Thermal Processes

How is energy transferred

Energy is transferred from a region of higher temperature (hotter object) to a region of lower temperature (cooler object) until both regions have the same temperature (thermal equilibrium).

Thermal equilibrium

Thermal equilibrium describes a state in which two or more objects have the same temperature and that there is no net transfer of energy between them.

Conduction

The transfer of heat through a medium without the physical movement of the medium.

• Metals are good conductors of heat.

• Most non-metals like glass, rubber and wood are poor conductors of heat. Water and air are poor conductors of heat.

How does conduction in solids work?

1. Particles at 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 electrons that move to cooler regions and transfer kinetic energy upon collisions with other particles.

Convection

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.

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Heating:

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.

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Cooling:

1. Air near the air-con gets cooled, contracts, becomes denser 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.

Radiation

The transfer of heat in the form of infrared radiation without the need for a medium.

• All objects emit infrared radiation – the hotter the object, the greater the rate of infrared radiation emitted.

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

   The higher the temperature of an object’s surface relative to the surrounding temperature, the higher the rate of emission of (infrared) radiation.

3. Surface area

   For two objects of the same mass and material, the object with the larger surface area will emit or absorb (infrared) radiation at a higher rate.

Answering Technique

S/N	Step	Examples
1	Identify the feature	Material (e.g. plastic, metal) Air spaces/pockets Lid/ Trapped air Surface colour and texture (e.g. white/ black/ silver/ shiny/ smooth/ dull surface, etc) Surface area
2	Increases/ reduces rate of heat gain/ loss from ____ to ____* by <process>	Process: Conduction/ convection/ radiation/ evaporation
3	State the reason	Conduction good/poor conductor of heat Convection • allows/ reduces air flow that is needed for convection • prevents hot air from escaping • limits the convection currents that can be set up Radiation good/poor absorber/emitter* of radiation *use absorber for heat gain and emitter for heat loss