may have actually performed it. A pendulum is swung and always seems to remember how high you released it from. Not only does it come back to the point it was released from, it will return even if interrupted. In my example, it is interrupted by a peg in the middle of its path. This shows that whatever it loses on its journey toward the bottom is not actually lost, but only converted from the possibility of falling into actual falling. The pendulum not only returns to the original height, but swings out to the same height, even when there is a peg. When you perform this experiment, you should ask your students the following questions: “We see that if a pendulum is swung from a specific height, it somehow always remembers the height it was released from. Very strange, how does it remember?”

Figure 3: Galileo's Pendulum is a good way to start off an energy unit. It's amazing that the peg doesn't interfere with the ball as it rises to the same height as it was released

“What if we put a barrier in the way? Somehow it still remembers. Where is this memory stored?” [in the motion] “What is it that the mass has that helps it remember, what carries it to this height?”

Newton’s Cradle as an Energy Demo?

Many people use the Newton’s Cradle to teach momentum, but it is also possible to use it to teach energy concepts – perhaps this is its best application! Of course, Newton himself used it to explain and demonstrate his third law. For example, when ball A swings to hit ball B , then ball a will stop and ball b will go. They hit each other with equal and opposite forces or with equal and opposite impulses. Yes, momentum is conserved in the collision, and in fact it is conserved in all collisions. The motion mv, of the first ball is transferred to the second, but are we giving Newton too much credit? Couldn’t two balls come out of the collision and not just one? This would NOT violate the conservation of momentum! If the two come out at half of the speed! Then the momentum mv would be equal to 2m x ½ v which is an acceptable result. WHY DOESN’T THIS HAPPEN?! The answer is energy. Specifically, energy would have to be lost for that to happen, and what makes this toy so fun to watch is that very little energy is lost in each collision.

Figure 4: Perhaps the Newton's cradle demonstration has more to say about energy than it does about momentum.

There is of course the exciting possibility that we could force the coupling of two balls of the Newton’s Cradle. I usually do this experiment with a hair tie, and I like to film it in slow motion. When the collision does occur, the two balls scream in protest. We not only witness that momentum is conserved in all collisions, but that energy is not! The mechanism of energy loss is twisting, vibration, and sound. This is an important demonstration because most people are unaware that energy is almost completely conserved in the newton’s cradle’s collisions. They usually only discuss momentum, but this is just as much a demonstration of energy. Regarding energy, when in normal operation – without hair ties – the newton’s cradle also demonstrates that it wears down to a lower energy state of five moving at once, the most boring of all situations.

Happy and Sad Ball Collisions

Momentum is transferred by collisions, but an interesting questions is, " Will more energy be lost in sticking or bouncing?" For example when a ball that doesn’t bounce hits a block, is it more or less likely to knock it over than a ball that sticks to the block? Make your prediction. This is a demonstration that can help us understand the idea that momentum is a vector, and that change in momentum is larger when the momentum is reverse. As many teachers will have correctly guessed, the bouncing transfers more momentum because it bounces backwards with negative momentum, the total change is larger than just coming to a stop. The sticking is only losing the momentum it brought into the situation. Rigging up your happy sad balls demonstration is a bit tricky. Be sure to practice before hand. The block must tip over decisively. Many people will be surprised by the results, even if they guess correctly!

Figure 5: Energy is conserved in both cases, but why doesn't the sad ball knock over the block?

Colliding Steel Spheres

The collision of heavy metal spheres transforms a lot of kinetic energy into heat. That energy has to go somewhere. It is cool that we can use this collision to singe paper and cause ripples in aluminum foil. We expect that momentum might be discussed when we think of wrecking balls, but more relevant is the discussion of energy imparted when motion is brought to a halt. Just think of slamming on the brakes–those tires will be hot! In the case of the spheres, most of if will be in this one tiny spot. Colliding Steel Spheres can illustrate the idea of energy being "lost" in a collision. Of course it is not lost, but only converted , and yet the conversion is into forms that are no longer available to us for anything useful.

Figure 6: Colliding steel spheres are a great demonstration of how energy converts from one form to another.

Relative Potential Energy and Relative Kinetic Energy

Most people will immediately get the idea that potential energy is RELATIVE to some arbitrary zero-point energy. For example, a mass might fall off a stack of books to the table top, but it could also fall all the way off the floor, or even further out the window onto the ground, then it might fall down a down a well which allows it to fall down a cavern all the way to the center of the earth, and maybe the earth will fall into the sun or the sun could fall all the way to the center of the milky way galaxy! Most people do not know that it is possible to show that Kinetic energy is also relative. Here is a way, if we take a swinging pendulum and I let it swing back and forth, it has its highest kinetic energy at the lowest moment.

