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Keeping homes warm, and how insulation works

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Heat energy can be transferred by conduction, convection, and radiation. The loss of heat from poorly insulated homes wastes energy resources and costs money. There are several ways to insulate homes against heat loss. It is possible to evaluate different ways to save energy by calculating their payback times.

# Transferring heat energy

Heat is thermal energy. It can be transferred from one place to another by conduction, convection and radiation. Conduction and convection involve particles, but radiation involves electromagnetic waves.

## Conduction

Thermogram of a pan being heated on a stove

Heat energy can move through a substance by conduction. Metals are good conductors of heat, but non-metals and gases are usually poor conductors of heat. Poor conductors of heat are called insulators. Heat energy is conducted from the hot end of an object to the cold end.

Metals consist of metal ions and free electrons. Ions are charged particles formed when the metal atoms lose their electrons - and these become free electrons. The ions are packed closely together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons. These move through the structure of the metal, colliding with ions as they go.

Heat transfer by conduction

## Convection

Liquids and gases are fluids. The particles in these fluids can move from place to place. Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy. Heat energy is transferred from hot places to cooler places by convection.

Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated than they do when they are cold. As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size.

The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up.

All objects give out and take in thermal radiation - also called infrared radiation. The hotter an object is, the more infrared radiation it emits. This infrared radiation can be reflected by shiny surfaces. Infrared radiation is absorbed best by black, dull - not shiny - surfaces.

Infrared radiation is a type of electromagnetic radiation. It involves waves, rather than particles. Unlike conduction and convection, radiation can even work through the vacuum of space. This is why we can still feel the heat of the Sun even though it travels through a vacuum for 150 million km to reach the Earth.

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