OCR GCSE Physics A: Dissipation, efficiency and reducing energy loss (J249) – Revision Notes

These notes condense statements P7.2a to P7.2g, Power and efficiency, from the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026). They are for the final weeks of revision and are assessed in both tiers. For full explanations and worked examples, read the study guide. When you are ready to test yourself, use the practice questions, the course hub and the printable checklist. Earlier ideas on work and power from forces are in work done, stored energy and power from forces. The free 10-minute diagnostics show which parts to revise first.

Key ideas in one place

Statement	Key point
P7.2a	Dissipation: energy spreads out and ends up stored in less useful ways, mostly the thermal store of the surroundings
P7.2b	Batteries start with a chemical store. The a.c. mains supplies energy electrically. Motors and heating devices waste some on the way in and inside
P7.2c	Power rating = energy transferred per second. A higher rating means stores change faster, not that the device is more efficient
P7.2d	efficiency = useful output energy transfer / input energy transfer
P7.2e	Increase efficiency by reducing the unwanted stores that fill
P7.2f	Lubrication cuts friction. Thermal insulation slows energy escaping
P7.2g	Thicker walls and lower thermal conductivity both slow the cooling of a building (qualitative)

Dissipation (P7.2a)

• Total energy stays the same. It does not get used up.
• Dissipated energy is thinly spread through the surroundings, so it cannot be gathered and used again.
• Typical wording: “The kinetic store of the car decreases and the thermal stores of the brakes, tyres, road and air increase.”
• Examples: warm charger, braking, a bouncing ball that settles, a rung bell going quiet, a light bulb that gets hot.

Energy in domestic devices (P7.2b)

Device type	Source	Useful store	Common wasted stores
Battery motor (toy, drill)	Chemical store of battery	Kinetic store of moving parts	Thermal in wires and windings, thermal from friction, sound
Mains motor (fan, washing machine)	a.c. mains	Kinetic store	Same three as above
Mains heater (kettle, toaster)	a.c. mains	Thermal store of water, bread or room	Thermal store of casing, nearby air and table, steam
Battery lamp or torch	Chemical store	Light	Thermal store of the lamp and its case

Must-know distinction: “wasted” means stored somewhere you did not want, not destroyed.

Power and energy (P7.2c)

Quantity	Symbol	Unit
Energy transferred	E	joule (J) or kilowatt-hour (kW h)
Power	P	watt (W) or kilowatt (kW)
Time	t	second (s) or hour (h)

• E = P t. P = E / t. t = E / P.
• 1 W = 1 J per second. 1 kW = 1000 W.
• 1 kW h = 3 600 000 J (1000 W for 3600 s).
• Pair kW with hours, W with seconds.
• A 3 kW heater changes its stores three times as fast as a 1 kW heater. It is not three times as efficient.
• Joulemeters measure the energy transferred electrically. Divide by time to get power.

Method in steps: energy from a power rating

1. Write the power in W (or kW).
2. Write the time in s (or h) to match.
3. Multiply: E = P t.
4. Give the unit: J (or kW h).

Reminder: a 0.9 kW toaster on for 2 minutes is 900 W × 120 s = 108 000 J.

Efficiency (P7.2d)

efficiency = useful output energy transfer (J) / input energy transfer (J)

• Percentage = efficiency × 100.
• Efficiency is always above 0 and below 1 (below 100%) for real devices.
• Input = useful output + wasted energy.
• Rearrangements: useful output = efficiency × input. Input = useful output / efficiency.
• With steady power, you may use useful power output / total power input.

Method in steps

1. Identify the useful output and the total input, in the same unit.
2. If only the wasted energy is given, subtract it from the input to find the useful output.
3. Divide useful by input.
4. Write as a decimal or percentage as asked, with a sensible number of significant figures.

Worked reminder. A fan takes 640 J and wastes 160 J. Useful = 640 − 160 = 480 J. Efficiency = 480 / 640 = 0.75 = 75%.

Check that useful and wasted energies add up to the input.

