Presentation on theme: "Write down the energy transfers in the following:"— Presentation transcript:
1 Write down the energy transfers in the following: SpeakersWind turbinesFanHydroelectric power station
2 Efficiency Learning objectives To be able to collect and use measurements of V and I to calculate power.To be able to calculate energy used from measurements of power and time.To be able to state what energy is ‘wasted’
3 Energy cannot be created or destroyed, Law of conservation of energyAll energy transfers follow the law of conservation of energy:Energy cannot be created or destroyed,just changed in form.This means that energy never just ‘disappears’.The total amount of energy always stays the same,i.e. total input energy = total output energy.In most energy transfers, the energy is transferred to several different forms, which may or may not be useful.Energy that is transferred to unwanted forms of energy is wasted.
4 useful output energy total input energy EfficiencyEfficiency is a measure of how good a device is at changing energy from one form to another.All devices waste energy, so the efficiency of a device is never 100%.Calculating Efficiency:energy efficiency =useful output energytotal input energyX 100
5 Calculating Efficiency This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light energy.How much energyis wasted?In what form is the energy wasted?What is the efficiencyof the bulb?Wasted energy = Total – Useful= 100 J – 45 J= 55 J55 JheatEfficiency = UsefulTotal= 45 J100 J= 0.45 or 45%0.45 or 45%
6 Energy Transfer diagrams Sankey diagramsEnergy Transfer diagramsConsider a light bulb. Let’s say that the bulb runs on 100 watts (100 joules per second) and transfers 20 joules per second into light and the rest into heat. We can show how energy is transferred into various forms using Sankey diagrams. Draw this as a diagram:“Input” energy“Output” energy20 J/s light energy100 J/s electrical energy80 J/s heat energy (given to the surroundings)
7 Example questions Consider a computer: Consider a kettle: 150 J/s electrical energy10 J/s wasted sound20 J/s wasted heatUseful light and soundHow much energy is converted into useful energy?What is the computer’s efficiency?Consider a kettle:Work out each energy value.What is the kettle’s efficiency?Sound energyWasted heatHeat to water2000 J/s electrical energy
8 In which two forms is energy usually wasted? Sound and lightHeat and soundHeat and lightLight and chemical
9 What is the intended energy transfer for a Bunsen burner? Sound lightHeat chemicalChemical lightChemical heat
10 What is the energy change for a falling rock? Kinetic gravitationalGravitational kineticKinetic speedMass speed
11 In what form is energy wasted for all electrical devices? HeatElectricalSoundLight
12 The cost of electricity Learning objectivesUnderstand that Power is calculated from IV and that energy consumed is calculated from:Energy = Power x time.Know that mains electricity is charged for in units of kilowatthours.Use the equation to calculate the cost of electricity: cost = power x time x cost of 1kWh.
13 Buying electricityIn your home will be an electricity meter.It records how much electricity you use in units.From the number of units you use your electricity bill is calculated.26How many units have been used here? ______Demo to look at meter and different appliances
14 electrical energy = power x time Calculating the units of electricityThe amount of electrical energy (i.e. the amount of electricity) used by an appliance depends on its power and how longthe electricity is used for.electrical energy = power x timePower is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in?1 unit of electricity = 1 unit of electrical energy= 1 kilowatt hour (kWh)
15 Cost of electricity = Number of units x cost per unit The cost of electricity is the number of units multiplied by the cost per unit:Cost of electricity = Number of units x cost per unitwhich is the same as……Cost of electricity = Power (kW) x time (h) x cost per kWh (p)Example:How much would 10 units of electricity cost at a price of 9p per unit?Cost = 10 x 9p= 90 p
16 Buying electricity – question 1: A kettle uses 45.2 kWhr of energy.If electricity costs 10p per unit, how much does it cost to use the kettle?Number of units:Number of units of electricity is 45.2 unitsCost of electricity:Cost = 45.2 x 10 p= 452p or £4.52
17 Buying electricity - question 2: An iron that operates at a power of 3 KW for 4 hours uses electricity that costs 8p per unit.How much does it cost for the electricity that it uses?Number of units:kWhr = 3kW x 4hr= 12 kWhrNumber of units of electricity is 12 unitsCost of electricity:Cost = 12 x 8 p= 96p
18 Amount of electrical energy Electricity is measured in units called “kilowatt hours” (kWh). For example…A 3kW fire left on for 1 hour uses 3kWh of energyA 1kW toaster left on for 2 hours uses 2kWhA 0.5kW hoover left on for 4 hours uses __kWhA 200W TV left on for 5 hours uses __kWhA 2kW kettle left on for 15 minutes uses __kWh
19 The Cost of Electricity To work out how much a device costs we do the following:Cost of electricity = Power (kW) x time (h) x cost per kWh (p)For example, if electricity costs 8p per unit calculate the cost of the following…A 2kW fire left on for 3 hoursA 0.2kW TV left on for 5 hoursA 0.1kW light bulb left on for 10 hoursA 0.5kW hoover left on for 1 hour48p8p8p4p
20 The cost of electricity Learning outcomesUse the equation to calculate the cost of electricity:cost = power × time × cost of 1kWhwhere power is measured in kilowatts and time is measured in hours.