Heat in the home
(General and Credit) In your Standard Grade Physics course you are expected to know that heat is something that flows from a hot to a cold body. Heat is a form of energy. Heat energy is measured in joules. You should know how heat energy is measured, how it is transferred and how to reduce heat losses in your home. You are also expected to know how heat energy can produce a change of state and what is meant by latent heat.This revision bite deals with the following topics: energy transfer by conduction convection and radiation, specific heat capacity and calculating heat energy, Change of state and latent heat.
You can revise each of these topics in turn or go to the one that you think needs most of your attention. You may wish to print a hard copy for your reference.
Heat energy can be moved from place to place by three processes: conduction [conduction: The transfer of heat energy through a material - without the material itself moving.] , convection [convection currents: Radioactive decay in the core of the Earth causes hot circular convection currents to rise and cold currents to fall. These currents cause the motion of tectonic plates.] and radiation [radiation: Energy carried by particles from a radioactive substance, or spreading out from a source.] .
Conduction of heat energy takes place mainly in solids.
We can explain the transfer of heat energy through a solid by thinking of the heat from the source as causing the atoms of the solid to vibrate and gain kinetic energy. These atoms cause neighbouring atoms to vibrate. Kinetic energy is transferred from one atom to the next. Heat energy is conducted through the solid in this way. As the atoms of the solid gain kinetic energy the temperature of the solid increases.
In liquids and gases (fluids) heat energy is transferred mainly by a process called convection.
The heat source causes the fluid to expand and rise upwards. The hot fluid gives up its heat energy to cooler parts of the fluid. A convection current of warm fluid is set up. A roomful of air can be heated by convection.
The third method of heat transfer is by radiation. In this process, infrared radiation is emitted by the hot object. Infrared waves are radiated from the hot object at the speed of light.
The infrared radiation is absorbed by a cooler body. The absorption of infrared radiation causes the temperature of the cooler body to increase. Infrared radiation is the means by which heat energy reaches us from the Sun.
In our home, we try to cut down heat losses by conduction, convection and radiation.
Loft insulation or under floor insulation reduce heat losses by conduction. The insulation material traps air. Air is a poor conductor of heat. The fibres of the insulation also prevent the trapped air from circulating and causing a convection current to be set up. Cavity wall insulation produces a similar effect. Double glazing is another example where the insulating properties of air reduces heat losses by conduction. Radiation losses can be reduced by using special plasterboard that is backed with a sheet of aluminium foil. The shiny metal foil acts as reflector of infrared radiation. Some of the infrared radiation is reflected back into the room and heat losses by radiation are therefore reduced. Metal foil placed behind a radiator that is fixed to an outside wall also helps to reduce heat losses by radiation.
Click this audio clip to learn about the science of heat.
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