OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply: thermal energy for a change in state (J) = mass (kg) × specific…
Full specification wording
Changes of state: apply: thermal energy for a change in state (J) = mass (kg) × specific latent heat (J/kg)
This equation gives the energy needed to change the state of a substance at constant temperature.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 2 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This equation gives the energy needed to change the state of a substance at constant temperature. Multiply the mass in kilograms by the specific latent heat in joules per kilogram, and the answer is in joules. For example, melting 0.50 kg of ice uses a specific latent heat of fusion of about 3.3 × 10⁵ J/kg, so the energy is 0.50 × 3.3 × 10⁵ = 1.65 × 10⁵ J. Use the latent heat of fusion for melting or freezing and the latent heat of vaporisation for boiling or condensing. Check units before substituting: convert grams to kilograms by dividing by 1000, and remember the temperature stays constant during the change of state.
What earns the marks
- Selects the equation thermal energy = mass × specific latent heat and substitutes values in the correct units.
- Converts a mass given in grams to kilograms by dividing by 1000 before multiplying.
- Uses the latent heat of fusion for melting or freezing and the latent heat of vaporisation for boiling or condensing.
- States the answer in joules and recognises that temperature remains constant during the change of state.
Common mistakes and how to put them right
Substitutes a mass in grams directly into the equation.
Put it right: Convert grams to kilograms first by dividing by 1000.
Uses the specific latent heat of vaporisation when a substance is melting.
Put it right: Select the latent heat of fusion for melting and freezing.
Adds a temperature change to the calculation.
Put it right: Recognise that the temperature stays constant during a change of state.
Omits the unit or gives the energy in the wrong unit.
Put it right: State the answer in joules.
Exam tips
- Write down the equation, then list mass and specific latent heat with their units before substituting any numbers.
- Convert every mass to kilograms and check that the specific latent heat is in J/kg so the answer comes out in joules.
- Use the value of specific latent heat given in the question rather than a remembered figure, and keep powers of ten in standard form.
Try the lesson
The start of the Changes of state: apply: thermal energy for a change in state (J) = mass (kg) × specific latent heat (J/kg) lesson, made from checked teaching notes for this specification point.
Have a go first
A 2 kg block of ice melts completely at 0 °C. The specific latent heat of fusion of ice is 334 000 J/kg. Calculate the thermal energy needed to melt the ice.
Listen
This equation gives the energy needed to change the state of a substance at constant temperature. Multiply the mass in kilograms by the specific latent heat in joules per kilogram, and the answer is in joules. For example, melting 0.50 kg of ice uses a specific latent heat of fusion of about 3.3 × 10⁵ J/kg, so the energy is 0.50 × 3.3 × 10⁵ = 1.65 × 10⁵ J. Use the latent heat of fusion for melting or freezing and the latent heat of vaporisation for boiling or condensing. Check units before substituting: convert grams to kilograms by dividing by 1000, and remember the temperature stays constant during the change of state. A strong answer earns marks for these points. Selects the equation thermal energy = mass × specific latent heat and substitutes values in the correct units. Converts a mass given in grams to kilograms by dividing by 1000 before multiplying. Uses the latent heat of fusion for melting or freezing and the latent heat of vaporisation for boiling or condensing. States the answer in joules and recognises that temperature remains constant during the change of state. Exam tip: Write down the equation, then list mass and specific latent heat with their units before substituting any numbers.
Key words
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Specific latent heat
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Thermal energy for a change of state
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Latent heat of fusion
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Latent heat of vaporisation
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See every definition
- Specific latent heat
- The value in joules per kilogram that is multiplied by the mass in kilograms to give the thermal energy for a change in state.
- Thermal energy for a change of state
- The mass in kilograms multiplied by the specific latent heat in joules per kilogram, giving an answer in joules.
- Latent heat of fusion
- The specific latent heat used for melting or freezing.
- Latent heat of vaporisation
- The specific latent heat used for boiling or condensing.
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