The Physical Science Project

Conduction, Convection, and Radiation

Conduction

Conduction is the transfer of heat by molecular collisions. This happens in solids.

How this works:

The KE of the molecules is transferred through collisions.

This means that liquids and gases are poor conductors because the molecules do not collide as often, due to the large amount of space between each molecule. Liquids and gases would be considered thermal insulators because they have poor thermal conductivity, or cannot conduct heat well. Other thermal insulators, like cloth, wood, and styrofoam, are poor conductors because they have large numbers of gas spaces.

The conductivity of a substance depends on the molecular bonding. For example, metals are particularly good conductors. Some metals are really good, like copper, and some are not as good, like brass. This is because each metal has different molecular bonding.

For example, think of pots and pans. Really good pans are made from copper because copper is a good conductor. But all pans will have handles made from rubber or brass, which are thermal insulators, so that you can carry the pot without burning yourself. 

Convection

Convection is the the transfer of heat by movement of substance or mass from one position to another. This happens in liquids and gases.

As the diagram above shows, the warm (orange) air is rising and the cool (blue) air is sinking. The warm air rises because it is less dense from all of the space between the molecules. 

 

As this picture shows, the warm air that was rising is now cold air because it is farther from Earth. Also, after the cold air sinks it is warmed up from being closer to the ground. This starts the cycle all over again and that cycle is convection.

Radiation

Radiation is the transfer of energy by electro magnetic waves. Be sure to remember that radiation needs no medium! This means that radiation can travel through a vacuum.

 For example, think of when you warm your hands by a fireplace. Air is a bad conductor (so conduction is not providing heat) and all of the warmed air is moving up the chimney (so convection can’t be providing the heat) so it must be radiation that is providing the heat for your hands. This is shown in the figure below.

Also know that dark objects are good absorbers of radiation, whereas light objects are poor absorber but good reflectors.This is why we tend to where lighter colored clothes in the summer and darker colors in the winter.

Now let’s examine the thermos, and object that uses knowledge of conduction, convection, and radiation to prevent all three of those from happening.

In a thermos-

The double glass and the air in between act as insulators, thus reducing conduction.

The space between the two glasses also reduces convection between the two glasses.

The inner surface is silvered to prevent radiation. 

All of this works together to keep as little heat as possible from transferring.