Figure   7: Relative to the table, the mass has no potential energy after it falls off the stack of books. But it does have potential energy relative to the floor.

But if I walk with it then I see from my perspective that it looks as though it has zero kinetic energy. My relative motion affects my calculation of the kinetic energy of this object. Kinetic energy depends on the relative motion of two objects. Usually it is an object relative to the laboratory. That is, we assume that the laboratory is not moving but everything else inside could be. This is not a very truthful assumption because we are on a rotating planet orbiting a star that is itself moving sinusoidally in an orbital plane of the galaxy. But this demo can serve as a discussion for the idea of center of mass motions and collisions. For example, if it is just one proton hitting another in an atom smasher, such as the Large hadron Collider at CERN. It would be completely arbitrary which proton is the one that is stationary. However, there is kinetic energy relative to the center of mass, which will also be the point of collision, or nearly so, as the proton has a finite radius.

The Proof of Potential and Kinetic Energy

One of the purposes of this video is to illustrate that energy is not momentum. Far too often I feel that we do momentum demonstrations that are very similar to our energy demonstrations and vice versa. Therefore, for the final demo, I like to show a simple lab that shows that kinetic energy is connected to height. Specifically, height lost will result in new kinetic energy being gained. ½ mv 2 . This experiment can be done with a marble, a car, or a rolling can, and the height of fall is not proportional to velocity, but velocity squared. This is true even if we take into account rotational kinetic energy. This lab shows it immediately. I have chosen an opaque marble which can immediately reveal the translational velocity as it passes through a photogate. As I move up the ramp, the vertical height gives me a faster translational velocity at the end. Twice the height does not however give twice the velocity. Rather only radical-two times as much.

Figure 8: If I want to double the speed, I have to start from four times the height… which is 2 squared. The height to which an object will rise or fall corresponds with the square of the velocity.

March 29, 2019 James Lincoln

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September 9, 2024

A New Quantum Cheshire Cat Thought Experiment Is Out of the Box

The spin of a particle seems to detach and move without a body—a strange experimental observation that’s stirring up debate

By Manon Bischoff

Quantum physics is often about cats—and this is also the case here.

Seregraff/Getty Images

Physicists seem to be obsessed with cats. James Clerk Maxwell, the father of electrodynamics, studied falling feline s to investigate how they turned as they fell. Many physics teachers have used a cat’s fur and a hard rubber rod to explain the phenomenon of frictional electricity. And Erwin Schrödinger famously illustrated the strangeness of quantum physics with a thought experiment involving a cat that is neither dead nor alive.

So it hardly seems surprising that physicists turned to felines once again to name a newly discovered quantum phenomenon in a paper published in the New Journal of Physics in 2013. Their three-sentence study abstract reads, “In this paper we present a quantum Cheshire Cat. In a pre- and post-selected experiment we find the Cat in one place, and its grin in another. The Cat is a photon, while the grin is its circular polarization.”

The newfound phenomenon was one in which certain particle features take a different path from their particle—much like the smile of the Cheshire Cat in Alice’s Adventures in Wonderland, written by Lewis Carroll—a pen name of mathematician Charles Lutwidge Dodgson—and published in 1865. To date, several experiments have demonstrated this curious quantum effect. But the idea has also drawn significant skepticism . Critics are less concerned about the theoretical calculations or experimental rigor than they are about the interpretation of the evidence. “It seems a bit bold to me to talk about disembodied transmission,” says physicist Holger Hofmann of Hiroshima University in Japan. “Instead we should revise our idea of particles.”

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Recently researchers led by Yakir Aharonov of Chapman University took the debate to the next level. Aharonov was a co-author of the first paper to propose the quantum Cheshire effect. Now, on the preprint server arXiv.org, he and his colleagues have posted a description of theoretical work that they believe demonstrates that quantum properties can move without any particles at all —like a disembodied grin flitting through the world and influencing its surroundings—in ways that bypass the critical concerns raised in the past.

A Grin without a Cat

Aharonov and his colleagues first encountered their quantum Cheshire cat several years ago as they were pondering one of the most fundamental principles of quantum mechanics: nothing can be predicted unambiguously. Unlike classical physics, the same quantum mechanical experiment can have different outcomes under exactly the same conditions. It is therefore impossible to predict the exact outcome of a single experiment—only its outcome with a certain probability. “Nobody understands quantum mechanics. It’s so counterintuitive. We know its laws, but we are always surprised,” says Sandu Popescu, a physicist at the University of Bristol in England, who collaborated with Aharonov on the 2013 paper and the new preprint.

