Thermal Energy Transfer: practice questions

Question 1

A school science technician is comparing how well four flat tiles of different material conduct thermal energy: copper, steel, glass and expanded polystyrene foam. Each tile is identical in size and thickness. An electric heating pad, set to the same power on every tile, is stuc…

Accepted Answer

A

Question 2

A tropical-fish keeper fits a fully submerged electric water heater near the bottom of a rectangular fish tank, close to one of the short end walls. The heater is switched on, and after some time a thermometer at the far end of the tank shows that the whole tank has warmed to th…

Accepted Answer

((a)) Convection.  ((b)) The water in immediate contact with the heater is warmed, so it expands and becomes less dense than the surrounding cooler water. This warmer, less dense water rises up through the tank, while cooler, denser water from elsewhere sinks and flows in near the heater to take its pla…  ((c)) With the heater near the bottom, the water it warms is already the least dense water in the tank, so it rises straight up through all the cooler, denser water above it, driving a current that mixes the whole tank. If the heater were near the surface instead, the water it warms w…

Question 3

An engineering team is designing a small satellite. The electronic instruments inside constantly generate waste thermal energy, which must be removed to stop the satellite overheating. Because the satellite orbits in the vacuum of space, there is no surrounding air or other matt…

Accepted Answer

((a)) (Thermal) radiation. This is the only method available because conduction and convection both require a medium of particles to carry energy between them, and there is no air or other matter surrounding the satellite in the vacuum of space; radiation is carried by infrared waves,…  ((b)) The rate at which the satellite receives (or generates) energy must equal the rate at which it transfers energy away. If the electronics began generating waste heat faster than before, the satellite would be gaining energy faster than it was losing it, so its temperature would r…  ((c)) Start both panels at the same high temperature in the vacuum chamber, keeping every other variable the same (size, position in the chamber, starting temperature), then measure the temperature of each panel at equal time intervals as they cool. The panel whose temperature falls f…  ((d)) Increasing the surface area of the panel, while keeping its temperature and finish the same, increases the rate at which it radiates thermal energy, because a larger emitting surface is exposed to space and can radiate infrared energy from more of its area at the same time.

Question 4

Aluminium window frames conduct thermal energy far more readily than uPVC (a rigid plastic) window frames of the same thickness, which is why many homes now use uPVC frames to reduce heat loss through windows in winter.

Which statement correctly explains, at the particle level, why the uPVC frame conducts thermal energy so much more slowly than the aluminium frame?

Accepted Answer

C

Question 5

A metal fire poker has one end resting in the glowing embers of a campfire for several minutes. The other end, which never touches the embers or the flames, is being held by a camper wearing a thick glove. The camper notices that this held end slowly becomes hot.

Which statement correctly explains how thermal energy travels along the solid metal poker, from the end in the embers to the end being held?

Accepted Answer

B

Question 6

A camping equipment company designs a new double-walled steel travel mug. It has an inner steel wall and an outer steel wall, with a narrow gap between them. Almost all of the air is pumped out of this gap, leaving a partial vacuum, and both surfaces facing the gap are polished…

Accepted Answer

((a)) Conduction needs particles in contact with one another to collide and pass on vibrational energy; the near-vacuum in the gap contains almost no particles, so almost no energy can be conducted across it. Convection needs a fluid (a liquid or gas) that can physically circulate, ca…  ((b)) Shiny, polished surfaces are poor emitters of infrared radiation, so the inner wall radiates much less energy across the gap than a dull surface would. Shiny surfaces are also poor absorbers (good reflectors), so any radiation that does cross the gap is mostly reflected back rat…  ((c)) Reducing conduction and convection works regardless of which side is hotter, since removing the particles/fluid from the gap prevents these transfers happening in either direction. Reducing radiation also works both ways, since both surfaces facing the gap are equally poor emitt…

Question 7

An engineer is choosing a surface finish for the metal absorber plate of a rooftop solar water heater. The plate is mounted in direct, unobstructed sunlight for most of the day, and its job is to absorb as much of the Sun's infrared radiation as possible so that water flowing through pipes attached to it warms up quickly.

Which surface finish should the engineer choose for the absorber plate?

Accepted Answer

C

Question 8

On a sunny afternoon at a coastal town, the sand on the beach becomes much hotter than the sea water nearby. A noticeable breeze is blowing from the sea towards the land. (a) Explain, in terms of air density and convection, why this breeze blows from the sea towards the land dur…

Accepted Answer

((a)) The hot sand warms the air immediately above it by conduction, so this air expands and becomes less dense than the air above the cooler sea. Being less dense, the air above the land rises. Denser, cooler air above the sea then flows in at ground level to take its place over the…  ((b)) The breeze reverses direction and blows from the land towards the sea (a land breeze). Once the sand is cooler than the sea, the air above the (relatively warmer) sea is warmed more, expands and becomes less dense than the air above the land, so it rises above the sea. Denser, c…

Question 9

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,…

Accepted Answer

((a)) Aluminium is a metal and so conducts thermal energy much better than a plastic, allowing thermal energy to pass quickly from the hot chip into the heat sink.  ((b)) The greater surface area means more air can be in direct contact with the heat sink at the same time, so both convection (more air being warmed and carrying energy away as it rises and is replaced) and radiation (more surface emitting infrared energy) can happen from a larger ar…  ((c)) Dull, dark surfaces are much better emitters of infrared radiation than shiny surfaces at the same temperature. A dark coating therefore increases the rate at which the heat sink loses thermal energy by radiation, compared with bare shiny aluminium, which would emit radiation po…  ((d)) Fit a small fan to blow air across or through the fins. This creates forced convection, continuously replacing the layer of warmed air next to the fins with cooler air, which removes thermal energy from the heat sink faster than relying on the slower natural convection currents…

Question 10

A clothing company is designing outdoor jackets: one range for a hot, sunny desert country, and another range for a cold, snowy mountain country. Both types of jacket are worn outdoors in direct sunlight for long periods.

Which choice of jacket colour correctly applies the effect of surface colour on thermal radiation?

Accepted Answer

A

Tile material	Temperature rise after 4 minutes
Copper	$38\text{ °C}$
Steel	$21\text{ °C}$
Glass	$4\text{ °C}$
Expanded polystyrene foam	$1\text{ °C}$

Thermal Energy Transfer: Physics 0625 (Cambridge O Level / IGCSE)

Syllabus coverage

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10