Presentation on theme: "AIMS State and apply the law of conservation of energy"— Presentation transcript:
1 AIMS State and apply the law of conservation of energy Fixed amount in closed systemsChange form not create or destroyUnderstand need to transform energyExplain any lossesUse systems diagrams to account for energy changesIdentify energy forms and changes within a systemCalculate energy transfers
2 The Law of Conservation of Energy The conservation of energy is a fundamental concept of physics.Along with the conservation of mass and momentum.Derived from first law of thermodynamics.Within a closed system, the amount of energy remains constant and energy is neither created nor destroyed.Energy can be converted from one form to another but the total energy within the domain remains fixed.
3 Energy Transformation How energy can be converted to other forms is important to technologistsSome forms are directly interchangeableDropping a stonePotential KineticOthers require several stagesCoal burnt in a power station to produce electrical powerChemical heat kinetic electrical
4 Systems ApproachSystems diagrams can be used to summarise energy changesConsider a light bulb (simplified)Produce system diagrams for an electric motor and an electric generatorLIGHT BULBELECTIRCALLIGHT
5 Energy Transformation Examples State the energy form at points A, B, C and DDA: Potential energyB: Kinetic Energy (linear motion)C: Kinetic Energy (rotary motion)D: Electrical EnergyBC
6 Energy Transformation Examples State the energy form at points A to H in the diagram opposite.Describe the energy changes that take place within the systemA: PotentialB: ElectricalC: SoundD: ElectricalE: LightF: ElectricalG: ElectricalH: Potential
7 Energy ‘losses’ during transformation We accept that energy cannot be created or destroyedThis tells us that the energy output of a system equals the energy inputHOWEVER, not all the energy is used to do USEFUL workWhen a conversion takes place there is always a lossExamples are sound, friction or heatGo back to the energy conversion diagrams for the bulb, motor and generator and add any losses to the output side
8 Energy Losses in a Wind Turbine A turbine can be used to generate electricity. The generator can be connected to it in two ways.coupled directly to vanescoupled via shafts and gears
9 Energy ‘losses’ during transformation List the energy conversions that take place during its operationDescribe the energy losses in both systemsWhich do you think is more efficient?
10 Calculating Energy Transfers: A Falling Ball E = EP1E = (EP2 + EK1) = EP1E = EK2 = (EP2 + EK1) = EP1If the mass is 5kg and the building is 25m tall calculate the final velocity and the kinetic energy at impact
11 Worked ExampleA body of mass 30 kg falls freely from a height of 20 metres. Find its final velocity and kinetic energy at impact.First calculate the initial potential energy.EP = mgh = 30 9.81 = 5.88 kJThis potential energy is converted or transferred into kinetic energy, which means that the kinetic energy at impact is equal to 5.9 kJ.To calculate the final velocity of the body we begin by taking EK = 5.9 kJ.EK = ½mv² 10³ = ½ 30 v²v² = 392.4v = 19.8 m/s
12 Pupil ProblemsA 5 kg mass is raised steadily through a height of 2 m. What work is done and what is the body’s potential energy relative to the start?A body of mass 30 kg is projected vertically upwards with an initial velocity of 20 m/s. What is the initial kinetic energy of the body and to what height will it rise?A mass of 20 kg is allowed to fall freely from a certain height above a datum. When the body is 16 m above the datum, it possesses a total energy of 3,531 J. What is the starting height of the object?
13 Efficiency Calculating efficiency The efficiency of an energy transformation is ameasure of how much of the input energy appearsas useful output energy.The efficiency of any system can be calculatedusing the equation:Efficiency, = Useful energy outputTotal energy input
14 Worked Example An electric lift rated at 110 V, 30 A raises a 700 kg load a height of 20 m in two minutes.By considering the electrical energy input and the potential energy gained by the mass, determine the percentage efficiency of this energy transformation.
15 Worked Problem Ee = ItV = 30 120 110 = 396 kJ Potential energy gained is calculated as follows.EP = mgh= 700 9.81 20= kJPercentage Efficiency = Useful Energy Output 100%Total Energy Input= 100% = 34.7%
16 Pupil Problems(1) An electric kettle is rated at 240 V, 10 A. When switched on it takes three minutes to raise the temperature of 0.5 kg of water from 20C to 100C.Determine:The electrical energy supplied in the three minutesThe heat energy required to raise the temperature of the waterThe efficiency of the kettle.
17 Pupil ProblemsBoxes in a factory are transferred from one floor to another using achute system as shown above. The boxes start from rest at the top ofthe chute and during the decent there is a 40 per cent loss of energy.The boxes weigh 10 kg each.Calculate the velocity of the boxes at the bottom of the chute.
18 Energy AuditsYour Teacher will show you how to construct an energy audit