Heat Transfer (G3 Lower Secondary)

Kelvins (K)

SI unit for temperature

Density

Mass per unit volume (Density = Mass / Volume)

Determines whether substances float (lower density) or sink (higher density)

Expansion

Increase in volume, a process with occurs upon heating

As temperature increases, particles become further apart.
Mass remains constant while volume increases, so density decreases.
This is why hot air rises.

Contraction

Decrease in volume, a process with occurs upon cooling

As temperature decreases, particles become closer together.
Mass remains constant while volume decreases, so density increases.
This is why cool air sinks.

Bimetallic strip

Strip consisting of two metals which bends when heated or cooled, due to different rates of expansion or contraction between the two metals

Conduction

Mode of heat transfer where particles do not move around. Rather, particles vibrate and collide with each other.

Rate of conduction

Conduction is faster when particles are closer together.
Therefore, rate of conduction: Solid > Liquid > Gas

Within solids: Metals (e.g., copper, steel, aluminum) > Non-metals (e.g., plastic)
Solids with pockets of still air trapped in them (e.g., wood, Styrofoam) are poorer conductors as air is a poor conductor of heat.

Rate of heat transfer is higher of the temperature difference between the emitter and absorber is higher.

Convection

Mode of heat transfer where particles move around.

Rate of convection

Only liquids and gases can transfer heat via convection as their particles can move around.

Rate of heat transfer is higher of the temperature difference between the emitter and absorber is higher.

Convection current

Motion of fluid which occurs when hot fluid rises and cool fluid sinks. Therefore, coolers should be placed on top, while heaters should be placed at the bottom.

Radiation

Mode of heat transfer that does not involve particles (of matter). Heat is transferred through electromagnetic waves.

Rate of radiation

An object is a better emitter and absorber of thermal radiation if:
- Its surface is black
- Its surface is rough

Rate of heat transfer is higher of the temperature difference between the emitter and absorber is higher.

Heat transfer through vacuum

Only radiation

Conduction and convection cannot happen through a vacuum as it contains no particles.

Heat transfer through solids

Only radiation and conduction

Convection cannot happen through a solid as its particles cannot move around.

Heat transfer through liquids and gases (fluids)

Radiation, conduction, and convection

Solar heaters

Application of transfer of heat energy through radiation (in this case, form the Sun)