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

This guide covers statements P7.2a to P7.2g of topic P7.2, Power and efficiency, in the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026). It explains how energy ends up stored in less useful ways, how devices transfer energy from a battery or the a.c. mains, how power ratings link to energy changes, how to calculate efficiency, and how lubrication and insulation cut waste. All of this content is assessed in both the Foundation and Higher Tier papers.

Course pages: the hub and the printable checklist. Companion pages: the revision notes and the practice questions. For the energy ideas that come before this, see work done, stored energy and power from forces. To find out where you stand, try the free 10-minute diagnostics.

What this guide covers

Statement	What you must be able to do
P7.2a	Describe, with examples, the process of dissipation, in which energy ends up stored in less useful ways
P7.2b	Describe how energy is transferred from batteries or the a.c. mains in different domestic devices, including how it is wasted in motors and heating devices
P7.2c	Describe, with examples, how power ratings of domestic appliances link to changes in stored energy during use
P7.2d	Calculate energy efficiency for any energy transfer
P7.2e	Describe ways to increase efficiency
P7.2f	Explain ways of reducing unwanted energy transfer, including lubrication and thermal insulation
P7.2g	Describe how the rate of cooling of a building depends on the thickness and thermal conductivity of its walls (qualitative only)

Dissipation (P7.2a)

Energy is never destroyed. In every process the total stays the same. What changes is where it is stored. Energy is dissipated when it spreads into the surroundings, mainly the thermal store of the air, walls or ground, where it is far less useful to you. Once it is spread thinly through a large amount of material, you cannot gather it up and use it again.

Examples to be ready with:

• A phone charger feels warm. Some of the energy from the mains is stored thermally in the charger and the air round it, not in the battery.
• A cyclist freewheels to a stop. Friction at the brakes and in the bearings, and air resistance, warm the brake blocks, the wheel hubs and the air. The kinetic store has become many tiny thermal stores.
• A bell rings, then goes quiet. The sound energy passes to the air and the walls, which warm very slightly.

Use the words “stored” and “less useful”, not “lost” or “used up”. An answer that says energy is “used up” contradicts conservation of energy.

Energy transfers from batteries and the mains (P7.2b)

Two sources are named in the specification.

Batteries start with energy in a chemical store. When a circuit is complete the battery does work on the charges, so energy is transferred electrically.

The a.c. mains supplies energy electrically too. The current keeps reversing direction, but energy is still transferred from the supply to the device.

The specification asks you to cover how energy may be wasted in the transfer to and within motors and heating devices.

A motor (a fan, a drill, a toy car):

• Useful: kinetic store of the moving parts, and from there the gravitational store if it lifts something.
• Wasted: the wires and windings have resistance, so some energy is stored thermally. Friction in bearings and gears warms them. Moving parts make air vibrate, so some energy leaves as sound.

A heating device (a kettle, an electric heater, a toaster):

• Useful: the thermal store of the water, the room or the bread.
• Wasted: the casing, the element supports and the air nearby also warm up. A kettle that is switched off still gives energy to the table and air as it cools. Some energy leaves as steam if the lid is open.

A heating device is often described as very efficient because nearly all the input ends up as thermal energy somewhere. It is only “wasted” if it is stored in the wrong place.

Worked example 1. A battery-powered toy crane lifts a 0.75 kg load through 1.6 m. The battery supplies 18 J. Take g = 9.8 N/kg.

1. Gain in gravitational potential energy = m g h = 0.75 × 9.8 × 1.6 = 11.76 J. That is the useful output.
2. Wasted energy = 18 − 11.76 = 6.24 J, stored thermally in the motor, wires and gears and as sound.
3. Efficiency = 11.76 / 18 = 0.653, which is 65% to 2 significant figures.

Power ratings and stored energy (P7.2c)

Power is the rate of energy transfer. The power rating on an appliance says how much energy it transfers from the supply each second when it is working. A 2 kW kettle takes 2000 J every second. This is measured in watts: 1 W is 1 joule per second.

energy transferred (J) = power (W) × time (s), written E = P t.

The same idea links the rating to the stores that change. A hair dryer rated 1.6 kW takes 1600 J from the mains each second. That energy goes into the thermal store of the warm air, the kinetic store of the moving air and, unavoidably, the thermal store of the surroundings. A higher rating means the stores change faster. It does not tell you how much of the energy ends up where you want it. Do not confuse a high power rating with high efficiency.

Energy can also be measured in kilowatt-hours (kW h). Use kW with hours, or W with seconds, but never mix them. A 1 kW appliance running for 1 hour transfers 1 kW h. Converting to joules: 1000 W for 3600 s is 3 600 000 J, so 1 kW h = 3.6 MJ.

