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What are conduction, convection and radiation?

Class 7 ScienceNCERT chapter: Heat Transfer in Natureइसे हिंदी में पढ़ें

Exam-ready answer

Conduction is the transfer of heat from the hotter part to the colder part of a material, mainly in solids, without the material itself moving. Convection is the transfer of heat in liquids and gases by the actual movement of the heated material. Radiation is the transfer of heat without any medium, as when heat from the Sun reaches Earth.

Explanation

Put one end of a steel spoon in hot tea and the other end soon becomes warm. The particles at the hot end vibrate faster and pass heat to their neighbours, particle by particle, though the spoon itself does not move. This is conduction. Metals like iron, copper and aluminium are good conductors of heat. Wood, plastic and air are poor conductors, or insulators, so pan handles are made of them.

When water is heated in a pan, the water at the bottom becomes warm, expands and becomes lighter, so it rises. Cooler, heavier water from above sinks to take its place and is heated in turn. This circulating flow, called a convection current, spreads heat through the whole liquid. Air is heated the same way, which is why warm air rises above a burning stove or candle.

Standing near a fire or in sunlight, we feel warm even though the air between us and the source is not hot. The heat reaches us by radiation, which does not need any medium and can travel even through empty space. Every hot object gives off heat by radiation. Dark-coloured surfaces absorb more radiated heat than light-coloured ones, which is why light-coloured clothes feel cooler in summer.

Points to remember

  1. Conduction: heat flows from the hotter to the colder part of a solid without the material moving.
  2. Conductors (metals) let heat pass easily; insulators (wood, plastic, air) do not.
  3. Convection: in liquids and gases, heated material rises and cooler material sinks, forming convection currents.
  4. Radiation: heat transfer that needs no medium, for example heat from the Sun reaching Earth.
  5. Dark surfaces absorb more heat by radiation than light-coloured surfaces.

Exam tip

Give one everyday example for each mode: a spoon in hot tea (conduction), boiling water (convection) and sunlight (radiation). Draw convection currents in a heated beaker with arrows showing warm water rising and cool water sinking.

Questions students also ask

Why are cooking pans made of metal but their handles of plastic or wood?
Metal is a good conductor, so heat from the flame quickly passes through the pan to the food. Plastic and wood are insulators, so heat does not travel through them to the hand, and the handle can be held safely.
Why can convection not take place in solids?
In convection, the heated material itself moves from one place to another. The particles of a solid are held in fixed positions and cannot flow, so solids transfer heat mainly by conduction, not by convection.
Why do woollen clothes keep us warm in winter?
Wool is a poor conductor of heat, and it traps air in the tiny gaps between its fibres. Air is also a poor conductor, so this trapped layer slows down the loss of body heat to the cold surroundings and keeps us warm.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.