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Convection occurs in fluids (liquids or gases) as the hot (lower density) fluid rises and the cooler (higher density) fluid sinks.
Convection cannot take place in solids because the particles are not free to move.
Heat can only pass through a vacuum by radiation since there are no particles in a vacuum to allow for conduction or convection.
Houses can be insulated against heat loss by wall insulation, double glazing, loft insulation and draft excluders.
Small objects (bigger surface area in relation to their size) lose heat faster than large objects
Forms of energy
Energy can exist in a variety of forms:
Chemical energy – stored in food or fuel.
Kinetic energy – in moving objects
Gravitational potential energy – due to position of an object above ground.
Elastic energy – stored in springy objects
Electrical energy – carried by moving charges in a current.
Thermal energy – energy in particles of a hot object
Transformation of energy
Energy can be transformed from one form to another.
When this happens no new energy is created or destroyed it simply changes form – this is called conservation of energy.
In many cases the energy is transformed into heat energy in the surroundings. As this happens the energy becomes more diluted and is very difficult to reuse.
Useful energy is energy which is transformed for the purpose for which it was intended. Example electrical energy can be usefully converted into light.
Wasted energy – this is energy which is transformed into a form which was not intended and often results in the surroundings becoming warmer. Example – the electric light also heats the room. Wasted energy generally results in an effective loss of money.
Energy is measured in Joules (or kilojoules)
Energy supplied = useful energy + wasted energy.
Efficiency = useful energy / total energy
Efficiency is often expressed as a percentage.
Electrical devices transform the kinetic energy of the moving charges in the current into various other energy types.
Examples are: light (lamps), heat (ovens), sound (radios), movement (motors).
Most electrical devices also waste energy – usually as thermal energy.
The unit of power is the watt (W), 1 watt = 1 Joule/second.
1 kilowatt (kW) = 1000 watts (W)
Power is the rate at which energy is transformed.
Power (W) = energy (J) / time (s)
Using electrical energy
Electrical energy used is measured in kilowatt hours (kWh).
1kWh = 1000 watt appliance used for 1 hour.
Cost = number of units x cost per unit.
1 unit costs about 15p at present – this is going up rapidly!
Example: Cost of using a 2000 watt heater for 3 hours will be:
Cost = 2kW x 3 hours x 15p = 90 pence
The National Grid
The National Grid is the cables and transformers that supply homes and factories with electrical power.
Step up transformers are used to produce very high voltages in overhead cables (about 130000V) – this reduces heat loss in the cables because the current is very small.
Step down transformers are used to provide safe voltages (230V) for homes.
Fuel for electricity
The diagram shows the basic parts of a power station. The heat is produced by often burning fossil fuels.
Energy from wind and water
Wind and water can spin an electrical generator in these ways:
Wind turns the turbine blades.
Waves produce a rocking motion.
Water flows through tidal barrages and hydroelectric dams.
All these produce renewable, pollution free electricity but all can damage the environment and are unsightly.
Energy from the Sun and the Earth
Solar cells can produce electricity when light falls on them – this is an expensive way to make electricity.
Solar panels can heat water.
Geothermal power stations use steam produced by radioactive substances underground drive a generator. This is a renewable energy source but is not available in many places.
Energy and the environment
Burning fossil fuels like coal produce carbon dioxide (a greenhouse gas) and acid rain.
Nuclear fuels do not produce greenhouse gases and use a very small volume of fuel but the radioactive waste is very difficult to store and remains dangerous for many years.