Energy Transfers, Conservation And Dissipation
Übersicht
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 to say precisely when and where its energy moved, to separate the part of a transfer that does the job you wanted from the part that merely warms the room, and to name friction and air resistance as the two forces that carry the second part away. By the end you will be able to write any change out as an account in joules: so much supplied, so much arriving where you wanted it, so much dissipated, and the three numbers balancing every single time.
Ziele
- 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.
- 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.
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- A device is supplied with 500 J of energy and usefully transfers 150 J of it. How much energy is dissipated? A. 150 J B. 350 J C. 500 J D. 650 J Answer: B
- 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
- Which pair of forces is named in the specification as dissipating energy by heating the surroundings? A. Weight and tension B. Friction and air resistance C. Magnetic and electrostatic forces D. Upthrust and the normal contact force Answer: B
- A cyclist freewheels down a hill at a constant speed. Which statement is correct? A. Her kinetic energy is increasing all the way down B. Her gravitational potential energy is increasing all the way down C. The gravitational potential energy she gives up is being dissipated by heating the surroundings D. Energy is being destroyed by air resistance Answer: C
- A hoist is supplied with 20 000 J of energy and raises a load, giving it 12 500 J of gravitational potential energy. Which statement is correct? A. 7500 J of energy has been destroyed B. 7500 J of energy has been dissipated, mostly by heating C. A total of 32 500 J has been supplied D. The load has gained 20 000 J of gravitational potential energy Answer: B
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Lehrbuchabschnitt
Conservation of Energy
Energy, as we have noted, is conserved, making it one of the most important physical quantities in nature. The law of conservation of energy can be stated as follows: Total energy is constant in any process. It may change in form or be transferred from one system to another, but the total remains th
"College Physics: Conservation of Energy" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/7-6-conservation-of-energy. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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Lehrbuchabschnitt
Nonconservative Forces
Forces are either conservative or nonconservative. Conservative forces were discussed in Conservative Forces and Potential Energy . A nonconservative force is one for which work depends on the path taken. Friction is a good example of a nonconservative force. As illustrated in Figure 7.14 , work don
"College Physics: Nonconservative Forces" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/7-5-nonconservative-forces. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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