Thermal Energy Transfer: Question 9
Syllabus 2.3.1, 2.3.3, 2.3.4
A laptop manufacturer fits a heat sink onto the powerful processor chip inside a laptop, to stop the chip overheating. The heat sink is a block of aluminium with many thin, closely spaced fins, and it is finished with a dark grey anodised coating rather than being left as bare, shiny metal.
(a) State why the heat sink is made from a metal such as aluminium rather than from a plastic. [1]
(b) The fins give the heat sink a much greater total surface area than a plain block of aluminium of the same mass. Explain why this increases the rate at which thermal energy is transferred from the heat sink to the surrounding air. [2]
(c) Explain why the heat sink is finished with a dark coating instead of being left as bare, shiny aluminium. [2]
(d) Suggest one change, other than adding fins or changing the surface coating, that would further increase the rate of thermal energy transfer away from the heat sink. Explain why it would have this effect. [2]
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Worked solution
Part (a): Why aluminium rather than plastic
The heat sink’s job is to carry thermal energy quickly away from the hot chip. Aluminium, being a metal, conducts thermal energy far better than a plastic would, so it can move energy from the chip into the fins rapidly, ready to be lost to the surrounding air.
Part (b): Why more fins increase the rate of transfer
A plain block of aluminium has a relatively small surface exposed to the air. Adding many thin fins greatly increases the total surface area of the heat sink without adding much extra mass.
With more surface area:
- More air can be in direct contact with the heat sink at any one moment, so more air is being warmed and carrying thermal energy away by convection at the same time.
- More surface is exposed to radiate infrared energy outward at once, increasing the rate of radiation as well.
Both effects mean that thermal energy is transferred to the surroundings faster than it would be from a plain block with the same mass but far less surface area.
Part (c): Why a dark coating is used
Dull, dark surfaces are far better emitters of infrared radiation than shiny, polished surfaces at the same temperature. Coating the aluminium fins dark grey therefore increases the rate at which the heat sink radiates thermal energy away, compared with leaving the aluminium bare and shiny, which would emit radiation only weakly and let more thermal energy build up near the chip.
Part (d): A further way to increase the rate of transfer
Convection normally relies on natural air currents forming slowly as air near the fins warms, expands and rises. Fitting a small fan to blow air directly across or through the fins forces a continuous stream of cooler air past the heat sink, sweeping away the warmed air layer far faster than natural convection alone would. This forced convection increases the overall rate at which thermal energy is carried away from the heat sink.
Final answers
- (a) Aluminium conducts thermal energy well (unlike a plastic), carrying energy quickly from the chip into the heat sink.
- (b) The fins give a much greater surface area, so more convection and radiation can happen at once, increasing the overall rate of transfer.
- (c) A dark coating emits infrared radiation faster than bare shiny aluminium, increasing the rate of radiation.
- (d) Adding a fan (forced convection) replaces warmed air next to the fins faster than natural convection, increasing the rate of transfer.