Series and parallel circuits, how current and potential difference are shared in each, the equations for electrical power and energy, and the UK mains supply.

What this dot point is asking

Edexcel wants you to describe series and parallel circuits and how current and potential difference are shared in each, use the equations for electrical power and energy, and describe the UK mains supply.

Series and parallel circuits

This is why household appliances are wired in parallel: each gets the full mains voltage and can be switched on or off independently. If they were wired in series, switching one off would break the circuit and turn them all off, and each would only get a share of the voltage, so they would run weakly.

Electrical power

Electrical energy

The energy transferred by an appliance depends on its power and how long it runs:

where $E$ is energy in joules ($J$), $P$ is power in watts ($W$) and $t$ is time in seconds ($s$). A more powerful appliance, or one left on for longer, transfers more energy.

The mains supply

The UK mains electricity supply is alternating current (a.c.) at a potential difference of about 230 V and a frequency of 50 Hz (it changes direction 50 times a second). Cells and batteries supply direct current (d.c.), which flows in one direction only.

A UK three-pin plug has three wires with a fixed colour code: the live wire (brown) carries the alternating potential difference from the supply, the neutral wire (blue) completes the circuit, and the earth wire (green and yellow stripes) is a safety wire that carries current away if a fault makes the case live. A fuse in the live wire melts and breaks the circuit if too large a current flows, protecting the appliance and the user. The live wire is the dangerous one, because it is at a high potential relative to earth, so touching it can give a fatal shock even when the appliance is switched off.

The energy and power equations explain why some appliances cost more to run than others. An appliance with a higher power rating, such as a $2000\,W$ heater, transfers energy much faster than a $100\,W$ light bulb, so it uses more energy in the same time and costs more. Knowing $E = Pt$ lets you compare appliances and calculate the energy used over a given time, which is the basis of an electricity bill (though bills use the kilowatt-hour as the unit of energy).

Try this

Q1. State the equation for electrical power in terms of current and potential difference. [1 mark]

• Cue. $P = IV$.

Q2. In a series circuit, what is true about the current at different points? [1 mark]

• Cue. It is the same everywhere.