Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Explain that mechanical processes become wasteful when they cause a rise…
Full specification wording
Topic 8 – Energy – forces doing work: 8.11 Explain that mechanical processes become wasteful when they cause a rise in temperature so dissipating energy in heating the surroundings
Mechanical processes such as rubbing, braking or a machine with moving parts transfer energy by forces doing work.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 11 of 15
Study notes
From the checked teaching notes for this specification point.
The idea
Mechanical processes such as rubbing, braking or a machine with moving parts transfer energy by forces doing work. Friction and air resistance oppose motion and warm the surfaces involved, so some of the input energy becomes thermal energy in the object and the surrounding air. Because that energy is spread into the surroundings, it is dissipated and cannot easily be recovered to do further useful work, so the process is wasteful. A bicycle brake heating the rim and the air is a clear case: the kinetic energy is not returned to the rider. Explain the link between the temperature rise, the transfer to the surroundings and the loss of useful energy, and note that reducing friction reduces the waste.
What earns the marks
- Identifies that friction or another resistive force does work and raises the temperature of the surfaces.
- Explains that the thermal energy is transferred to the surroundings and dissipated.
- States that the process is wasteful because the dissipated energy cannot easily be recovered for useful work.
- Links a named mechanical process, such as braking or rubbing, to the temperature rise and the energy lost to the surroundings.
Common mistakes and how to put them right
Says the energy disappears when surfaces warm.
Put it right by stating the energy is transferred to the thermal store of the surroundings.
Describes a temperature rise without linking it to wasted useful energy.
Put it right by explaining the dissipation makes the process wasteful.
Assumes all mechanical energy remains available for work.
Put it right by identifying the portion dissipated as heating.
Exam tips
- Name the resistive force and the surfaces that warm up in your example.
- Use the sequence: work done against friction, temperature rise, energy dissipated to surroundings, process wasteful.
- Explain why the dissipated energy is not easily recovered rather than just saying it is 'lost'.
Try the lesson
The start of the Topic 8 – Energy – forces doing work: 8.11 Explain that mechanical processes become wasteful when they cause a rise in temperature so dissipating energy in heating the surroundings lesson, made from checked teaching notes for this specification point.
Have a go first
A cyclist applies the brakes and the bicycle slows down. The brake blocks and wheel rims become warm. Which statement describes the energy transfer that has taken place?
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