15. Thermal Processes

Question 1

How Energy is Transferred by Heating

Answer 1

Energy is transferred from a region of higher temperature to a region of lower temperature until both regions reach thermal equilibrium.
Thermal equilibrium describes a state in which 2 or more objects have the same temperature and there is no net transfer of energy between them.

Question 2

Conduction

Definition

Answer 2

Conduction is the transfer of heat through a medium without the physical movement of the medium.

Fastest in solids and slowest in gases (due to the distance between particles)
Metals are good conductors and non-metals are poor conductors (due to the presence of free moving electrons)

Question 3

Describe in terms of particles how conduction occurs

Answer 3

1. Particles at the hotter end gain kinetic energy and vibrate more vigorously.
2. These particles collide with neighbouring particles, transferring kinetic energy from one particle to another.
3. [for metals] Metals have free moving electrons that move to cooler regions and transfer kinetic energy upon collisions with other particles.

Question 4

Convection

Definition

Answer 4

Convection is the transfer of heat from one place to another by the physical movement of a medium.

Occurs in fluids (liquid and gas) by convection currents due to a difference in density

Question 5

Describe how a fluid is heated by convection in terms of density

e.g. beaker of liquid heated by a flame at the bottom

Answer 5

1. Liquid near the heat source gets heated, expands, becomes less dense and rises.
2. Cooler, denser liquid at the top sinks to take its place.
3. This sets up a convection current which heats up the whole liquid.

Question 6

Describe how a fluid is cooled by convection in terms of density

e.g. room cooled by air conditioner near the ceiling

Answer 6

1. Air near the air conditioner gets cooled, contracts, becomes more dense and sinks.
2. Warmer, less dense air at the bottom rises to take its place.
3. This sets up a convection current which cools down the whole room.

Question 7

Radiation

Definition

Answer 7

Radiation is the transfer of heat in the form of infrared ratdiation without the need for a medium.

Hotter object has a greater rate of emission of infrared radiation

Question 8

3 Factors affecting the rate of emission and absorption of infrared radiation

Answer 8

Factors affecting the rate of emission and absorption of infrared radiation:
1. Colour and texture of surface: Dull/rough/black surfaces are better emitters and absorbers of infrared radiation than shiny/smooth/silvery/white surfaces
2. Surface temperature: Larger difference in temperature between object and surroundings means higher rate of emission and absoption of infrared radiation
3. Surface area: Larger surface area means higher rate of emission and absorption of infrared radiation