OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe the changes in energy involved when a system is changed by…
Full specification wording
Work done: describe the changes in energy involved when a system is changed by heating (in terms of temperature change and specific heat capacity), by work done by forces, and by work done when a current flows
A system can be changed in three ways, and each moves energy into or out of a store.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A system can be changed in three ways, and each moves energy into or out of a store. Heating a substance raises its temperature; the energy needed depends on the mass, the temperature change and the specific heat capacity, using E = m × c × Δθ. A larger specific heat capacity means more energy is needed for the same temperature rise. Work done by a force transfers energy mechanically, for example when a force moves an object through a distance, using W = F × s. Work done when a current flows transfers energy electrically; the energy transferred depends on current, potential difference and time, using E = V × I × t. In each case the total energy is conserved and the change shows as a rise or fall in a store such as thermal, kinetic or chemical.
What earns the marks
- States that heating raises temperature and the energy needed depends on mass, temperature change and specific heat capacity.
- Uses E = m × c × Δθ to relate energy transferred by heating to temperature change.
- Explains that a larger specific heat capacity means more energy for the same temperature rise.
- States that work done by a force transfers energy mechanically, related by W = F × s.
- States that work done when a current flows transfers energy electrically, related by E = V × I × t.
- States that energy is conserved and appears as a change in a store.
Common mistakes and how to put them right
Uses temperature change alone and ignores mass.
Put it right: Include mass and specific heat capacity in the calculation.
Confuses specific heat capacity with the total energy.
Put it right: State that it is the energy per kilogram per degree Celsius.
Adds the three routes as if energy were created.
Put it right: State that each route transfers energy between stores.
Exam tips
- Write the equation you use before substituting values.
- Keep units consistent: mass in kg, temperature change in °C, energy in J.
- Check whether the question asks about heating, work by a force or work by a current before choosing an equation.
Try the lesson
The start of the Work done: describe the changes in energy involved when a system is changed by heating (in terms of temperature change and specific heat capacity), by work done by forces, and by work done when a current flows lesson, made from checked teaching notes for this specification point.
Have a go first
Two identical masses of water are heated, one gaining 4200 J and the other gaining 8400 J. Assuming no energy is lost, how do the temperature rises compare?
Listen
A system can be changed in three ways, and each moves energy into or out of a store. Heating a substance raises its temperature; the energy needed depends on the mass, the temperature change and the specific heat capacity, using E = m × c × Δθ. A larger specific heat capacity means more energy is needed for the same temperature rise. Work done by a force transfers energy mechanically, for example when a force moves an object through a distance, using W = F × s. Work done when a current flows transfers energy electrically; the energy transferred depends on current, potential difference and time, using E = V × I × t. In each case the total energy is conserved and the change shows as a rise or fall in a store such as thermal, kinetic or chemical. A strong answer earns marks for these points. States that heating raises temperature and the energy needed depends on mass, temperature change and specific heat capacity. Uses E = m × c × Δθ to relate energy transferred by heating to temperature change. Explains that a larger specific heat capacity means more energy for the same temperature rise. States that work done by a force transfers energy mechanically, related by W = F × s. Exam tip: Write the equation you use before substituting values.
Key words
Say what each word means before you turn the card.
Specific heat capacity
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Work done by forces
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Electrical work
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Conservation of energy
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See every definition
- Specific heat capacity
- The energy needed for a temperature rise depends on the mass, the temperature change and this quantity, and a larger value means more energy is needed for the same temperature rise.
- Work done by forces
- A way of transferring energy mechanically, for example when a force moves an object through a distance, related by W = F × s.
- Electrical work
- Work done when a current flows, transferring energy electrically, where the energy transferred depends on current, potential difference and time, related by E = V × I × t.
- Conservation of energy
- In each case the total energy is conserved and the change shows as a rise or fall in a store such as thermal, kinetic or chemical.
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