Power and efficiency — OCR GCSE Physics

Dissipation is the spreading of energy into the surroundings, usually by heating, so that it becomes stored in less useful ways and is harder to recover for a intended purpose. When a device transfers energy, some is always lost to the environment. A filament lamp, for example, transfers most of its electrical input as thermal energy to the air rather than as light; a moving car transfers energy by friction and air resistance, warming the brakes, tyres and surrounding air. In each case the energy is not destroyed but becomes spread out among many particles, raising their temperature slightly. Because it is dispersed over a large area at a low temperature, it is described as stored in a less useful way and cannot easily be harnessed to do useful work again.

Domestic devices take energy from a battery (chemical store) or from the mains a.c. supply and transfer it to other stores. In a battery torch, chemical energy in the battery becomes electrical transfer along the wires and then light and thermal energy in the bulb; the thermal energy is wasted to the surroundings. In a mains kettle, the a.c. supply drives current through the heating element, transferring energy electrically to the element's thermal store, then to the water; some heats the kettle body and air. In a motor, such as a fan, electrical transfer becomes kinetic energy of the rotating parts, but friction in bearings and resistance heating in the coils waste energy as thermal energy. Identify the input store, the transfer pathway and the useful and wasted output stores for each device.

A power rating tells you the rate at which an appliance transfers energy: a 2000 W kettle transfers 2000 J each second, while a 100 W lamp transfers 100 J each second. The energy transferred equals power multiplied by time, so a higher-rated appliance changes the stored energy of its surroundings faster. A 2 kW kettle switched on for 300 s transfers 2000 × 300 = 600 000 J, heating the water's thermal store; the same kettle for 60 s transfers 120 000 J. Compare appliances by their ratings: a 2500 W heater warms a room faster than a 1000 W heater because it transfers energy at a greater rate. Match each rating to the store that changes, such as thermal energy of water or kinetic energy of a fan, and use E = P × t to compare.

Efficiency compares the useful energy delivered by a transfer with the total energy supplied to it. For any transfer, efficiency = useful energy output ÷ total energy input, and multiplying by 100 gives a percentage. A lamp supplied with 500 J of energy that transfers 100 J as light has efficiency 100 ÷ 500 = 0.20, or 20%; the remaining 400 J is dissipated, mainly by heating the surroundings. The same ratio applies to power, so efficiency = useful power output ÷ total power input. Always identify which quantity is useful and which is total before dividing, keep the units consistent, and check that the answer is below 1 (or below 100%).

Efficiency rises when a smaller share of the input energy is dissipated, usually as heat to the surroundings. Ways to achieve this include lubricating moving parts so friction transfers less energy to thermal stores, reducing air resistance by streamlining shapes, and insulating or lagging a device so less heat escapes. Improving the design of a machine, such as using bearings or tighter seals, also cuts wasted transfers. Each measure works by reducing the wasted energy for the same useful output, so the ratio of useful output to total input increases. Explain the mechanism, not just the change: state which wasteful transfer is reduced and why efficiency then rises.

Unwanted energy transfer happens whenever energy moves from a useful store into the surroundings, usually by heating. Lubrication reduces unwanted transfer where surfaces rub: oil or grease between moving parts reduces friction, so less energy is transferred by heating to the parts and surroundings and more stays in the kinetic store of the moving object. Thermal insulation reduces unwanted transfer by conduction, convection and radiation from a warm object such as a house. Materials with low thermal conductivity, trapped air and reflective surfaces slow the rate at which energy passes to the cooler surroundings. In both cases the method works by reducing the rate of transfer, not by creating energy, and the total energy is conserved.

A warm building cools because energy is transferred through its walls to the cooler outside. The rate of cooling depends on the walls' thickness and thermal conductivity, treated qualitatively here. A thicker wall slows the rate of energy transfer, so the building cools more slowly. A wall made from a material of lower thermal conductivity, such as one containing trapped air or insulating foam, also slows the transfer and so reduces the cooling rate. Combining greater thickness with lower thermal conductivity gives the slowest cooling. A thin wall of a good conductor lets energy pass quickly, so the building cools fastest. The comparison is about rate, not total energy, and energy is conserved throughout.