# Energy Dissipation Efficiency Do now activity 1 Describe

Energy Dissipation & Efficiency Do now activity: 1. Describe a simple energy transfer 2. What types of energy stores does a bungee jumper have: a) Before he jumps b) During his fall Keywords Energy Dissipation Waste Efficiency

Progress indicators GOOD PROGRESS: State where energy is lost in a particular system Identify the correct calculation to use on a particular problem OUTSTANDING PROGRESS: To consider the useful and wasted energy in a system and complete calculations on efficiency calculations

A machine transfers energy for a purpose. Friction between the moving parts of a machine causes the parts to warm up. So, not all of the energy supplied to the machine is useful. Useful energy is energy transferred to be where it is wanted in the way that is wanted Wasted energy is the energy that is not usefully transferred This energy is dissipated to the surroundings

Example: Useful energy transfer: • Kinetic Energy • Gravitational Potential Energy Chemical energy store of fuel Wasted energy transfer: • Thermal Energy • Sound Energy

Task: Complete the flow diagrams on the worksheet to represent the useful and wasted energy transfers for the following systems: Riding a bike Running Washing machine

When energy spreads out it gets less and less useful Thermal energy store of the water decreases as it transfers energy by heating to thermal energy store of the radiators. The thermal energy store of the radiators gets transferred to thermal energy store of the air in the room and eventually this is transferred through the walls to the air outside the house.

Quick Check: a) Describe the energy transfers and changes to energy stores of a pendulum as it swings from one side to the middle, and then to the opposite side. (4 marks) b) Explain why a swinging pendulum eventually stops (3 marks)

Self-assessment: a) As the pendulum is raised to point A the pendulum gains gravitational potential energy. When the pendulum is released the gravitational potential energy is transferred to the kinetic energy store of the object. As the pendulum moves past point B to point C some of the kinetic energy is transferred back to the gravitational potential energy store. Some energy is lost from the pendulum to the surroundings as thermal energy, due to the frictional force of air resistance between the pendulum and the surrounding air. b) In air, the pendulum will eventually stop moving. As the pendulum is moving through the air the frictional force of air resistance means that the kinetic energy store of the object is slowly being transferred to thermal energy store of the surrounding air. Until eventually the kinetic energy store is completely dissipated as thermal energy to the surroundings and the pendulum comes to a stop.

Energy and efficiency Energy supplied to a device is called the input energy The useful energy transferred by the device is useful output energy Input energy = Some of the input energy doesn’t get usefully transferred as output energy, this is the wasted energy of the system. Useful energy = 80 J of light energy Wasted energy = 20 J of heat energy

For any device that transfers energy: Efficiency = Useful output energy transferred by the device (J) Total input energy supplied to the device (J) Work out the efficiency for the following devices: a) A petrol engine is supplied with a total of 1000 J of chemical energy, 250 J of this energy is usefully transferred to kinetic energy. b) A diesel engine is supplied with 6000 J of chemical energy and usefully transfers 2100 J of chemical energy to kinetic energy. c) An electric motor uses 50 J of electrical energy and transfers 40 J of useful energy

For any device that transfers energy: Efficiency = Useful output energy transferred by the device (J) Total energy supplied to the device (J) Work out the efficiency for the following devices: a) 250 J/1000 J = 0. 25 or (x 100) = 25% b) 2100 J/6000 J = 0. 35 or (x 100) = 35% c) 40 J/50 J = 0. 80 or (x 100) = 80%

Task: Complete the worksheet on energy efficiency calculations

Self-assessment: 1. 0. 375 or 37. 5% 1. An electric motor takes 1200 J of electrical energy and does 450 J of work 2. A cyclist has 200 J of energy and does 30 J of work on his/her bike 3. A petrol engine takes 3400 J of chemical energy and transfers 2200 J of energy as kinetic energy, 800 J of energy as thermal energy and 400 J as sound energy. 4. A rocket engine takes in 800 J of chemical energy and changes this into 450 J of kinetic energy and 350 J of heat energy 5. A jet engine gas turbine takes in 1200 J of chemical energy and give out 960 J of kinetic, 180 J of heat and 60 J of sound energy. 6. A TV takes in 700 J of electrical energy and gives out 300 J of light energy, 240 J of sound energy and 160 J of heat energy. 2. 0. 15 or 15% 3. 0. 647 or 64. 7% 4. 0. 56 or 56% 5. 0. 8 or 80% 6. 0. 77 0 r 77%

Task: Copy and complete the following table:

Task: Copy and complete the following table: 60 J 60 % 75 % 160 J 20 J 420 J 325 J 8 J 142 J 108 J 580 J 175 J Self-assessment

Exam-style question: Exit Card Task: Complete the past-paper question, as you leave the room you will need to hand the question to me. I will mark it and grade it for next lesson.

1. The following diagram shows the energy transferred each second by a television set. Heat 180 J Electricity 500 J Heat 180 J Light 260 J Electricity 500 J Sound 60 J i) What form of energy is transferred as waste energy by the television set? Light 260 J Sound 60 J i) What form of energy is transferred as waste energy by the television set? ii) What percentage of energy was wasted by the television set? iii) Work out the efficiency of the television set, as a percentage.

Mark scheme for exit card: i) Heat energy ii) 36% of energy wasted as heat energy (180 J/500 J = 0. 36, x 100 = 36%) iii) Useful energy = light and sound energy = 260 J + 60 J = 320 J/500 J = 0. 64, x 100 = 64%.

1. An electric motor takes 1200 J of electrical energy and does 450 J of work 2. A cyclist has 200 J of energy and does 30 J of work on his/her bike 3. A petrol engine takes 3400 J of chemical energy and transfers 2200 J of energy as kinetic energy, 800 J of energy as thermal energy and 400 J as sound energy. 4. A rocket engine takes in 800 J of chemical energy and changes this into 450 J of kinetic energy and 350 J of heat energy 5. A jet engine gas turbine takes in 1200 J of chemical energy and give out 960 J of kinetic, 180 J of heat and 60 J of sound energy. 6. A TV takes in 700 J of electrical energy and gives out 300 J of light energy, 240 J of sound energy and 160 J of heat energy.

Extension: Mark your work

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