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Conservation of Energy

Dissipation and wasteful energy transfer

  • 3 things to know
  • Paper 1

Overview

Why friction makes mechanical processes wasteful, warming on slides and rubbed hands, why dissipated energy is hard to recover while the total is conserved, and dissipation in lamps, loudspeakers, mobile phones and bicycles.

What you need to know

Answer each one before you reveal it.

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Key term

Why does wasted energy not change the total energy, and how is energy dissipated in a filament lamp and a loudspeaker?

Process

Explain why mechanical processes involving friction are described as wasteful.

Key term

Explain why a child slowing down on a playground slide leaves the slide slightly warmer.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Explain why mechanical processes involving friction are described as wasteful.

    Model answer

    Friction raises the temperature of the surfaces and then the surroundings, dissipating energy that can no longer be used.

  2. 2

    Describe what happens to the temperature of two surfaces that rub together as an object slides.

    Model answer

    Their temperature rises, because energy is transferred to their thermal stores.

  3. 3

    Explain why a child slowing down on a playground slide leaves the slide slightly warmer.

    Model answer

    Friction transfers energy from the kinetic store of the child to the thermal stores of the slide and the child.

  4. 4

    Explain why energy that has dissipated into heating the surroundings is very difficult to recover.

    Model answer

    It becomes spread through a large volume of material at a low temperature, so it cannot be usefully gathered again.

  5. 5

    Explain why rubbing your hands together makes them feel warm.

    Model answer

    Work done against friction transfers energy to the thermal store of the skin, raising its temperature.

  6. 6

    State what happens to the total energy of a mechanical process even though some of the energy is wasted by heating.

    Model answer

    The total energy stays the same, because the wasted energy has been transferred rather than destroyed.

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Key Concepts of Physics
Motion and Forces
Conservation of Energy
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ALength is measured in metres, with the symbol m.
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