When a gas or liquid is heated it expands. The expansion means that the density of the fluid gets less so the warmer fluid floats upwards as it is pushed out of the way by colder, and therefore denser, fluid
E.g. Explain why the heating element in a kettle is placed as close to the bottom as possible
Water is a poor thermal condutor. If the heater was at the top, only that layer of water would heat quickly,and, being less dense, would float on top of the colder, denser water at the bottom.
Placing the heater at the bottom means that as the water in contact with it warms up, the water expands and becomes less dense and it is pushed upwards by colder, denser water which sinks lower from above.This results in a circulating current which heats up all of the water much more quickly.
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