# Energy Transfer

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Energy can never be created or destroyed, it is only ever converted from one store to another usefully or dissipated into the surroundings.

It is important that you can identify key energy transfers in common electrical appliances including both useful and wasted energy. For example, in a kettle, food processor or lamp.

Energy transfers as useful energy, or it is stored, or it is dissipated into the surroundings. Questions can include common transfers you are familiar with, or new machinery that you will have to read the information given carefully to write your answers.

Sankey diagrams are used to visually represent the efficiency of an energy transfer, for example the efficiency of an electronic device.

In a closed system energy and matter cannot leave the system, therefore the energy input will equal the energy output.

Efficiency is the ratio of useful work done by a machine compared to the total energy supplied. It is calculated using the equation in the shown image.

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A battery powered electric motor is used to lift a load. Fully describe the energy transfer that occurs from the battery to the load lifted by the electrical motor.

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Describe the energy transfer that happens in a mobile phone. Include how the energy is useful, stored and wasted.

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A coal fired power station burns coal to create electrical energy. Describe the series of energy transfers that take place to generate the electricity in the power station.

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A drone flies up to a height of 16 m and it weighs 4 kg. How much gravitational potential energy does the drone have? Gravity = 9.8 N/kg

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As ball is thrown into the air kinetic energy is transferred into gravitational potential energy as it rises. Suggest a reason why the maximum amount of kinetic energy is greater than the maximum gravitational potential energy when the ball gets to its peak.

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Describe the energy transfers that take place when an arrow is shot from a bow.

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When an oven is switched on it heats up quickly and then it remains at a set constant temperature. Explain why the oven remains at the constant temperature.

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When mechanical parts of a machine rub together energy is lost through friction between the parts. Suggest a way that the energy lost can be reduced through friction in the machine.

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How much thermal energy is created by the electric motor shown by the Sankey diagram?

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The input energy of a filament lamp is 75 J, 5 J of this is converted into light and the rest is wasted as heat. Draw a Sankey diagram to show this flow of energy.

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Calculate the efficiency of an LED light which has an input energy of 8 J and transfers 5.5 J of that into light energy?

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A washing machine has an input energy of 4000 J and it is 75% efficient, what is value of the useful output energy of the washing machine?

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An LED lamp has an energy input of 17 J, and has a useful output of 10 J. A halogen lamp has an energy input of 46 J and has a useful output of 24 J. Which one is the most efficient?

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Draw a Sankey diagram of an electric motor that has an energy input of 2300 J and is 27% efficient.

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An electric fan has the power of 3 W and is running for 20 minutes. In the time it is on 2400 J of energy is transferred usefully. Draw a Sankey diagram to illustrate the energy transfer of the fan. You are not required to find the wasted energy in this case.

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A pan of water is put on a cooker to boil. Matter and energy are lost as the water evaporates; how can this system be made into a closed energy system?

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What kind of change in energy can occur in a closed system?

A) energy can be dissipated to the surroundings,

B) energy can be transferred from one form to another,

C) energy can be gained from the surroundings.

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If 5000 J of energy is transferred into a closed system and 3500 J is transferred usefully, how much energy is wasted?

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A tennis ball has 600 J of kinetic energy and it is hit by a racket. A third of the ballβs energy is transferred to the racket. How much energy do the ball and the racket have once itβs been hit?

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A ball is thrown up in the air and there is no air resistance. Describe the energy transfers that take place as the ball moves vertically upwards and back down again.

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If a ball is given 24 J of energy as it is thrown into the air and there is no air resistance, how much gravitational potential energy will it have when it has reached its highest point?

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A girl sits on top of a 1.5 m high slide and weighs 40 kg as shown in the diagram. Assuming that it is a closed energy system, how much kinetic energy does she have when she reaches the bottom of the slide?

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A kettle is a closed energy system and has a power rating of 8 W. If the kettle boils in 2 minutes how much thermal energy is required to raise the temperature of the water to boiling?

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Calculate the efficiency of an electric motor which converts 5 kJ of electrical energy into 3.5 kJ of kinetic energy.

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An LED converts 80% of the energy it is supplied to light. If the light is supplied with 150 J of energy, how much of that is converted into light energy?

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A solar panel is 15% efficient at converting the energy supplied to it into electrical power. If it creates 60 J of energy, how much energy is it supplied with?

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A washing machine converts electrical energy into useful kinetic and heat energy, but energy is also dissipated to the surroundings in the form of heat and sound. The washing machine is 65% efficient, if 500 J are converted into useful energy, how much energy is wasted?

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A halogen bulb is supplied with 24 J of energy and transfers 20 J into light. An LED bulb is supplied with 15 J of energy and 13 J are transferred into light. Which bulb is the most efficient?

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A hairdryer is on for 600 seconds and has a power rating of 8 W. The hairdryer transfers 300 J of useful energy over the time it is running, by using the equations shown, what is the efficiency of the hairdryer?

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A phone charger converts electrical energy into chemical energy stored in the phoneβs battery. 270 J is transferred to the phoneβs battery every minute and the charger is 90% efficient. By working out the efficiency of the phone charger, calculate the power rating of the charger.

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Two televisions are both supplied with the same amount of energy and they transfer energy at the same rate, but television A wastes more energy than television B. Which one of the two televisions (A or B) is the most efficient? Explain your answer.

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As an apple falls from a tree, how is energy is transferred to the surroundings?

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If an object is travelling and energy is transferred to its kinetic energy store, what happens to its speed?

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Describe the energy transfer taking place when a toy car moves as a spring is released.

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When a ball is thrown in the air, as shown in the diagram, it is given 30 J of energy. Using the conservation of energy, what type of energy and how much energy does the ball have of different energy stores at points A, B and C shown in the diagram? Assume that the ball stops at the point C and B is exactly in the middle of A and C.

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A simple pendulum swings and small amounts energy is lost to the surroundings as it continues to swing. When the pendulum reaches the top of its swing, point x shown in the diagram, all its energy is stored as gravitational potential energy. How is the energy in the pendulum transferred as the pendulum swings, and how does this effect the speed it is travelling? Use points y and z in the diagram as reference points in your answer.

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When an electrical appliance is working energy is often wasted, what happens to this wasted energy?

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A car travels from rest to a speed of 30 mph. Describe the energy transfers that occur.

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Explain the energy transfer that occurs as a man runs along a flat road.

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