Convection occurs when a fluid is heated: the heated region becomes less dense and rises, while cooler, denser fluid moves in to replace it. This creates a convection current.
Convection cannot occur in solids (particles are fixed) or in a vacuum (no fluid medium).
A heater is placed at the bottom of a room so that warm air rises and cooler air sinks to be heated, creating a circulation that warms the whole room.
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