Convection Currents

Convection: heat transfer by bulk motion of a FLUID (liquid or gas).

Natural convection: driven by density differences (hot fluid rises, cold fluid sinks).

Forced convection: pump or fan moves the fluid.

Doesn't occur in solids (no bulk fluid motion) or vacuum.

Examples: boiling water, atmosphere circulation (winds), ocean currents, room heating with radiator.

Newton's law of cooling: rate of heat loss ∝ (T_body − T_surroundings) — assumes convective cooling.

Convection coefficient h depends on fluid speed, geometry, fluid properties. Typical: natural air h ~ 5 W/m²·K; forced air h ~ 50; water h ~ 500.

Rayleigh number determines whether convection occurs vs pure conduction in a fluid layer.