Differentiate between natural and forced convection.

Natural (free) convection occurs due to buoyancy forces arising from density differences caused by temperature gradients; fluid motion is driven by gravity (e.g., warm air rising from a radiator). Forced convection involves external means like fans, pumps, or wind to move the fluid (e.g., car radiator with a fan). Forced convection typically has higher heat transfer coefficients (h = 25-250 W/m²-K for air) compared to natural convection (h = 5-25 W/m²-K for air) due to higher fluid velocities.