Transfer of heat: Class 9 Physics

Key points

Heat always moves from a region of higher temperature to a region of lower temperature. It travels in three ways: conduction, convection and radiation. Often more than one way acts together. Heat transfer stops when both bodies reach the same temperature.

Conduction is the transfer of heat through a substance from particle to particle, without the substance as a whole moving. It is the main way of heat transfer in solids. When one end of a metal rod is heated, its particles vibrate more strongly and pass energy to their neighbours.

In metals there are also free electrons. These gain kinetic energy at the hot end, move quickly through the metal and pass energy to atoms by collisions. This is why metals conduct heat much better than other solids. Conduction in non-metals depends on the vibration of particles only.

Materials are grouped by how well they conduct heat:

• Good conductors: silver, copper, aluminium and iron. They are used for cooking pots, radiators and heat sinks.
• Bad conductors or insulators: wood, plastic, glass, air, wool, cork and water. They are used for handles of pots, house walls and winter clothes.
• Liquids and gases are poor conductors. Air trapped in wool, feathers and foam makes them good insulators.

A simple test of conduction uses a metal rod with wax and pins. When one end is heated, the wax melts and the pins fall one by one, starting from the hot end. A metal feels colder than wood at the same temperature because metal conducts heat away from your hand faster.

The rate of heat flow by conduction through a slab of area A, thickness L and temperature difference ΔT is Q/t = kAΔT/L. Here k is the thermal conductivity of the material, in W/(m K). It is large for copper and small for wood. The rate increases with a larger area and temperature difference and decreases with a thicker slab.

Convection is the transfer of heat in liquids and gases by the actual movement of the heated fluid. When a fluid is heated, it expands, becomes less dense and rises. Cooler, denser fluid sinks to take its place. This sets up a circulation called a convection current. Convection cannot happen in solids because their particles cannot flow.

Convection is seen in many places:

• Boiling water in a pot: hot water rises from the bottom and cold water goes down.
• Sea breeze: in the daytime the land heats faster than the sea. Air over land rises, and cool air from the sea moves in. At night the land cools faster, so the breeze blows from land to sea.
• Room heaters are placed near the floor and air conditioners high on the wall, so that currents carry heat or cold through the room.
• Ventilation: warm air rises through a ventilator near the ceiling and fresh air enters from below.

Radiation is the transfer of heat as electromagnetic waves, mostly infrared. It does not need any medium. Heat from the Sun reaches the Earth through empty space by radiation. It travels at the speed of light, 3 × 10⁸ m/s, and in a straight line. You feel the warmth of a fire from the side through the air, but the air is heated only a little, so the heat is not coming by conduction.

All bodies emit radiation, and the hotter the body the more it radiates. The amount of radiation absorbed or emitted depends on the surface. Dull black surfaces are the best absorbers and the best emitters. Shiny, white or silvered surfaces are poor absorbers and poor emitters, but good reflectors. So we wear white or light clothes in summer, and the outer surface of an aircraft or a fuel tank is often painted silver.

A vacuum flask keeps hot things hot and cold things cold. It has double glass walls with a vacuum between them. The vacuum stops conduction and convection. The inner surfaces are silvered, which reduces the loss of heat by radiation. The stopper is made of plastic or cork, which is a bad conductor, to reduce the loss by conduction and by convection at the opening.

The greenhouse effect is also an example of radiation. Short wave radiation from the Sun passes through glass or the atmosphere and warms the ground. The ground gives out longer wavelength infrared waves, and glass or gases such as carbon dioxide trap them. This keeps the greenhouse or the Earth warmer. An increase in such gases makes the Earth warmer than before, which is called global warming.

Study tip: In theory questions name the method of heat transfer first, then say why it takes place in that medium. For flask or sea breeze questions, draw a labelled diagram and write each feature with its reason. In the conduction formula keep all quantities in SI units and use the temperature difference, not the individual temperatures.

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Common mistakes

• Saying heat travels from cold to hot, or that cold flows into a body. Only heat flows, from hot to cold.
• Saying that metals feel cold because they are cold. They conduct heat away from the hand faster.
• Stating that convection occurs in solids, or that radiation needs a medium.
• Believing that black surfaces absorb heat but do not emit it. Good absorbers are also good emitters.
• Writing that the vacuum in a flask stops radiation. It stops conduction and convection. The silvering reduces radiation.

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