Power is defined as the rate of transferring energy. The word rate means per second. Another way of saying this is : ‘Power is the energy transferred per second’. Power is measured in Watts, W, Energy in Joules, J and time in seconds, s.

An appliance rated at 100 watts transfers 100 Joules of energy every second. I watt equals 1 joule per second ( 1W = 1J/s or J s -1 ). This can be written in equation form : Energy = Power x time E = P x t

Aim : To verify that Energy = Power x time. This is an example of a direct relationship. If energy doubles then time doubles,if power remains the same. A graph of energy v time will be a straight line graph through the origin. The gradient or slope equals the Power of the appliance.

Calculate the energy used by a 2 kW kettle in 5 minutes. P = 2 kW t = 5 mins E = ? P = 2000 Wt = 300sE = ?

Power 3 The power of a component can also be calculated if the current flowing and voltage across it is known: Power = Voltage x Current Power, P : Watts, W Voltage, B : Volts, V Current, I : Amps, A

Measuring Power The ammeter is connected in SERIES with the bulb and the voltmeter is connected in PARALLEL with the bulb. Current flows THROUGH the bulb and the Voltage is ACROSS the bulb.

P = V x I

Cost of Electrical Appliances The electricity meter in a house is an ENERGY meter. One unit of ‘electricity’ is the energy used by a 1kilowatt appliance switched on for 1 hour. ( 1 kilowatt hour, 1 kWh ) E = P x t P = 1 kW = 1000 W T = 1 hr = 60 x 60 s = s E = 1000 x = J or 3.6 x 10 6 J

Example cost of appliance Mr Smith’s neat TV has a power rating of 200 W. He watches a football match lasting 2 hrs. How much does it cost him if 1 unit of ‘electricity’ costs 10 p? Cost = No of units x cost of 1 unit No of units = Power ( kW ) x time ( hrs) P = 200 W = 200/1000 = 0.2 kW t = 2 hrs No of units = Power x time = 0.2 x 2 = 0.4 units ( kwh ) Cost = No of units x cost of 1 unit = 0.4 x 10 = 4.0 p

Energy Efficiency Efficiency : This is a measure of how well a machine does its job i.e. carries out its useful energy change e.g. The useful energy change in a motor is electrical energy to kinetic energy ( E E E K ) But it also produces some waste heat and sound energy ( E H + E S )

Efficiency 2 Example : Calculate the efficiency of a power station producing 600 MJ of electrical energy per second if it is supplied with 1500 MJ of chemical energy. Useful work = 600 MJWork input = 1500 MJ

Energy As power is the rate of doing work ( work transferred per second ) then we can write:

Energy 1.Fossil Fuels : Coal, oil and gas are our main energy source. 2.The supply of these fuels is finite. 3.Burning fossil fuels causes pollution. 4.Fossil fuels are formed from the remains of dead plants / animals

Energy Energy can be conserved by: 1.Turning off lights when not in use, turning down thermostats, 2.Using public transport more often instead of our cars, walk, cycle, 3.Using hot air from fridges/ freezers in shops to heat water instead of the air outside the shop.

Energy Renewable sources will not run out e.g. Wind, wave, solar, tidal, geothermal, hydro electric Non renewable sources will run out e.g. coal, oil and gas.

Energy Renewable sources have advantages / disadvantages e.g SourceAdvantageDisadvantage SolarFree to useNot always available WindNo air pollution caused Expensive to connect up to national grid HydroelectricFree to useNeed to flood a valley

Energy Power Stations Power stations are energy changers : A coal power station turns the chemical energy stored in coal into electrical energy. Coal, Oil, Gas and Nuclear Power stations are called Thermal power stations as they produce vast quantities of heat energy

Energy The output of a power station tells us how much electrical energy it produces per second ( power ): Longannet power station has an output of 1200 MW ( mega watts,million watts). Every second it produces 1200MJ of electrical energy. Longannet power station is 40% efficient. For every 100 J of chemical energy supplied it produces 40 J of electrical energy.

Coal Power Stations Burning coal In boiler Heat exchanger Turbinegenerator

Coal Power Station Chemical Energy Heat Energy ( burning coal ) Heat EnergyKinetic Energy ( steam moving through pipes ) Kinetic EnergyElectrical Energy ( generator turning )

Hydro - Electric Power Station Water has potential energy Water flows through pipes simulation

Hydro - Electric Power Station Potential EnergyKinetic Energy ( water flowing through pipes) Kinetic EnergyElectrical Energy ( turbine turns generator )

Nuclear Power Stations The Uranium fuel is bombarded by neutrons. This causes the uranium nuclei to break up producing : energy, radioactive waste and more neutrons. These neutrons can then be used to split further uranium nuclei. This is called a chain reaction. The break up of one uranium nucleus is called a FISSION reaction.

Chain Reaction Neutron Uranium nucleus The neutrons produced in a fission reaction are used to produce more fission reactions.

Nuclear Power Station Fuel Rods Coolant circulates Heat exchanger : liquid turned into steam Turbinegenerator

Nuclear Power Station Nuclear EnergyHeat Energy ( fission reactions in core ) Heat EnergyKinetic Energy ( steam moves through pipes) Kinetic EnergyElectrical Energy ( turbine turns generator )

Nuclear Power Stations Whilst nuclear power stations produce large quantities of electrical energy from a small mass of fuel the waste produced is highly active and has a long half life. The waste will be harmful for a long time. At present this waste is stored above ground. There are plans to dispose of it underground.

Pumped Storage schemes During periods of peak demand they operate as conventional hydro electric stations. During early hours of morning the excess electrical energy generated by the coal and nuclear stations is used to pump water back behind the dam. The pumps are inefficient. This allows the coal and nuclear stations to be run at peak efficiency 24 / 7.

Pumped Storage schemes