OCR GCSE Physics A: Energy stores, transfers and energy calculations (J249) – Revision Notes

These revision notes follow the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), for first assessment in 2018, topic P7.1 Work done (statements P7.1a to P7.1e). Every statement here is assessed in both Foundation and Higher Tier papers. For full explanations read the study guide, then try the practice questions.

Course links: OCR GCSE Physics hub, printable checklist, free diagnostics, OCR GCSE Physics: Forces and OCR GCSE Physics: Matter.

Conservation of energy (P7.1a)

• Energy is never created or destroyed. It moves from store to store.
• In a closed system there is no net change to the total energy.
• Wasted energy is not lost. It spreads into thermal stores of the surroundings and is hard to use again.
• Energy is not a fuel that gets used up, and objects at rest still have energy.
• This statement is qualitative only: describe it in words.

Changes in stores (P7.1b)

Situation	Goes down	Goes up
Thrown upwards or up a slope	Kinetic	Gravitational potential (and thermal if friction)
Moving object hits an obstacle	Kinetic	Thermal, sound, some elastic or deformation
Constant force accelerating an object	Store supplying the force	Kinetic
Vehicle slowing down	Kinetic	Thermal (brakes, tyres, road)
Electric kettle heating water	Electrical (current doing work)	Thermal in water, then kettle and surroundings
Wind-up toy or compressed spring released	Elastic	Kinetic
Object falling	Gravitational potential	Kinetic

Method: describing a change

1. Name the system or object.
2. Say which store decreases.
3. Say which store increases.
4. Say how it moved: heating, work done by a force, or work done by a current.
5. State that the total is unchanged.

Three ways a system changes (P7.1c)

Process	What happens
Heating	Energy moves from hot to cold, or raises the temperature of a material
Work done by forces	A force moves an object through a distance
Work done by a current	Charge moves through a potential difference

Equations (P7.1d, P7.1e)

Quantity	Equation	Units
Kinetic energy	E = ½mv²	J, kg, m/s
Gravitational potential energy	E = mgh	J, kg, N/kg, m
Elastic energy	E = ½kx²	J, N/m, m
Work done	W = Fs	J, N, m
Thermal energy change	ΔE = mcΔθ	J, kg, J/kg °C, °C
Charge	Q = It	C, A, s
Electrical energy	E = QV	J, C, V
Power	P = VI	W, V, A
Energy from power	E = Pt	J (W and s) or kW h (kW and h)

Conversion: 1 kW h = 3 600 000 J. Cost = kW h × price per kW h.

Method: any energy calculation

1. Write the equation.
2. Convert units: g to kg, cm to m, minutes to seconds or hours.
3. Substitute and calculate.
4. Give the unit, and a sensible number of significant figures.

Worked reminders

Kinetic energy. A cyclist and bike of mass 80 kg move at 9.0 m/s.

E = ½ × 80 × 9.0² = 3240 J

Elastic energy. A spring with k = 200 N/m is stretched by 4.0 cm.

x = 0.040 m
E = ½ × 200 × 0.040² = 0.16 J

Kilowatt-hours. A 60 W lamp is on for 2.0 hours.

E = 0.060 kW × 2.0 h = 0.12 kW h
In joules: 60 W × 7200 s = 432 000 J

Heating. A mug holds 0.25 kg of water, warmed from 20 °C to 90 °C (c = 4200 J/kg °C).

ΔE = 0.25 × 4200 × 70 = 73 500 J

Model sentences for common situations. Learn the pattern, then swap in the situation.

• A skier slides down a slope: “The gravitational potential store decreases. The kinetic store increases, and the thermal store of the skis and snow increases because of friction. The total energy is unchanged.”
• A ball hits a wall and stops: “The kinetic store decreases. The thermal store of the ball and wall increases, and some energy is transferred to the surroundings by sound.”
• A kettle is switched on: “Work is done by the current. The electrical energy is transferred to the thermal store of the water, and some also warms the kettle and the air.”

More worked reminders follow.

Rising object. A 0.50 kg ball leaves the hand at 8.0 m/s and rises with no air resistance (g = 9.8 N/kg).

