AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy stores and systems: use calculations to show on a common scale how the overall energy in a system is redistributed when the system is changed.
Energy is conserved overall, but within a system it can move between stores when the system changes.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Energy is conserved overall, but within a system it can move between stores when the system changes. To show redistribution on a common scale, list every store before and after the change, calculate each energy value in joules, then compare them using the same axis or bar scale.
For example: A ball held 2 m above the ground has gravitational potential energy E = mgh = 0.5 × 9.8 × 2 = 9.8 J. When released, air resistance is negligible, so at the ground the store is kinetic: 9.8 J. A bar chart with both stores on one 0–10 J scale shows the gravitational bar falling to zero as the kinetic bar rises to 9.8 J. If friction transfers 2 J to the thermal store of the surroundings, the kinetic bar reaches only 7.8 J and a thermal bar of 2 J appears, keeping the total at 9.8 J.
What earns the marks
- Identifies all relevant energy stores in the system before and after the change, such as gravitational potential, kinetic, elastic potential, thermal and chemical stores.
- Calculates each store's energy in joules using the correct equation, for example E = mgh for gravitational potential energy or E = ½mv² for kinetic energy.
- Uses one common scale, such as a single axis or a consistent bar-chart scale, so that before-and-after values can be compared directly.
- Shows that the total energy is unchanged when stores are added, demonstrating conservation of energy across the change.
- Explains any difference between the initial and final mechanical energy as transfer to a thermal store by heating, for example due to friction or air resistance.
Common mistakes and how to put them right
Drawing before-and-after bars on different scales, which hides the redistribution.
Put it right: Use one axis or one stated scale for every bar.
Forgetting to include a thermal store when friction or air resistance acts.
Put it right: Add the missing energy as a thermal transfer so the totals match.
Mixing units, such as entering grams in E = mgh instead of kilograms.
Put it right: Convert all masses to kilograms and heights to metres before calculating.
Exam tips
- Write the store names and their calculated values in a table before drawing any chart, so every store is accounted for.
- State the common scale explicitly, for example 'each grid square represents 2 J', so the comparison is clear to the examiner.
- Check conservation by adding the final store values and comparing with the initial total; show this check in your answer.
Try the lesson
The start of the Energy stores and systems: use calculations to show on a common scale how the overall energy in a system is redistributed when the system is changed. lesson, made from checked teaching notes for this specification point.
Have a go first
A ball is held 2 m above the ground. The gravitational potential energy store of the ball is 40 J. The ball is then dropped and, just before it hits the ground, its kinetic energy store is 30 J. How much energy has been transferred to the thermal energy store of the surroundings?
Listen
Energy is conserved overall, but within a system it can move between stores when the system changes. To show redistribution on a common scale, list every store before and after the change, calculate each energy value in joules, then compare them using the same axis or bar scale. For example, a ball held 2 m above the ground has gravitational potential energy E = mgh = 0.5 × 9.8 × 2 = 9.8 J. When released, air resistance is negligible, so at the ground the store is kinetic: 9.8 J. A bar chart with both stores on one 0–10 J scale shows the gravitational bar falling to zero as the kinetic bar rises to 9.8 J. If friction transfers 2 J to the thermal store of the surroundings, the kinetic bar reaches only 7.8 J and a thermal bar of 2 J appears, keeping the total at 9.8 J. A strong answer earns marks for these points. Identifies all relevant energy stores in the system before and after the change, such as gravitational potential, kinetic, elastic potential, thermal and chemical stores. Calculates each store's energy in joules using the correct equation, for example E = mgh for gravitational potential energy or E = ½mv² for kinetic energy. Uses one common scale, such as a single axis or a consistent bar-chart scale, so that before-and-after values can be compared directly. Shows that the total energy is unchanged when stores are added, demonstrating conservation of energy across the change. A common slip: Drawing before-and-after bars on different scales, which hides the redistribution. Instead, use one axis or one stated scale for every bar. Exam tip: Write the store names and their calculated values in a table before drawing any chart, so every store is accounted for.
Key words
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Conservation of energy
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Energy stores and transfers
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Quantitative energy calculations
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Representing data on a common scale
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See every definition
- Conservation of energy
- The total energy is unchanged when stores are added, so the overall energy in a system is conserved across the change.
- Energy stores and transfers
- Energy can move between stores when the system changes, and any difference in mechanical energy is transfer to a thermal store by heating.
- Quantitative energy calculations
- Each store's energy is calculated in joules using the correct equation, such as E = mgh for gravitational potential energy.
- Representing data on a common scale
- One common scale, such as a single axis or a consistent bar-chart scale, lets before-and-after values be compared directly.
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