Increasing efficiency and reducing waste (P7.2e, P7.2f)

Method	How it helps
Lubrication (oil, grease)	Layer of liquid separates the surfaces, friction falls, less thermal energy produced
Streamlining	Less air resistance, less work against drag
Thicker or lower-resistance wires	Less thermal energy stored in the wires
Loft insulation	Trapped air is a poor conductor and cannot circulate, so conduction and convection are reduced
Cavity wall filling	Foam or trapped air slows conduction through the wall
Double glazing	Layer of trapped air or gas slows conduction and convection across the window
Draught-proofing	Stops warm air leaking out
Switching off standby	No energy transferred when the device is not needed

Insulation slows energy transfer. It never stops it.

Cooling of a building (P7.2g)

• Thicker walls: lower rate of cooling.
• Lower thermal conductivity material: lower rate of cooling.
• Higher thermal conductivity or thinner walls: faster cooling.
• A bigger temperature difference between inside and outside makes cooling faster.
• Investigation: two copper cans, one insulated. Keep volume, starting temperature, lid and room the same, and record the temperature at regular times.
• Thermograms show hot areas of a house where energy leaves fastest.

Must-know distinctions

Often confused	Difference
Power and energy	Power is how fast energy is transferred (W). Energy is the amount transferred (J or kW h)
Power rating and efficiency	The rating gives the rate of transfer. Efficiency gives the fraction that is useful
Wasted and destroyed	Wasted energy still exists, stored in the surroundings. Nothing is destroyed
Slowing and stopping	Insulation and lubrication reduce unwanted transfers, they do not remove them
Thickness and conductivity	Thickness changes the path length. Conductivity is a property of the material. Both change the cooling rate

Worked reminder with a rating and efficiency together. A 500 W motor runs for 40 s and is 80% efficient. Input = 500 × 40 = 20 000 J. Useful = 0.80 × 20 000 = 16 000 J. Wasted = 4000 J.

Quick self-test

1. What does it mean when we say energy is dissipated?
2. Give two ways a motor wastes energy.
3. A heater takes 120 J and gives 84 J of useful thermal energy. Calculate the efficiency as a percentage.
4. Convert 0.5 kW h to joules.
5. A device transfers 7200 J in 60 s. What is its power?
6. A device is 30% efficient and gives 90 J of useful output. What is the input energy?
7. Define 1 watt.
8. Explain how oil in a hinge makes a device more efficient.
9. Why is a layer of trapped air a good insulator?
10. How does doubling the thickness of a wall affect the rate at which a building cools?
11. A 3 kW heater and a 1 kW heater are on for the same time. A friend says the 3 kW heater is more efficient. Is this correct?
12. A device takes 200 J and gives 50 J of useful output. What energy is wasted, and what is the efficiency?

Answers

1. It spreads out to the surroundings and is stored in less useful ways, mainly the thermal store of the surroundings.
2. Any two of: resistance heating the wires and windings, friction in bearings or gears, sound from vibrating parts.
3. 84 / 120 = 0.70, which is 70%.
4. 0.5 × 3 600 000 = 1 800 000 J.
5. 7200 / 60 = 120 W.
6. Input = 90 / 0.30 = 300 J.
7. A rate of energy transfer of 1 joule per second.
8. The oil separates the surfaces, which reduces friction. Less work is done against friction, so less energy is stored thermally and more goes into the useful motion.
9. Air is a poor conductor, and in small pockets it cannot circulate, so convection is limited too.
10. It slows the cooling, because energy has a longer path to travel through the wall.
11. No. The rating gives the rate of energy transfer, not the fraction that is useful. Nothing is said about efficiency.
12. Wasted = 200 − 50 = 150 J. Efficiency = 50 / 200 = 0.25, or 25%.

Where marks are usually lost

• Writing “energy is lost” or “used up” instead of “dissipated to the surroundings” or “stored thermally”.
• Saying a higher power rating means higher efficiency.
• Dividing the wasted energy by the input instead of dividing the useful energy by the input.
• Giving an efficiency over 100%, or leaving it as a fraction when a percentage was asked.
• Mixing kW with seconds or W with hours.
• Saying insulation “stops” or “traps” energy rather than slowing the transfer.
• Naming only “heat” for the wasted store, without saying where it ends up (wires, casing, air).
• Describing the cooling investigation without saying what is kept the same.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), for first assessment in 2018. Topic P7.2, Power and efficiency, statements P7.2a to P7.2g.