Worked example 2. A 2.4 kW kettle runs for 150 s.

1. P = 2.4 kW = 2400 W.
2. E = P t = 2400 × 150 = 360 000 J, or 360 kJ.

Worked example 3. A 1.8 kW heater is on for 3.5 hours. E = 1.8 × 3.5 = 6.3 kW h. No conversion to joules is needed.

The specification suggests using joulemeters to investigate the power of different appliances. A joulemeter reads the energy transferred electrically, so you can divide it by the time to find the power.

Efficiency (P7.2d)

Efficiency compares the useful energy you get with the energy you put in:

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

The answer is a number between 0 and 1 (never above 1). Multiply by 100 to give a percentage. Because the total energy is conserved, input = useful output + wasted energy. Every real device has efficiency below 1, as some energy always dissipates.

If a device runs steadily, dividing the energy by the same time gives power. So you may also use useful power output / total power input.

Worked example 4. A pump takes 850 J from its supply and delivers 612 J as useful kinetic energy of water.

1. Efficiency = 612 / 850 = 0.72.
2. As a percentage this is 72%.
3. Wasted energy = 850 − 612 = 238 J.

Worked example 5 (rearranging). A heating system is 35% efficient (0.35). It takes 4200 J from its fuel store.

1. Useful output = efficiency × input = 0.35 × 4200 = 1470 J.
2. Wasted = 4200 − 1470 = 2730 J.

Worked example 6 (power and efficiency together). A cordless drill has an input power of 45 W and is 60% efficient. It runs for 20 s.

1. Input energy = P t = 45 × 20 = 900 J.
2. Useful output = 0.60 × 900 = 540 J of kinetic energy.
3. Wasted = 900 − 540 = 360 J.

In each case the useful and wasted energies add up to the input.

Increasing efficiency (P7.2e)

Efficiency rises when less of the input is stored in unwanted ways. Ideas to describe:

• Lubricate moving parts to cut friction.
• Add thermal insulation so less energy escapes from a hot object.
• Make shapes more streamlined to reduce air resistance.
• Use thicker wires, which have lower resistance, in circuits where the wires warm up.
• Replace a device with a better design. An LED lamp gives off the same light as a filament lamp for much less input energy, because less of the input is stored thermally.
• Use the device only when needed and avoid leaving it on standby.

Reducing unwanted energy transfer (P7.2f)

Lubrication. Rubbing surfaces have tiny bumps that catch on each other. Work done against friction makes the surfaces warm, and that thermal energy dissipates. A layer of oil or grease keeps the surfaces apart so they slide over a liquid layer. Friction is smaller, so less energy is stored thermally, and more of the input goes into the useful motion. Lubricating a bicycle chain is a standard example.

Thermal insulation. Energy leaves a hot object by conduction, convection and radiation. Insulation reduces this transfer. Loft insulation is a thick layer of fibres that trap air. Trapped air is a poor conductor, and because it is held in small pockets it cannot circulate, so convection is stopped as well. Cavity walls can be filled with foam, double glazing traps a layer of air or gas between two panes, and draught-proofing seals gaps so warm air cannot leak out. Each measure reduces the rate at which energy is transferred out of the house, so the heating needs less energy to keep the temperature up.

The specification suggests examining thermograms of houses. These are images that show surface temperature by colour. Bright areas of a wall or roof mean energy is leaving faster there.

Cooling of a building (P7.2g)

This is qualitative. Two properties of the walls matter.

• Thickness. A thicker wall makes the path longer, so energy is transferred through it more slowly. Thicker walls give a lower rate of cooling.
• Thermal conductivity. This is how well a material conducts thermal energy. A wall made of a material with low thermal conductivity (like brick or foam) transfers energy slowly. A wall of high conductivity material (like metal) transfers energy quickly. Low conductivity gives a lower rate of cooling.

So the best wall for keeping a building warm is thick and made of a low-conductivity material. Two other factors matter in practice: the bigger the temperature difference between inside and outside, the faster the building cools, and windows, doors and gaps can let much more energy through than the walls.

The specification suggests investigating rate of cooling with insulated and non-insulated copper cans. For a fair test, use the same volume of hot water at the same starting temperature, the same lid and the same room, and change only the insulation.

Common errors

• Writing that energy is “used up” or “lost”. Say it is dissipated to the surroundings.
• Calling a high-power appliance efficient. Power and efficiency are different ideas.
• Giving efficiency above 100% or above 1. Check that the useful energy is smaller than the input.
• Mixing units: kW with seconds, or W with hours.
• Saying insulation “stops” energy transfer. It slows it.
• Saying thicker walls with high conductivity cool slowly. Thickness and conductivity must both be considered.

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.