READ: Conduction, Convection, and Radiation

1. Heat Conduction

Yummy! These cookies look delicious. But watch out! They just finished baking in a hot oven, so the cookie sheet is too hot to handle without an oven mitt. Touching the cookie sheet with bare hands could cause a painful burn. Do you know why? The answer is conduction.

What Is Conduction?

Conduction is the transfer of thermal energy between particles of matter that are touching. Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Conduction is one of three ways that thermal energy can be transferred (the other ways are convection and thermal radiation). Thermal energy is always transferred from matter with a higher temperature to matter with a lower temperature.

Pass It On

To understand how conduction works, you need to think about the tiny particles that make up matter. The particles of all matter are in constant random motion, but the particles of warmer matter have more energy and move more quickly than the particles of cooler matter. When particles of warmer matter collide with particles of cooler matter, they transfer some of their thermal energy to the cooler particles. From particle to particle, like dominoes falling, thermal energy moves through matter. Click on the animation “Conduction” at this URL to see an animation of conduction:

http://www.hk-phy.org/contextual/heat/hea/condu/conduction_e.html

In the opening photo above, conduction occurs between particles of metal in the cookie sheet and anything cooler that comes into contact with it—hopefully, not someone’s bare hands! For a deeper understanding of this method of heat transfer, watch the animation “Conduction”:

Examples of Conduction

The cookie sheet above transfers thermal energy to the cookies and helps them bake. There are many other common examples of conduction. The figure below shows a few situations in which thermal energy is transferred in this way.

Q: How is thermal energy transferred in each of the situations pictured above?

A: Thermal energy is transferred by conduction from the hot iron to the shirt, from the hot cup to the hand holding it, from the flame of the camp stove to the bottom of the pot as well as from the bottom of the pot to the food inside, and from the feet to the snow. The shirt, hand, pot, food, and snow become warmer because of the transferred energy. Because the feet lose thermal energy, they feel colder.

2. Convection

Do you see the water bubbling in this pot? The water is boiling hot. How does all of the water in the pot get hot when it is heated only from the bottom by the gas flame? The answer is convection.

Defining Convection

Convection is the transfer of thermal energy by particles moving through a fluid (either a gas or a liquid). Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Convection is one of three ways that thermal energy can be transferred (the other ways are conduction and thermal radiation). Thermal energy is always transferred from matter with a higher temperature to matter with a lower temperature.

How Does Convection Occur?

The Figure below shows how convection occurs, using hot water in a pot as an example. When particles in one area of a fluid (in this case, the water at the bottom of the pot) gain thermal energy, they move more quickly, have more collisions, and spread farther apart. This decreases the density of the particles, so they rise up through the fluid. As they rise, they transfer their thermal energy to other particles of the fluid and cool off in the process. With less energy, the particles move more slowly, have fewer collisions, and move closer together. This increases their density, so they sink back down through the fluid. When they reach the bottom of the fluid, the cycle repeats. The result is a loop of moving particles called a convection current. You can learn more about convection currents by watching the cartoon video called “Convection”:

Examples of Convection

Convection currents transfer thermal energy through many fluids, not just hot water in a pot. For example, convection currents transfer thermal energy through molten rock below Earth’s surface, through water in the oceans, and through air in the atmosphere. Convection currents in the atmosphere create winds. You can see one way this happens in the Figure below. The land heats up and cools off faster than the water because it has lower specific heat. Therefore, the land gets warmer during the day and cooler at night than the water does. During the day, warm air rises above the land and cool air from the water moves in to take its place. During the night, the opposite happens. Warm air rises above the water and cool air from the land moves out to take its place.  

Q: During the day, in which direction is thermal energy of the air transferred? In which direction is it transferred during the night?

A: During the day, thermal energy is transferred from the air over the land to the air over the water. During the night, thermal energy is transferred in the opposite direction.

This video shows a convection current. A tank of water is put over a bowl of hot water and a bowl of ice water. The blue coloring is added over the ice water and the red is added over the hot water. 

3. Thermal Radiation

These three friends are warming their hands over a bonfire. They don’t have to touch the fire to feel its warmth. How is warmth from the fire transferred to their hands? In this article, you’ll find out.

Introducing Thermal Radiation

The bonfire from the opening image has a lot of thermal energy. Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Thermal energy from the bonfire is transferred to the friends’ hands by thermal radiation. Thermal radiation is the transfer of thermal energy by waves that can travel through air or even through empty space, as shown below. When the waves of thermal energy reach objects, they transfer the energy to the objects, causing them to warm up. This is how the fire warms the hands of the friends sitting near the bonfire. This is also how the sun’s energy reaches Earth and heats its surface. Without the energy radiated from the sun, Earth would be too cold to support life as we know it.

Thermal radiation is one of three ways that thermal energy can be transferred. The other two ways are conduction and convection, both of which need matter to transfer energy. Radiation is the only way of transferring thermal energy that doesn’t require matter. To learn more about thermal radiation, watch “Radiation”:

Sources of Thermal Radiation

You might be surprised to learn that everything radiates thermal energy, not just really hot things such as the sun or a fire. For example, when it’s cold outside, a heated home radiates some of its thermal energy into the outdoor environment. A home that is poorly insulated radiates more energy than a home that is well insulated. Special cameras can be used to detect radiated heat. In the Figure below, you can see an image created by one of these cameras. The areas that are red are the areas where the greatest amount of thermal energy is radiating from the home. Even people radiate thermal energy. In fact, when a room is full of people, it may feel noticeably warmer because of all the thermal energy the people radiate!

Q: Where is thermal radiation radiating from the home in the picture?

A: The greatest amount of thermal energy is radiating from the window on the upper left. A lot of thermal energy is also radiating from the walls of the home.

4. Summary and Vocabulary

Summary

• Conduction is the transfer of thermal energy between particles of matter that are touching. Thermal energy is always transferred from particles of warmer matter to particles of cooler matter.
• When particles of warmer matter collide with particles of cooler matter, they transfer some of their thermal energy to the cooler particles.
• Convection is the transfer of thermal energy by particles moving through a fluid. Thermal energy is always transferred from an area with a higher temperature to an area with a lower temperature.
• Moving particles transfer thermal energy through a fluid by forming convection currents.
• Convection currents move thermal energy through many fluids, including molten rock inside Earth, water in the oceans, and air in the atmosphere.
• Thermal radiation is the transfer of thermal energy by waves that can travel through air or even through empty space. This is how thermal energy from a fire is transferred to your hands and how thermal energy from the sun is transferred to Earth.
• Everything radiates thermal energy, even objects that aren’t very warm.

Vocabulary

• conduction: Transfer of thermal energy between particles of matter that are touching.
• convection: Transfer of thermal energy by particles moving through a fluid.
• convection current: Flow of particles in a fluid that occurs because of differences in temperature and density.
• thermal radiation: Transfer of thermal energy by waves that can travel through air or across space.