Kinetic energy = ½ × 0.50 × 8.0² = 16 J
Greatest height: h = 16 / (0.50 × 9.8) = 3.27 = 3.3 m

Braking. A van of mass 2000 kg slows from 10 m/s to rest with a constant braking force of 5000 N.

Energy transferred = ½ × 2000 × 10² = 100 000 J
Braking distance = 100 000 / 5000 = 20 m

Electrical. A 12 V supply drives 0.50 A through a motor for 30 s.

Q = It = 0.50 × 30 = 15 C
E = QV = 15 × 12 = 180 J
Check: P = VI = 6.0 W, E = Pt = 6.0 × 30 = 180 J

Kettle time. A 2.0 kW kettle heats 1.0 kg of water through 60 °C (c = 4200 J/kg °C).

ΔE = 1.0 × 4200 × 60 = 252 000 J
t = 252 000 / 2000 = 126 s (if no energy is wasted)

Practical links

The specification suggests these investigations. Know what is measured and what controls matter.

Investigation	What you measure	Energy idea
Joulemeter with different appliances	Energy used by each appliance	Work done by current, E = Pt
Light gates and trolleys	Speed of a trolley	Kinetic energy, E = ½mv²
Joulemeter and motor lifting a weight	Energy supplied and height gained	Gravitational potential energy, E = mgh
Bouncy balls	Drop and rebound heights	Energy changes and wasted energy

Repeating measurements and calculating a mean improves reliability. Measure rebound height at eye level against a fixed rule, and drop each ball from the same height.

Must-know distinctions

• Work done and energy transferred are one and the same amount, in joules.
• Kinetic energy depends on v². Doubling speed gives four times the energy.
• Gravitational potential energy depends on vertical height only, not the length of a slope.
• Power is a rate (J/s). Energy is an amount (J or kW h).
• Charge (C) is not energy. E = QV links them through potential difference.
• Dissipated is not destroyed.

Quick self-test

1. State the law of conservation of energy.
2. Calculate the kinetic energy of a 2.0 kg trolley moving at 5.0 m/s.
3. Calculate the gravitational potential energy gained when a 4.0 kg box is lifted 2.5 m (g = 9.8 N/kg).
4. A spring has k = 80 N/m and is stretched 0.10 m. How much energy is stored?
5. Calculate the energy needed to heat 0.40 kg of copper by 25 °C (c = 400 J/kg °C).
6. How much energy is transferred when 20 C of charge passes through a 6.0 V potential difference?
7. A 3 kW fire runs for 20 minutes. How many kW h is that?
8. Convert 0.50 kW h into joules.
9. A 0.50 kg ball has 40 J of gravitational potential energy at the top of its path. How high is it (g = 9.8 N/kg)?
10. A car slows down. Name the store that decreases and the store that increases.
11. A ball’s speed doubles. What happens to its kinetic energy?

Answers

1. Energy is never created or destroyed; it only moves from one store to another. In a closed system the total stays the same.
2. ½ × 2.0 × 5.0² = 25 J.
3. 4.0 × 9.8 × 2.5 = 98 J.
4. ½ × 80 × 0.10² = 0.40 J.
5. 0.40 × 400 × 25 = 4000 J.
6. E = QV = 20 × 6.0 = 120 J.
7. 3 kW × (20/60) h = 1.0 kW h.
8. 0.50 × 3 600 000 = 1 800 000 J (1.8 × 10⁶ J).
9. h = 40 / (0.50 × 9.8) = 8.16 = 8.2 m.
10. Kinetic store decreases. Thermal store (brakes, tyres, road) increases.
11. It becomes four times as large.

Where marks are usually lost

• Saying energy is “used up” or “lost” instead of naming the new store.
• Naming a store that goes down without naming the one that goes up.
• Leaving the speed unsquared, or squaring the product ½mv instead of just v.
• Using extension in cm, or mass in g, without converting.
• Putting the distance along a slope into mgh in place of the vertical rise.
• Giving kW h and joules mixed up, or forgetting to convert minutes into hours.
• Rounding too early, then giving an answer that is not to a sensible number of significant figures.
• Leaving out units, or writing J when the answer is in kW h.
• Writing “the kettle uses 2 kW” when asked for energy rather than power.

Official syllabus

This page is based on the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), for first assessment in 2018, published by OCR (Oxford Cambridge and RSA). Topic P7.1 Work done.