AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy changes in systems: change in thermal energy, ∆E, in joules, J
The change in thermal energy, ∆E, is the energy transferred to or from a system when its temperature changes, measured in joules, J. You calculate it with ∆E = mc∆θ, where m is mass in kilograms,…
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The change in thermal energy, ∆E, is the energy transferred to or from a system when its temperature changes, measured in joules, J. You calculate it with ∆E = mc∆θ, where m is mass in kilograms, c is specific heat capacity in J/kg°C, and ∆θ is the temperature change in °C. For example, heating 2 kg of water (c = 4200 J/kg°C) from 20 °C to 70 °C gives ∆E = 2 × 4200 × 50 = 420 000 J. A positive ∆E means energy is transferred into the system and its temperature rises; a negative ∆E means energy leaves and it cools. The joule is the unit of energy, so ∆E is always in J, not °C or kg. In calculations, rearrange carefully and keep units consistent.
What earns the marks
- States that ∆E is measured in joules, J, and represents energy transferred to or from a system during a temperature change.
- Uses the equation ∆E = mc∆θ correctly, substituting mass in kg, specific heat capacity in J/kg°C and temperature change in °C.
- Calculates a numerical value of ∆E and gives the unit J, for example 420 000 J for 2 kg of water heated by 50 °C.
- Explains that a positive ∆E corresponds to a temperature rise and a negative ∆E to a temperature fall, linking sign to direction of energy transfer.
- Rearranges the equation to find m, c or ∆θ when ∆E is known, maintaining correct units throughout.
Common mistakes and how to put them right
Using mass in grams instead of kilograms.
Put it right: Convert grams to kilograms by dividing by 1000 before substituting.
Misusing units: ∆E is in J, so do not label the answer in °C or J/kg°C.
Put it right: Check the final unit matches the quantity calculated.
Using the final temperature instead of the temperature change.
Put it right: Calculate ∆θ = final temperature − initial temperature, and keep the sign.
Exam tips
- Write the equation ∆E = mc∆θ before substituting numbers so the examiner can see your method.
- Check that every quantity has the correct unit before calculating, and carry the unit J through to your final answer.
- If a temperature falls, expect a negative ∆E and state that energy is transferred out of the system.
Try the lesson
The start of the Energy changes in systems: change in thermal energy, ∆E, in joules, J lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following correctly identifies the symbol and standard unit for a change in thermal energy?
Listen
The change in thermal energy, ∆E, is the energy transferred to or from a system when its temperature changes, measured in joules, J. You calculate it with ∆E = mc∆θ, where m is mass in kilograms, c is specific heat capacity in J/kg°C, and ∆θ is the temperature change in °C. For example, heating 2 kg of water (c = 4200 J/kg°C) from 20 °C to 70 °C gives ∆E = 2 × 4200 × 50 = 420 000 J. A positive ∆E means energy is transferred into the system and its temperature rises; a negative ∆E means energy leaves and it cools. The joule is the unit of energy, so ∆E is always in J, not °C or kg. In calculations, rearrange carefully and keep units consistent. A strong answer earns marks for these points. States that ∆E is measured in joules, J, and represents energy transferred to or from a system during a temperature change. Uses the equation ∆E = mc∆θ correctly, substituting mass in kg, specific heat capacity in J/kg°C and temperature change in °C. Calculates a numerical value of ∆E and gives the unit J, for example 420 000 J for 2 kg of water heated by 50 °C. Explains that a positive ∆E corresponds to a temperature rise and a negative ∆E to a temperature fall, linking sign to direction of energy transfer. Exam tip: Write the equation ∆E = mc∆θ before substituting numbers so the examiner can see your method.
Key words
Say what each word means before you turn the card.
Change in thermal energy
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Specific heat capacity
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Joule
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Temperature change
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See every definition
- Change in thermal energy
- Energy transferred to or from a system when its temperature changes, measured in joules, J.
- Specific heat capacity
- A quantity in J/kg°C used in the equation ∆E = mc∆θ, alongside mass in kilograms and temperature change in °C.
- Joule
- The unit of energy, so ∆E is always in J, not °C or kg.
- Temperature change
- ∆θ in °C, calculated as final temperature minus initial temperature, and kept with its sign.
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