Conduction is heat passing through a material, molecule to molecule. A steel spoon in hot chai gets hot at the handle. Copper and aluminium conduct well, which is why heat exchangers use them.

Convection is heat carried by a moving fluid, such as air or water. Natural convection happens on its own: hot water rises in a geyser tank. Forced convection uses a fan or pump, like the fan on an outdoor unit.

Radiation is heat travelling as infrared waves. It needs no medium. You feel it from the sun on a terrace, or from a hot tandoor wall.

Insulation works by slowing conduction. Materials like PUF (polyurethane foam) and nitrile rubber trap tiny pockets of still gas. Still air and gas conduct heat poorly.

Worked example — heat loss through a pipe wall vs insulation (conduction)

Compare 1 m² of 2 mm steel with 1 m² of 25 mm PUF, each with a 40 K temperature difference.

• Steel: Q = (50 × 1 × 40) ÷ 0.002 = 10,00,000 W. In practice, the air film outside limits this, but steel offers almost no resistance.
• PUF: Q = (0.025 × 1 × 40) ÷ 0.025 = 40 W.
• The insulation cuts conduction by a factor of tens of thousands. This is why uninsulated hot pipes waste so much energy.

Remember

• Conduction: through solids. Convection: by moving fluid. Radiation: by infrared waves.
• Good conductors (copper, aluminium) go where we want heat to pass.
• Insulation works by trapping still gas.
• Fans and pumps turn natural convection into faster forced convection.