Power and efficiency: power as the rate of energy transfer, the power equations, useful versus wasted energy, the efficiency equation and ways to reduce unwanted transfers.

What this dot point is asking

AQA wants you to define power as the rate of energy transfer, use both power equations, distinguish useful from wasted energy, calculate efficiency and describe ways to reduce unwanted energy transfers.

Power

The two equations are really the same idea, because work done is just one way of transferring energy, so both measure how many joules are transferred each second. To compare two devices fairly, look at the power: a $2\,kW$ kettle transfers twice as much energy per second as a $1\,kW$ one, so it boils the same water in half the time. Power and energy are often confused in exams; energy (in joules) is the total transferred, while power (in watts) is how fast it is transferred.

Useful and wasted energy

In every device, only some of the input energy is transferred to the useful output store; the rest is wasted, usually dissipated to the thermal store of the surroundings. For example, a filament lamp wastes most of its electrical input as heat, with only a small fraction as light. Because energy is conserved, the useful output plus the wasted output always equals the total input, which gives you a quick way to find one quantity from the other two. The wasted energy is not destroyed, but once it is spread out in the thermal store of the surroundings it is no longer useful, because it cannot easily be gathered back to do a useful job.

Efficiency

Reducing unwanted transfers

Each method targets a particular wasteful transfer. Lubrication with oil or grease reduces the friction between surfaces that slide over one another, so less energy is dissipated to the thermal store as heat and sound. Thermal insulation uses materials with a low thermal conductivity (such as foam, fibreglass or trapped air) to slow the rate at which energy is transferred by heating from a warm region to a cooler one; thicker or more numerous layers slow it further, which is why loft insulation and cavity-wall insulation reduce heating bills. Streamlining shapes a vehicle so that air or water flows smoothly past it, reducing drag and so the energy wasted overcoming resistance. Improving efficiency matters because the higher the efficiency, the more of the input energy ends up doing the useful job, which saves fuel, money and resources.

Try this

Q1. Define power and state its unit. [2 marks]

• Cue. Power is the rate of energy transfer; the unit is the watt ($W$), equal to $1\,J/s$.

Q2. A lamp transfers $100\,J$ of electrical energy, of which $5\,J$ becomes light. Calculate its efficiency. [2 marks]

• Cue. $\text{efficiency} = \dfrac{5}{100} = 0.05$, or $5\%$.