Transferring thermal energy (conduction, convection, radiation)
In this topic you will:
- describe how thermal energy can be transferred by conduction
- describe how thermal energy can be transferred by convection
- describe how thermal energy can be transferred by radiation
Key words
conduction
convection
convection current
emit
expand
radiation
vigorously
Heat and particle movement
When any substance is heated, the particles in that substance start to move faster.
In solids, the particles move by vibrating around fixed positions. This vibration gets faster when the solid is heated. As the particles vibrate more vigorously, they take up more space. Vigorously means with more energy, more speed and more force. That means solids expand when heated. Expand means to increase in size.
In liquids, the particles also vibrate, but the particles are not in fixed positions. When the liquid is heated, the particles vibrate faster and move around faster. Again, as the thermal energy increases, the particles take up more space. That means the liquid expands when heated.
In gases, the particles are far apart and move in straight lines until they collide with another particle or the walls of the container. When the gas is heated, the particles move faster and collide with more force and more frequently. The particles will take up more space in a heated gas, so gases also expand when heated.
Conduction
When particles are vibrating, they will push against the particles beside them. The faster and more vigorous the vibration of the particles, the greater the push. This will cause the particles beside to vibrate more vigorously as well.
This is how thermal energy is transferred by conduction.
Conduction of thermal energy works best in solids where the particles are close together and can only move by vibrating. The diagram shows how thermal energy is transferred by conduction through a solid.
Metals are the best conductors of heat because of the way the particles are arranged. The electrons that are free to move in a metal help to pass along the vibrations.
Wood, plastics and fabrics, such as wool and cotton are poor conductors.
Just as with electricity, the opposite of a conductor is an insulator. We say that poor conductors of thermal energy are good thermal insulators.
Some objects are made from materials that are chosen because of their ability to conduct thermal energy.
Key Definition: Conduction
Conduction is the transfer of thermal energy through a substance by the vibration of particles. It happens best in solids where particles are closely packed and can pass on energy by colliding or vibrating in place.
Common Mistake
Students often think that particles expand when heated. In reality, it's the increased movement between particles that causes the substance to expand — the particles themselves do not get larger.
Quick Fact
Metals like copper and aluminium are excellent thermal conductors, which is why they are used in cooking pans and heat sinks.
Look at the pan in the picture. The pan is made from metal because metal is a good conductor of thermal energy. That helps the food to heat faster. The handle is made from plastic because plastic is a good thermal insulator. That makes the handle comfortable to touch. The spoon is made from wood, which is also a good insulator.
Clothes for cold weather are made from materials that are good insulators. That slows the conduction of thermal energy away from the body.
Quick Fact
Good thermal conductors, like metals, transfer heat quickly — ideal for cooking. Good insulators, like plastic and wood, slow heat transfer and help prevent burns.
Common Mistake
Students sometimes assume all cooking tools should be metal. But handles and spoons are often made from insulators to prevent burns.
Conduction does not work well in liquids because the particles move around more when heated, rather than just vibrating.
Conduction also does not work well in gases because the particles are far apart and the collisions are not very frequent.
Conduction cannot happen in a vacuum as there are no particles to vibrate in a vacuum.
Convection
When liquids and gases are heated, the particles move faster and take up more space.
Taking up more space means the volume increases. The number of particles does not change, so the mass of the liquid or gas does not change.
You will remember from Topic 3.1 that density = mass ÷ volume
From this equation, you can see that when volume increases and mass stays the same, then density will decrease.
You will also remember from Topic 3.1 that less dense substances float in more dense substances.
Imagine a gas or liquid that is heated at the bottom. The particles at the bottom will start to move faster and take up more space. This part of the liquid or gas will become less dense and start to float up through the colder, more dense parts. This upward movement of warmer liquid or gas is called convection.
Colder, more dense parts of the liquid or gas then sink down to occupy the space. These colder parts are then closer to the source of heat and they get heated. The particles in these parts then occupy more space and start to float.
This movement around the heated liquid or gas is called a convection current. The convection current causes all of the liquid or gas to eventually become heated.
The diagram shows how a convection current heats all the air in a room.
Key Definition: Convection Current
A convection current is the circular movement of fluid (liquid or gas) caused by heating, where warmer, less dense regions rise and cooler, denser regions sink.
Common Mistake
Students sometimes think convection only happens in air. In fact, convection occurs in both liquids and gases, but never in solids or vacuums.
The next diagram shows how a convection current heats all the water in a cooking pan.
Convection can happen in liquids and gases because the particles are free to move.
Convection cannot happen in a solid because the particles are not free to move. The particles in a solid can only vibrate about fixed positions.
Convection cannot happen in a vacuum as there are no particles to move.
Radiation
There is a vacuum in the space between the Sun and the Earth, so conduction or convection would not transfer thermal energy from the Sun to Earth. That means thermal energy must be transferred in a different way.
This different way is called radiation. Radiation is a type of wave that we cannot see. Thermal energy can be transferred by radiation very quickly. The thermal energy from the Sun only takes about eight minutes to reach Earth!
All objects give out thermal energy through radiation. The hotter the object, the more radiation it will emit. Emit means to give out.
The Sun and other hot objects emit thermal energy by radiation. Cooler objects absorb thermal energy by radiation from hotter ones.
Radiation can pass through a vacuum because radiation does not need particles.
Radiation can also pass through transparent solids, liquids and gases.
The colour and texture of an object affects its ability to emit or absorb thermal energy by radiation.
The best emitters and absorbers of radiation:
- are dull
- are black
- have a large surface area
The worst emitters and absorbers of radiation:
- are shiny
- are white or silver
- have a small surface area
Shiny, white or silver surfaces reflect radiation away.
