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Conservation of Energy

Conservation of Energy. Energy is the ability to do work. When work is done energy is transferred . Energy can only be turned from one form to another. Energy cannot be created or destroyed. Consider a car running down a slope :.

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Conservation of Energy

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  1. Conservation of Energy Energy is the ability to do work. When work is done energy is transferred . Energy can only be turned from one form to another. Energy cannot be created or destroyed. Consider a car running down a slope : At the top of the slope the car has potential energy. As it runs down the slope the potential energy ( Ep ) gets transformed into kinetic energy. (Ek)

  2. Conservation of energy 2 If there are NO frictional forces to overcome then Ep at top = Ek at bottom of slope ( Expts on this Loop the Loop, Car running down ramps) In the world outside NAT 5 Physics frictional forces act and so work must be done to overcome friction and there is always some ‘waste’ heat and sound energy produced

  3. Turbinegenerator Heat exchanger Coal Power Stations( Revision ) Burning coal In boiler

  4. Coal Power Station Chemical Energy Heat Energy ( burning coal ) Heat Energy Kinetic Energy ( steam moving through pipes ) Kinetic Energy Electrical Energy ( generator turning )

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

  6. Hydro - Electric Power Station • Potential Energy Kinetic Energy ( water flowing through pipes) • Kinetic Energy Electrical Energy ( turbine turns generator )

  7. Pumped Storage schemes

  8. Energy 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. Expt to determine efficiency of pump storage schemes.

  9. Energy Generation of electrical energy There must be a changing magnetic field about a conductor to induce a voltage across it: Move a magnet towards and away from a wire Or move the wire so that the magnetic field changes

  10. Energy The size of the induced voltage can be increased by : Increasing the number of coils of wire Increasing the magnetic field strength Increasing the rate at which the field is cut North pole wire South pole As the wire is moved up / down an a.c. current / voltage is induced

  11. Model generator Voltage is induced across stator coils.

  12. Working Generator The rotating coil is an electromagnet unlike the permanent magnet in a model generator. There is a voltage induced across the fixed coils ( stator coils ). To increase the size of the induced current the current flowing through the electromagnet is increased. If the coil spins faster then a bigger voltage is induced but the frequency also increases.

  13. The stator coils are fixed. The voltage is induced across these. The rotor is a spinning electromagnet.

  14. Heat Energy Temperature of a substance is a measure of the mean kinetic energy of its particles. Units : Degrees Celcius, oC Heat is a form of energy. The heat stored in a substance is the total kinetic energy of all its particles. Units are Joules, J. 100cm3 of water at 1000C contains more heat energy than 10cm3 water at the same temperature. Heat energy flows from areas of high temperature to low temperature.

  15. Heat Energy Specific Heat Capacity The energy required to raise the temperature of 1 kg of a substance by 1 0C. The same quantity of energy is released if 1 kg cools down by 10C. Units are J kg-10C-1 Water has the highest specific heat capacity : 4180 J kg-10C-1. i.e. 4 180 J of energy are required to raise the temperature of 1 kg of water by 10C. Eexpts on c of water and metal blocks.

  16. Heat Energy Eh = heat energy : J C = specific heat capacity : J kg-10C-1 M = mass : kg T = change in temperature : 0C

  17. Heat Energy Calculate the final temperature if 20 kJ of heat energy are absorbed by 10 kg of water at an initial temperature of 20 0C. Eh = 20 kJ m = 10 kg = 20 000 J c = 4180 J kg-1oC-1

  18. Heat Energy Working

  19. Temp (oC ) Change of State Liquid to gas Solid to liquid When a material changes state there is NO temperature change. Time ( s ) http://www.bbc.co.uk/learningzone/clips/latent-heat-and-changes-of-state/4551.html Expts on cooling by evaporation and measure latent heat of fusion / vapourisation of water

  20. Change of State When we sweat the liquid water evaporates, the energy for this comes from our skin and we cool down. Ice packs in cool boxes melt. The energy for this comes from the air and food in the cool box and it cools down. Coffee makers inject steam into cold milk. The steam turns into liquid giving out energy which is absorbed by the milk, raising its temperature http://www.bbc.co.uk/learningzone/clips/what-energy-changes-happen-during-freezing/8759.html

  21. Change of State Fridges :A refrigerant is pumped around the ice box. It turns from liquid into gas. The energy required for this is taken from the food / air and it cools down. The refrigerant is pumped to the back of the fridge and compressed. When it turns from gas into liquid it gives out energy. The fridge is a heat pump. Energy is transferred from inside the fridge to the outside.

  22. Change of State Specific latent heat of fusion ,lf , The energy required to turn 1 kg of a solid at its melting point into 1 kg of liquid at the same temperature. Units are joules per kilogram , J kg –1 Eh = m x lf heat energy mass latent heat of fusion

  23. Change of State Example Calculate how much ice cream is melted at 0oC if it absorbs 400 kJ of heat energy. Lf = 3.34 x 105 J kg-1 Eh = 400 kJ m =?

  24. Change of State Specific latent heat of vapourisation , lv, The energy required to turn 1 kg of a liquid at its boiling point into 1 kg of gas at the same temperature. Units :Joules per kilogram, Jkg-1 Eh = lv x m Heat energy latent heat of vapourisation mass ( J ) ( J kg -1) ( kg )

  25. Change of State Example An 12 V electric heater draws a current of 2.5A. It is immersed into a beaker of boiling water. Calculate the mass of water turned into gas if it is switched on for 5 minutes. Lv = 2.26 x 106 J kg-1. V = 12 V I = 2.5 A T = 5 mins = 300 s

  26. Calculate power of heater then energy supplied P = V x I = 12 x 2.5 = 30 Watts E = P x t = 30 x 300 = 9000 Joules This energy is used to turn the liquid into gas Use Eh = m x lv Eh = 9000 J M = ? Lv = 2.26 x 106 J kg-1 9000 = m x 2.26 x106 m = 4.0x10-3 kg or 4 g

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