Energy transfers

You should be able to explain how the power transfer in any circuit device is related to the potential difference across it and the current through it, and to the energy changes over time:

power = potential difference × current P = V I

power = current² × resistance P = I² R

power, P, in watts, W; potential difference, V, in volts, V; current, I, in amperes, A; resistance, R, in ohms, Ω.

You should be able to recall and apply both equations.

Which of the two to use is decided by what you are given. If you have the potential difference and the current, use P = V I. If you have the current and the resistance, use P = I² R. The two are consistent with each other, because V = I R — substituting that into P = V I gives P = I R × I, which is P = I² R.

The squared term in P = I² R matters. Doubling the current through a component quadruples the power dissipated in it, which is why a small increase in current can make a wire dangerously hot — and why the National Grid works the way it does.

Everyday electrical appliances are designed to bring about energy transfers. The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance.

Work is done when charge flows in a circuit. The amount of energy transferred by electrical work can be calculated using the equations:

energy transferred = power × time E = P t energy transferred = charge flow × potential difference E = Q V

energy transferred, E, in joules, J; power, P, in watts, W; time, t, in seconds, s; charge flow, Q, in coulombs, C; potential difference, V, in volts, V.

You should be able to recall and apply both equations.

You should be able to describe how different domestic appliances transfer energy from batteries or ac mains to the kinetic energy of electric motors, or the energy of heating devices.

You should be able to explain how the power of a circuit device is related to the potential difference across it and the current through it, and to the energy transferred over a given time. You should also be able to describe, with examples, the relationship between the power ratings for domestic electrical appliances and the changes in stored energy when they are in use.

The power rating printed on an appliance tells you how much energy it transfers each second. A 2 kW kettle transfers 2000 J every second, so it fills the thermal store of the water quickly; a 40 W lamp transfers 40 J every second. Over the same time, the higher-rated appliance brings about a much larger change in the stored energy.

The National Grid is a system of cables and transformers linking power stations to consumers. Electrical power is transferred from power stations to consumers using the National Grid.

Step-up transformers are used to increase the potential difference from the power station to the transmission cables. Step-down transformers are then used to decrease, to a much lower value, the potential difference for domestic use.

You should be able to explain why the National Grid system is an efficient way to transfer energy. The construction and operation of transformers is covered under electromagnetic induction, and is Higher tier only — here you need only what a step-up and a step-down transformer do.

The explanation rests on the two power equations from earlier in this sub-topic, used together.

The power that has to be delivered is fixed. Because P = V I, transmitting that same power at a much higher potential difference means a much smaller current in the cables.

The power wasted heating the cables is given by P = I² R. Because the current is squared, reducing the current reduces the wasted power dramatically — cutting the current to a tenth cuts the wasted power to a hundredth.

So stepping the potential difference up for transmission, and back down for use, wastes far less energy than transmitting at a low potential difference would. The potential difference is stepped down again at the end because 400 000 V would be far too dangerous to bring into a house.