Energy Transfers, Conservation And Dissipation Of Energy
Overview
Push a shopping trolley along a level floor and let go. It rolls, slows and stops. Something was clearly spent to get it moving, and a moment later that something has gone. Where? Not out of existence: the wheel bearings, the axle and a thin sheet of air along the floor are all a fraction of a degree warmer than they were, and if you added up every one of those warmings you would recover, to the last joule, exactly what the trolley had. Nothing was lost. It was scattered so thinly that no machine you could build would ever gather it back.
That is the whole of this lesson in one sentence, and it is one of the few ideas in physics that never has an exception. You will learn to treat any object or group of objects as a system and say precisely when and where its energy moved, to follow a pendulum bob trading height for speed and back again, to separate the part of a transfer that does the job you wanted from the part that merely warms the room, and to turn that split into a number using efficiency. By the end you will be able to draw the accounting out as a Sankey diagram, where the width of every arrow is the size of a transfer and the widths coming out must always add up to the width going in.
Objectives
- When a system changes, energy is transferred. A system is an object or group of objects. Students should be able to identify when and where energy has been transferred using concepts such as kinetic energy, gravitational potential energy and elastic potential energy.
- A simple pendulum is an example of oscillating motion and energy is transferred between kinetic energy and gravitational potential energy.
- Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed.
- When energy is transferred only part of it may be usefully transferred; the rest is dissipated so that it is stored in less useful ways. This energy is often described as being ‘wasted’.
- Friction and air resistance are forces that dissipate energy by heating the surroundings.
- The efficiency of a device can be calculated using: useful energy out efficiency = (×100%) total energy in and useful power out efficiency = (×100%) total power in Students may be required to calculate efficiency as a decimal or as a percentage.
- The energy flow in a system can be represented using Sankey diagrams. Students should be able to draw and interpret Sankey diagrams to show how the overall energy in a system is redistributed when the system is changed but there is no net change to the total energy.
Mind map
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Lesson Note
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Lesson Evaluation
Congratulations on completing the lesson on Energy Transfers, Conservation And Dissipation Of Energy. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.
You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- A device is supplied with 500 J of energy and usefully transfers 150 J of it. What is the efficiency of the device? A. 0.30 B. 0.35 C. 3.3 D. 350 Answer: A
- What happens to the energy that a device does not transfer usefully? A. It is destroyed B. It is dissipated into the surroundings and stored in less useful ways C. It is returned to the electricity supply D. It is converted into extra mass Answer: B
- A pendulum bob is momentarily at rest at the far end of its swing. Which statement about its energy at that instant is correct? A. Its kinetic energy is at a maximum and its gravitational potential energy is zero B. Its gravitational potential energy is at a maximum and its kinetic energy is zero C. Both its kinetic energy and its gravitational potential energy are at a maximum D. Both its kinetic energy and its gravitational potential energy are zero Answer: B
- In a Sankey diagram, what does the width of an arrow represent? A. The time taken for the transfer B. The temperature of the device C. The amount of energy transferred D. The efficiency of the device Answer: C
- An electric motor has a useful power output of 240 W and a total power input of 800 W. What is its efficiency as a percentage? A. 3.3% B. 30% C. 33% D. 560% Answer: B
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