Conduction, convection and radiation — GCSE Physics practice questions

Question 1

A laptop stand has a flat plate that the laptop rests on. The plate must carry thermal energy away from the bottom of the laptop quickly. The stand rests on the user's legs on four small pads, which must let very little thermal energy through to the legs.

Which row gives the best material for the plate and for the pads?

Accepted Answer

B: aluminium | rubber. The plate must transfer energy away from the laptop quickly, so it should be a good thermal conductor such as aluminium. The pads must let little energy reach the legs, so they should be a thermal insulator such as rubber.

Question 2

A student compares how well rods of different metals conduct. She heats one end of each rod in turn in the same flame and measures the time for a temperature sensor at the other end to show a rise of 5.0 °C.

| rod | metal | length (cm) | diameter (mm) | time (s) |
|---|---|---|---|---|
| P | aluminium | 20 | 6.0 | 45 |
| Q | brass | 20 | 6.0 | 95 |
| R | brass | 30 | 6.0 | 210 |
| S | steel | 20 | 4.0 | 160 |

Which conclusion is supported by a fair comparison of two of the rods?

Accepted Answer

C: Aluminium is a better conductor than brass.. Only rods P and Q differ in metal alone: both are 20 cm long and 6.0 mm in diameter. Energy reached the far end of the aluminium rod in 45 s but took 95 s in the brass rod, so aluminium is the better conductor. Rods R and S differ from the others in length or diameter as well, so they cannot be used to compare metals fairly.

Question 3

An egg incubator is warmed by an electric heater of power 40 W. All the energy from the heater goes into the incubator. The graph shows the temperature inside the incubator after the heater is switched on in a room at 20 °C.

The 40 W heater is then replaced by a 60 W heater.

Which row gives the rate at which the incubator loses energy to the room once its temperature is constant with the 40 W heater, and what then happens to its temperature with the 60 W heater?

Accepted Answer

D: 40 W · rises to a new constant value. Once the temperature is constant, the incubator's energy is not changing, so it loses energy to the room at the same rate as the heater supplies it: 40 W. With a 60 W heater it gains energy faster than it loses it, so its temperature rises; a hotter incubator loses energy faster, so the temperature levels off when the loss reaches 60 W.

	rate of energy loss with the 40 W heater	temperature with the 60 W heater
A	0 W	keeps rising for as long as the heater is on
B	0 W	rises to a new constant value
C	40 W	keeps rising for as long as the heater is on
D	40 W	rises to a new constant value