Currents in electrical circuits

An electrical circuit can be shown in a circuit diagram. Standard symbols are used to represent different electrical components.

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Circuit symbols

For your exam you’ll need to be able to draw the following symbols and be able to interpret circuit diagrams.

• A cell is needed to push electrons around in a completed circuit.
• A battery is made up of two or more cells which are joined together. This increases the circuit’s power supply.
• A bulb is a way of showing a current passing through a circuit. If it lights up then the circuit is complete.
• An ammeter measures electric current.
• A voltmeter measures voltage (or potential difference).
• A switch allows the circuit to be turned on or off.
• A fixed resistor limits the amount of current flowing through a circuit.
• A variable resistor varies the current.
• A diode lets the current flow in only one direction.
• A fuse melts thereby breaking a circuit if the current reaches over a certain limit.
• A heater transfers electrical energy into heat energy.

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Electric current

The flow of electric charge is called the electric current. If a circuit is broken then a current can’t flow. For example, if a switch is open.

Electric current is measured in amperes (amps or A) using an ammeter. In order to measure the current the ammeter must be placed in series with the component it’s measuring.

The size of an electric current represents the rate at which an electric charge is flowing. Current is calculated with the following equation:

I = Q / t

• – I is the current in amps (A)
• – Q is the charge in coulombs (C)
• – t is the time in seconds (s)

A coulomb is a unit of electric charge: one coulomb is the amount of charge which is transferred in one second by a current measuring one amp.