Heat transfer mechanisms

Heat moves from hotter to cooler by three genuinely different mechanisms, and telling them apart is what makes a heat-transfer problem tractable rather than a single vague notion of "heat flows".

Conduction transfers heat through direct contact, molecule to molecule, without any bulk motion of the material itself — a hot pan handle heats your hand because vibrating atoms in the metal pass their energy along neighbour to neighbour. How fast this happens depends enormously on the material: metals conduct heat far more readily than wood or plastic, which is exactly why a metal spoon feels colder than a wooden one even though both sit at the same room temperature. Nothing about their actual temperature differs; what differs is how quickly each pulls heat away from your significantly warmer hand, and metal simply does it faster.

Convection transfers heat by the bulk motion of a fluid — warmer, less dense fluid rises and is replaced by cooler fluid sinking in, carrying thermal energy along with the moving material itself. This is why convection needs a fluid that can flow; it does not occur in rigid solids, where the material itself cannot circulate. A radiator warms a room mostly by setting up convection currents in the air, not primarily by direct contact with everyone in the room.

Radiation is the odd one out: it transfers energy via electromagnetic waves, which need no medium at all to propagate, and it is the only one of the three mechanisms that works across a vacuum. This is why sunlight warms the Earth across 150 million kilometres of empty space, and why radiation is the dominant heat-loss mechanism from any warm object even in the absence of surrounding air. In most everyday situations all three mechanisms operate simultaneously, and solving a real heat-transfer problem starts with identifying which one dominates.