Key Definition: Radiation
Radiation is the transfer of thermal energy by electromagnetic waves. It does not require particles, so it can pass through vacuums and transparent materials.
Common Mistake
Some students confuse radiation with radioactive materials. Radiation here refers to thermal radiation, not nuclear radiation or radioactive decay.
Conduction, convection and radiation
Most objects gain or lose thermal energy by a combination of conduction, convection and radiation.
Look at the picture of the room heater.
The room heater in the picture has hot water flowing through the inside. Thermal energy from the hot water is transferred to the metal of the heater by conduction. Thermal energy from the metal is transferred to the air, also by conduction. When the warm air expands, this warm air rises, which heats the air above by convection. The metal surface of the heater also emits thermal energy by radiation.
Some objects are designed to reduce the effects of conduction, convection or radiation.
Vacuum flasks are containers used to store hot liquids such as tea, coffee or soup. The flask is designed to slow the transfer of thermal energy by reducing conduction and radiation.
Inside a vacuum flask there is a glass bottle with a silver surface. The glass bottle is surrounded by a vacuum, and the bottle only makes contact with the outside of the flask at the top, as shown in the diagram.
The silver surface of the bottle reflects thermal energy in the form of radiation back into the hot drink. The vacuum surrounding the bottle prevents conduction of thermal energy from the drink to the outside.
Double-glazed windows reduce thermal energy transfer into or out of buildings. The picture shows double-glazed windows. There are two layers of glass separated by a thin layer of argon gas. Argon is a poor conductor of heat. In the picture, you can also see insulation inside the wall.
Key Concept: Thermal Energy Control
Devices like vacuum flasks and double glazing are designed to control the transfer of thermal energy by blocking conduction, reducing radiation, and minimizing convection.
Common Mistake
Some students assume that a vacuum stops all heat transfer. While it stops conduction and convection, radiation can still pass through a vacuum.
There is a space between the two layers of the brick wall that is filled with foam. The foam is a poor conductor and also fills the space to prevent convection currents from forming inside the wall.
Car windows can be covered with silver material to reflect radiation from the Sun away from the inside of the car. This helps to stop the car becoming too hot inside when parked on a sunny day.
Quick Fact
Foam is often used in insulation because it slows down conduction and blocks air movement, which reduces convection. Silver materials reflect infrared radiation away from hot environments.
Questions
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Conduction, convection, and radiation.
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Conduction and convection cannot happen in a vacuum because they require particles to transfer energy.
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Conduction occurs best in solids because the particles are closely packed and can easily transfer energy by vibration.
Explain any difference in the heat that the two boys will feel when they spend the same time in the Sun.
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Arun will feel hotter because black absorbs more radiation than white. Marcus’s white shirt reflects more of the Sun’s radiation, so he stays cooler.
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Conduction.
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Radiation.
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The heater warms water at the bottom. As this water becomes less dense, it rises and is replaced by cooler water. This circulation spreads heat through convection.
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Hot water rises to the top due to convection, so the hottest water is found at the top of the tank.
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It should be placed higher up in the tank to heat only the top portion of water that is used most often.
Think Like a Scientist
You will compare how different materials conduct thermal energy using rods and wax-attached paperclips. This is now an individual investigation.
• Metal rods (30 cm) made from different metals
• Candle wax
• Paperclips
• Heat source (e.g. candle)
• Clamp stands
• Write a risk assessment for this investigation.
• Check your plan with your teacher before starting.
• Be careful with hot wax, flames, and metal rods.
Step 2. Clamp the rod so that one end can be heated as shown.
Step 3. Heat one end using a candle.
Step 4. Observe what happens to the paperclips and wax.
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Length of rods, amount of wax, paperclip spacing, time of heating, and distance of flame from rod.
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Position of the flame, exact wax thickness, or uneven metal surfaces may vary between trials.
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In good conductors, the wax melted faster and paperclips fell off sooner. In poorer conductors, changes were slower or did not occur.
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Thermal energy is transferred by particles vibrating and passing energy along. Metals have free electrons that transfer energy more efficiently.
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Wood and plastic are poor thermal conductors and may burn or melt before enough energy is transferred through them.
Reflect on the following:
- Did I control variables like spacing and wax amount?
- Was I accurate in observing when paperclips fell?
- Could I explain differences using particle theory?
Think Like a Scientist
You will compare how different colours emit thermal radiation by observing how fast hot water cools in coloured cans.
• Three or more identical food/drink cans
• Matt black, white, silver paint (or other colours)
• Three or more thermometers
• Hot water
• Measuring cylinder
• Funnel
• Timer
• Graph paper
Step 2. Measure the volume of water needed to fill one can 1 cm from the top.
Step 3. Heat enough water to fill all cans. Stop heating at about 60 °C.
Step 4. Pour equal volumes of hot water into each can using a funnel.
Step 5. Place a thermometer in each can.
Step 6. Record the temperature of each can every 2 minutes.
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The black can is predicted to cool fastest because dark, matte surfaces emit thermal radiation more effectively.
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Create a table with time intervals and temperatures for each can in separate columns.
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Plot temperature on the y-axis and time on the x-axis. Use a key to distinguish each line by can colour.
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The black can cooled fastest, followed by the white and then silver can. This shows how surface colour affects emission of radiation.
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Yes. The black can emitted thermal energy more quickly, matching the prediction based on radiation theory.
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Look for data points that don’t follow the expected pattern of cooling or that are inconsistent with other cans’ trends.
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Volume of water, starting temperature, can size, location, and measurement intervals must all be controlled.
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Expose each coloured can to a heat lamp or sunlight for the same time. Measure how much the temperature rises in each.
Reflect on:
- Was your prediction accurate?
- Did you control variables well?
- Was your graph neat, clear, and correctly labelled?