OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply the relationship between change in internal energy of a material and…
Full specification wording
Changes of state: apply the relationship between change in internal energy of a material and its mass, specific heat capacity and temperature change to calculate the energy change involved
When a material is heated without changing state, its change in internal energy equals the energy supplied, given by ΔE = m × c × Δθ, where m is mass in kg, c is specific heat capacity in J/(kg…
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 7 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When a material is heated without changing state, its change in internal energy equals the energy supplied, given by ΔE = m × c × Δθ, where m is mass in kg, c is specific heat capacity in J/(kg °C) and Δθ is the temperature change in °C. For example, heating 2 kg of water (c ≈ 4200 J/(kg °C)) through 10 °C needs ΔE = 2 × 4200 × 10 = 84 000 J, or 84 kJ. Work out the temperature change by subtracting the initial temperature from the final temperature, keep mass in kilograms, and match the units of your answer to the options. The same relationship lets you find c or Δθ by rearranging.
What earns the marks
- Selects the relationship ΔE = m × c × Δθ linking energy change to mass, specific heat capacity and temperature change.
- Substitutes mass in kilograms, specific heat capacity in J/(kg °C) and the temperature change in °C into the relationship.
- Calculates the temperature change by subtracting the initial temperature from the final temperature before multiplying.
- Computes the energy change with correct arithmetic and consistent units, for example 2 × 4200 × 10 = 84 000 J.
Common mistakes and how to put them right
Uses the final temperature instead of the temperature change.
Put it right by subtracting the initial temperature from the final temperature first.
Leaves mass in grams and multiplies directly.
Put it right by converting grams to kilograms before substituting.
Mixes units so the answer is out by a factor of 1000.
Put it right by keeping c in J/(kg °C) with mass in kg and stating the answer in joules or kilojoules consistently.
Exam tips
- Write down m, c and Δθ separately before multiplying so a slip in one value is easy to spot.
- Convert any mass given in grams to kilograms, since c is quoted per kilogram.
- Estimate the answer roughly first (for example 2 × 4000 × 10 = 80 000 J) to reject implausible options quickly.
Try the lesson
The start of the Changes of state: apply the relationship between change in internal energy of a material and its mass, specific heat capacity and temperature change to calculate the energy change involved lesson, made from checked teaching notes for this specification point.
Have a go first
How much energy is needed to raise the temperature of 2 kg of water by 10 °C? The specific heat capacity of water is 4200 J/(kg °C).
Listen
When a material is heated without changing state, its change in internal energy equals the energy supplied, given by ΔE = m × c × Δθ, where m is mass in kg, c is specific heat capacity in J/(kg °C) and Δθ is the temperature change in °C. For example, heating 2 kg of water (c ≈ 4200 J/(kg °C)) through 10 °C needs ΔE = 2 × 4200 × 10 = 84 000 J, or 84 kJ. Work out the temperature change by subtracting the initial temperature from the final temperature, keep mass in kilograms, and match the units of your answer to the options. The same relationship lets you find c or Δθ by rearranging. A strong answer earns marks for these points. Selects the relationship ΔE = m × c × Δθ linking energy change to mass, specific heat capacity and temperature change. Substitutes mass in kilograms, specific heat capacity in J/(kg °C) and the temperature change in °C into the relationship. Calculates the temperature change by subtracting the initial temperature from the final temperature before multiplying. Computes the energy change with correct arithmetic and consistent units, for example 2 × 4200 × 10 = 84 000 J. A common slip: Uses the final temperature instead of the temperature change. Instead, subtracting the initial temperature from the final temperature first. Exam tip: Write down m, c and Δθ separately before multiplying so a slip in one value is easy to spot.
See it
Put it in order
Put these steps in the order they happen.
- 1. Calculate Δθ = final temperature − initial temperature
- 2. Convert mass in grams to kilograms
- 3. Write down m, c and Δθ separately
- 4. State answer in joules or kilojoules
- 5. Compute energy change with consistent units
- 6. Substitute into ΔE = m × c × Δθ
Key words
Say what each word means before you turn the card.
Energy change and internal energy
Try answering aloud or in your head before turning the card.
Specific heat capacity relationship
Try answering aloud or in your head before turning the card.
Temperature change calculation
Try answering aloud or in your head before turning the card.
Unit consistency in energy calculations
Try answering aloud or in your head before turning the card.
See every definition
- Energy change and internal energy
- When a material is heated without changing state, its change in internal energy equals the energy supplied.
- Specific heat capacity relationship
- The relationship ΔE = m × c × Δθ links energy change to mass, specific heat capacity and temperature change.
- Temperature change calculation
- Work out the temperature change by subtracting the initial temperature from the final temperature.
- Unit consistency in energy calculations
- Keep mass in kilograms, specific heat capacity in J/(kg °C) and state the answer in joules or kilojoules consistently.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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