Conduction, convection and radiation

Heat always flows from hot to cold

When two things at different temperatures touch, or are near each other, heat flows from the hotter one to the colder one. It can travel in three ways: conduction, convection and radiation.

Conduction: passing heat along

Fix a metal strip to a stand, stick four pins to it with wax, and heat the end away from the stand with a candle. The pin nearest the flame falls first, then the next, and so on. Heat travels along the strip, and as it reaches each pin the wax melts. Each particle of the metal passes heat to the particle next to it, but the particles themselves do not move from their places. This is conduction, and it is the main way heat moves through solids.

Good and poor conductors

Materials that let heat pass easily are good conductors: copper, aluminium, iron and steel, that is, metals. Materials that do not are poor conductors, also called insulators: wood, plastic, glass, clay, porcelain, wool and air. A strip of wood or glass would not make the pins fall, because heat hardly moves along it.

We use both kinds every day. Cooking pans are made of metal so the food heats quickly, while their handles are made of a poor conductor. Tea stays hot longer in a clay or porcelain cup. Air trapped in the pores of wool, or between two thin blankets, slows the heat leaving our body, so we stay warm. Houses in very cold or very hot places use the same idea, with hollow bricks or walls of two wooden layers filled with mud.

Convection: warm fluid rises

Hang two paper cups upside down from the ends of a balanced stick and put a candle under one. That cup rises, because the air inside it warms, expands, becomes lighter and goes up. A balloon left in the Sun swells for the same reason, and smoke, a mix of hot gases and tiny particles, rises because it is warmer than the air around it.

Liquids behave the same way. Put a grain of potassium permanganate at the bottom of a beaker of water and heat it from below: a coloured streak rises in the middle and comes down at the sides. Warm water rises, cooler, heavier water sinks to take its place, and the cycle goes on until all the water is hot. Heat carried by the actual movement of the particles of a liquid or gas is convection.

Radiation: no medium needed

Sitting near a fire, we feel warm even though nothing flows between us and the flames. The heat reaches us directly by radiation, the same way the Sun’s heat crosses space to reach the Earth. Radiation needs no medium. Every object radiates heat: a hot pan kept away from the flame cools down because it radiates heat to its surroundings. Light-coloured clothes reflect most of the heat falling on them, while dark ones absorb more, so we wear light colours in summer and dark ones in winter.

Comparing the three

Way	How heat moves
Conduction	particle to particle; particles stay in place (solids)
Convection	particles move; warm fluid rises (liquids, gases)
Radiation	directly from a hot object; no medium needed

All three can happen at once. When water is heated in a pan, the pan heats by conduction, the water by convection, and our hands feel warm by radiation. Read how convection makes the wind change direction at the seaside in land and sea breezes. Answers to the textbook questions are in the NCERT solutions for Chapter 7; see all of Chapter 7 notes.