The immersion heater

An immersion heater heats water — but where it sits in the jug matters.

Figure to be added

Figure 9.31 — an immersion heater (a) only half-dipped near the top of a jug of water, and (b) lowered to the bottom.

Heater at the top

The water at the top warms quickly but the water below warms slowly.

Convection currents do not travel down, so heating all the water takes a long time.

Heater at the bottom

Heated water rises and cooler, denser water sinks to take its place.

The convection currents keep circulating, so all the water heats quickly.