Skoool Kerala

Heat energy (thermal energy ) is transferred when different parts of a substance are at different temperatures. It is transferred from an area of higher temperature to one at a lower temperature.

There are three types of thermal transfer: conduction, convection and radiation.
 

Conduction is the transfer of energy without the substance moving.

Metals are good conductors because the hotter the metal is the more kinetic energy the vibrating ions in the metal structure have.

This energy is transferred to cooler parts of the metal by free electrons which diffuse through the metal and collide with ions and with other electrons.

Non-metals are usually poor conductors (insulators).
 

Convection occurs in liquids and gases.

The particles can flow from an area at high temperature to one at a lower temperature.

When heated the particles in a fluid move faster and further apart, causing the gas or liquid to expand.

Warm regions are then less dense than cooler ones, so they rise up and are replaced by cooler, more dense fluid.

Convection currents are set up in the fluid which circulates the heat.
 

Thermal radiation is the transfer of energy by waves.

It can occur across a vacuum because it does not involve the movement of particles.

Infra-red Radiation

Infra-red radiation is heat.

All hot bodies emit infra-red radiation. The hotter the object is, the more energy it radiates.
 

Dark, matt surfaces emit more radiation than light shiny ones that are at the same temperature.
 

Dark, matt surfaces are also good absorbers (poor reflectors) of radiation.

Light, shiny surfaces are good reflectors (poor absorbers) of radiation.

This is why black cars get much hotter in summer than light coloured ones.

Energy Efficient Buildings

The diagram below show the ways in which heat energy is lost from a typical house.

The losses can be reduced in several ways:

• Loft insulation uses materials which trap air in tiny spaces. Air is a poor conductor, so less heat escapes through the roof.
• Double glazed windows have two layers of glass. Between the layers air is trapped. This reduces heat loss by conduction and convection.
• Cavity wall insulation traps air between two layers of bricks to reduce heat loss through the walls.
• Draught excluders around windows and doors stops warm air escaping, and stops cold air getting into the house.
• Carpets also help to prevent draughts, so reduce heat loss by convection. They also trap air, so reduce heat loss by conduction.

Energy consumption in a building can also be reduced by:

• switching lights off
• using low energy bulbs
• reducing the setting on central heating thermostats by a degree
• lagging heating pipes and hot water tanks
• keeping internal doors closed