OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply: change in thermal energy (J) = mass (kg) × specific heat capacity…
Full specification wording
Changes of state: apply: change in thermal energy (J) = mass (kg) × specific heat capacity (J/kg °C) × change in temperature (°C)
The equation ΔE = m × c × Δθ gives the thermal energy needed to raise a mass by a temperature change without a change of state.
- Point 1 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The equation ΔE = m × c × Δθ gives the thermal energy needed to raise a mass by a temperature change without a change of state. Specific heat capacity c is the energy to raise 1 kg by 1 °C, in J/kg °C, so units cancel to leave joules. Worked method: heat 2.0 kg of water (c = 4200 J/kg °C) through 15 °C, giving ΔE = 2.0 × 4200 × 15 = 126 000 J, or 1.26 × 10⁵ J. Use the change in temperature, not the final temperature, and keep mass in kilograms. A larger c means more energy for the same rise, which is why water heats slowly. The equation applies while the substance stays in one state; melting or boiling needs latent heat instead.
What earns the marks
- Selects ΔE = m × c × Δθ and substitutes mass in kilograms, c in J/kg °C and temperature change in °C.
- Calculates the temperature change by subtracting the initial temperature from the final temperature.
- Computes a thermal energy value in joules and expresses it with a sensible power of ten.
- Explains that a substance with a larger specific heat capacity needs more energy for the same temperature rise.
- Recognises that the equation applies to a temperature change within one state rather than to melting or boiling.
Common mistakes and how to put them right
Substitutes the final temperature instead of the change in temperature.
Put it right: Subtract the starting value first.
Leaves mass in grams and multiplies directly.
Put it right: Convert grams to kilograms by dividing by 1000.
Uses the equation to find energy for melting ice at constant temperature.
Put it right: Apply latent heat reasoning there instead.
Mixes units so the answer is not in joules.
Put it right: Keep c in J/kg °C with mass in kg and temperature in °C.
Exam tips
- Write the equation, then fill in m, c and Δθ in a row before calculating.
- Estimate the answer first, for example 2 × 4000 × 10 ≈ 80 000 J, to catch unit slips.
- Check the sign and size of the temperature change and state the energy in joules with a power of ten.
Try the lesson
The start of the Changes of state: apply: change in thermal energy (J) = mass (kg) × specific heat capacity (J/kg °C) × change in temperature (°C) lesson, made from checked teaching notes for this specification point.
Have a go first
3 kg of a liquid with specific heat capacity 2000 J/kg °C is cooled so that its temperature falls by 5 °C. What is the change in thermal energy released?
Listen
The equation ΔE = m × c × Δθ gives the thermal energy needed to raise a mass by a temperature change without a change of state. Specific heat capacity c is the energy to raise 1 kg by 1 °C, in J/kg °C, so units cancel to leave joules. Worked method: heat 2.0 kg of water (c = 4200 J/kg °C) through 15 °C, giving ΔE = 2.0 × 4200 × 15 = 126 000 J, or 1.26 × 10⁵ J. Use the change in temperature, not the final temperature, and keep mass in kilograms. A larger c means more energy for the same rise, which is why water heats slowly. The equation applies while the substance stays in one state; melting or boiling needs latent heat instead. A strong answer earns marks for these points. Selects ΔE = m × c × Δθ and substitutes mass in kilograms, c in J/kg °C and temperature change in °C. Calculates the temperature change by subtracting the initial temperature from the final temperature. Computes a thermal energy value in joules and expresses it with a sensible power of ten. Explains that a substance with a larger specific heat capacity needs more energy for the same temperature rise. Exam tip: Write the equation, then fill in m, c and Δθ in a row before calculating.
Key words
Say what each word means before you turn the card.
Specific heat capacity
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Change in temperature
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Thermal energy change
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Latent heat
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See every definition
- Specific heat capacity
- The energy needed to raise 1 kg of a substance by 1 °C, measured in J/kg °C.
- Change in temperature
- Found by subtracting the initial temperature from the final temperature, and used in the equation rather than the final temperature.
- Thermal energy change
- Calculated as mass in kilograms multiplied by specific heat capacity in J/kg °C multiplied by change in temperature in °C, giving joules.
- Latent heat
- What melting or boiling needs instead of the equation, which applies only while the substance stays in one state.
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