IGCSE Physics 0625: Convection Currents in a Heated Fish Tank

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 the same temperature, even though the heater itself only directly warms the water in immediate contact with it.

(a) Name the process by which thermal energy spreads from the heater to every part of the tank. [1]

(b) Explain, in terms of the density of the water, how this process carries thermal energy away from the heater and around the whole tank. [3]

(c) The fish keeper's friend suggests it would work just as well to hang the heater near the surface of the water instead of near the bottom. Explain why fitting the heater near the bottom of the tank is more effective at warming the whole tank. [2]

Part (a): Naming the process

Thermal energy spreads through the bulk of the water itself, not along a solid path, so the process is convection.

Part (b): Explaining the convection current

Follow the water near the heater step by step:

Water in direct contact with the heater absorbs thermal energy and gets warmer.
As it warms, this water expands, so a given mass of it takes up more space. It becomes less dense than the surrounding, cooler water.
Because it is now less dense, this warm water rises upwards through the tank.
Cooler, denser water from elsewhere in the tank sinks down and flows in near the heater to take the place of the water that rose.
This newly arrived water is then warmed by the heater in turn, and the cycle repeats.

This continuous loop of rising warm water and sinking cool water is a convection current, and it is what carries thermal energy from the heater to every part of the tank, not just the water immediately next to it.

Part (c): Why the heater’s position matters

For a convection current to form, the warmed water must be able to rise through cooler, denser water above it.

With the heater near the bottom, the water it warms becomes the least dense water in the tank and rises freely through the cooler water that fills the rest of the tank above it. This sets up a current that keeps circulating and mixing the whole tank.
With the heater near the surface, the water it warms is already at the very top of the tank. There is no cooler water above it for the warm water to rise through, so no convection current is driven downward. The warmed water would simply stay near the top, and the water lower down would remain cool.

This is why placing the heater near the bottom of the tank warms the whole tank far more effectively.

Final answers

(a) Convection
(b) Water at the heater warms, expands and becomes less dense, so it rises; cooler, denser water sinks in to replace it and is warmed in turn, setting up a convection current that carries thermal energy around the tank.
(c) A heater near the bottom warms the least dense water in the tank, which rises through all the cooler water above it, driving a convection current; a heater near the surface has no cooler water above it to rise through, so no current forms to warm the water below.