AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Calculations involving transfer of energy.
Full specification wording
Thermal energy transfer (A-level only): Calculations involving transfer of energy.
Energy transfer calculations in thermal physics use two main relationships.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 4 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Energy transfer calculations in thermal physics use two main relationships. When a substance changes temperature without changing state, use ΔE = mcΔθ, where m is mass, c is specific heat capacity and Δθ is the temperature change. When a substance changes state at constant temperature, use ΔE = mL, where L is specific latent heat. For example, heating 0.50 kg of water from 20 °C to 100 °C requires ΔE = 0.50 × 4200 × 80 = 1.68 × 10⁵ J. Boiling away 0.10 kg of water at 100 °C requires ΔE = 0.10 × 2.26 × 10⁶ = 2.26 × 10⁵ J. In mixed problems, calculate each stage separately and add the energies, checking units and significant figures. Power is energy transferred per unit time, so P = ΔE ÷ t links these calculations to rates of heating.
What earns the marks
- Select ΔE = mcΔθ for a temperature change and ΔE = mL for a change of state at constant temperature.
- Substitute mass in kg, specific heat capacity in J kg⁻¹ K⁻¹, specific latent heat in J kg⁻¹ and temperature change in K or °C.
- For multi-stage heating, calculate the energy for each stage separately and sum the values.
- Use P = ΔE ÷ t to find power, time or energy transferred when heating at a constant rate.
- Rearrange equations correctly before substituting numerical values, and quote the final answer with an appropriate unit.
- Recognise that temperature change in kelvin equals temperature change in degrees Celsius, so Δθ may be used directly.
Common mistakes and how to put them right
Using the specific heat capacity equation during a change of state.
Put it right by using ΔE = mL when temperature is constant.
Forgetting to convert grams to kilograms or minutes to seconds.
Put it right by converting all values to SI units before substitution.
Adding temperatures instead of finding the temperature change.
Put it right by calculating Δθ as final temperature minus initial temperature.
Exam tips
- Write down the equation, then substitute values with units before calculating to reduce arithmetic and unit errors.
- For multi-stage problems, draw a simple energy bar or flow diagram showing each stage and label the equation used.
- Check that the final answer is sensible: latent heat terms are often much larger than specific heat capacity terms for the same mass.
Try the lesson
The start of the Thermal energy transfer (A-level only): Calculations involving transfer of energy. lesson, made from checked teaching notes for this specification point.
Have a go first
A heater transfers 3600 J of energy to a block in 5.0 minutes. What is the average rate of energy transfer to the block?
Listen
Energy transfer calculations in thermal physics use two main relationships. When a substance changes temperature without changing state, use ΔE = mcΔθ, where m is mass, c is specific heat capacity and Δθ is the temperature change. When a substance changes state at constant temperature, use ΔE = mL, where L is specific latent heat. For example, heating 0.50 kg of water from 20 °C to 100 °C requires ΔE = 0.50 × 4200 × 80 = 1.68 × 10⁵ J. Boiling away 0.10 kg of water at 100 °C requires ΔE = 0.10 × 2.26 × 10⁶ = 2.26 × 10⁵ J. In mixed problems, calculate each stage separately and add the energies, checking units and significant figures. Power is energy transferred per unit time, so P = ΔE ÷ t links these calculations to rates of heating. A strong answer earns marks for these points. Select ΔE = mcΔθ for a temperature change and ΔE = mL for a change of state at constant temperature. Substitute mass in kg, specific heat capacity in J kg⁻¹ K⁻¹, specific latent heat in J kg⁻¹ and temperature change in K or °C. For multi-stage heating, calculate the energy for each stage separately and sum the values. Use P = ΔE ÷ t to find power, time or energy transferred when heating at a constant rate. A common slip: Using the specific heat capacity equation during a change of state. Instead, using ΔE = mL when temperature is constant. Exam tip: Write down the equation, then substitute values with units before calculating to reduce arithmetic and unit errors.
See it
Key words
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Specific heat capacity
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Specific latent heat
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Power
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Multi-stage heating
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See every definition
- Specific heat capacity
- Used in ΔE = mcΔθ when a substance changes temperature without changing state, where c is specific heat capacity.
- Specific latent heat
- Used in ΔE = mL when a substance changes state at constant temperature, where L is specific latent heat.
- Power
- Energy transferred per unit time, so P = ΔE ÷ t links these calculations to rates of heating.
- Multi-stage heating
- In mixed problems, calculate the energy for each stage separately and sum the values.
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