But Aharonov was not satisfied with this uncertainty. So, since the 1980s , he has been exploring ways to investigate fundamental processes despite the probability-based nature of quantum mechanics. Aharonov—now age 92—employs an approach that involves intensively repeating an experiment, grouping results and then examining what came out before and after the experiment and relating these events to each other. “To do this, you have to understand the flow of time in quantum mechanics,” Popescu explains. “We developed a completely new method to combine information from measurements before and after the experiment.”

The researchers have stumbled across several surprises with this method—including their theoretical Cheshire cat. Their idea sounds simple at first: send particles through an optical tool called an interferometer, which causes each particle to move through one of two paths that ultimately merge again at the end. If the setup and measurements were carried out skillfully, Aharonov and his colleagues theorized, it could be shown that the particle traveled a path in the interferometer that differed from the path of its polarization. In other words, they claimed the property of the particle could be measured on one path even though the particle itself took the other—as if the grin and the cat had come apart.

Inspired by this theory, a team led by Tobias Denkmayr, then at the Vienna University of Technology, implemented the experiment with neutrons in a study published in 2014. The team showed that the neutral particles inside an interferometer followed a different path from that of their spin, a quantum mechanical property of particles similar to angular momentum: Denkmayr and his colleagues had indeed found evidence of the Cheshire cat theory. Two years later researchers led by Maximilian Schlosshauer of the University of Portland successfully implemented the same experiment with photons. The scientists saw evidence that the light particles took a different path in the interferometer than their polarization did.

Weak Measurements and Illusions

But not everyone is convinced. “Such a separation makes no sense at all. The location of a particle is itself a property of the particle,” Hofmann says. “It would be more accurate to talk about an unusual correlation between location and polarization.” Last November Hofmann and his colleagues provided an alternative explanation based on widely known quantum mechanical effects .

And in another interpretation of the Cheshire cat results, Pablo Saldanha of the Federal University of Minas Gerais in Brazil and his colleagues argue that the findings can be explained with wave-particle duality . “If you take a different view, there are no paradoxes,” Saldanha says, “but all results can be explained with traditional quantum mechanics as simple interference effects.”

Much of the controversy surrounds the way in which particles’ properties and positions are detected in these experiments. Disturbing a particle could alter its quantum mechanical properties. For that reason, the photons or neutrons cannot be recorded inside the interferometer using an ordinary detector. Instead scientists must resort to a principle of weak measurement developed by Aharonov in 1988. A weak measurement makes it possible to scan a particle very lightly without destroying its quantum state. This comes at a price, however: the weak measurement result is extremely inaccurate. (Thus, these experiments must be repeated many times over, to compensate for the fact that each individual measurement is highly uncertain.)

In the quantum Cheshire cat experiments, a weak measurement is made along a path in the interferometer, the paths then merge, and the emerging particles are measured with an ordinary detector. Along one path of the interferometer, a weak measurement of the particle’s position can be taken and, along the other, its spin. Using detectors, physicists can more definitively characterize the particles that traveled through the interferometer and potentially reconstruct what occurred during the particle’s journey. For example, only certain particles will appear in certain detectors, helping the physicists piece together which path their neutron or photon previously took. According to Aharonov, Popescu and their colleagues, the Cheshire cat experiments ultimately reveal that the particle’s position can be confirmed on one path even as its polarization or spin was measured on the other.

Melissa Thomas Baum/Buckyball Design; Source: “Observation of a Quantum Cheshire Cat in a Matter-Wave Interferometer Experiment,” by Tobias Denkmayr et al., in Nature Communications , Vol. 5, Article No. 4492; July 29, 2014

Saldanha and his co-authors assert that it is impossible to make claims about quantum systems in the past given their measurements in the present. In other words, the photons and neutrons measured in the final detectors cannot tell us much about their previous trajectory. Instead the wave functions of particles passing through the paths of the interferometer could overlap, which would make it impossible to trace which path a particle had taken. “Ultimately, the paradoxical behaviors are related to the wave-particle duality,” Saldanha says. But in the papers that report evidence of the quantum Cheshire cat, he asserts, the findings “are processed in a sophisticated way that obscures this simpler interpretation.”

Hofmann, meanwhile, has stressed that the results will differ if you measure the system in a different way. This phenomenon is well-known in quantum physics: if, for example, you first measure the speed of a particle and then its position, the result can be different than it would be if you first measured the position of the same particle and then its speed. He and his colleagues therefore contend that Aharonov and his team’s conclusions were correct in themselves—that the particle moved along one path and the polarization followed the other—but that such differing paths do not apply simultaneously.

As Hofmann’s co-author Jonte Hance, also at Hiroshima University, told New Scientist , “It only looks like [the particle and polarization are] separated because you’re measuring one of the properties in one place and the other property in the other place, but that doesn’t mean that the properties are in one place and the other place, that means that the actual measuring itself is affecting it in such a way that it looks like it’s in one place and the other place.”