Electricity Notes

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Circuits

Circuit symbols and diagrams are used to represent the components in a circuit. This is because they are easier to see and understand.

The symbols that need to be known are: a cell, a battery, a bulb, an ammeter, a voltmeter and a variable resistor.

Circuits have an energy source (cells, batteries or main electricity) and components that use the energy (lamps, buzzers, motors, etc.). Components are connected with wires.

A complete circuit is required for any current to flow. Any gaps will prevent the current from flowing.

Current

Current is the rate of flow of charge (from electrons) around a circuit.

Current has the units of amps (A).

The current is measured using an ammeter, placed in series with the circuit.

A short circuit occurs when a wire is placed across a component. It causes all the current to flow through the wire and none to flow through the component.

Conductors and insulators

Conductors allow electric current to flow through them. This is because they contain free electrons

Most metals are conductors. Graphite is a non-metal conductor.

Insulators do not have free electrons and therefore do not allow current to flow through them freely.

Voltage

The voltage is the electrical energy transferred per unit of charge. It can be referred to as the ‘potential difference’.

It has the units of volts.

The voltage across a component is measured with a voltmeter placed in parallel with the component.

A cell supplies voltage (electric energy) to a circuit. When more than one cell is linked together it is called a battery.

Increasing the voltage supplied, increases the current. For a bulb in a circuit, this would make it brighter.

Resistance

Resistance is the ability of a component to ‘prevent’ or ‘slow down’ the flow of charge (i.e. reduce the current). This is caused by electrons bumping into atoms (ions) as they travel through the material, converting electrical energy to heat.

Good conductors have a very low resistance while insulators have a very high resistance.

The longer a piece of wire, the greater its resistance.

Variable resistors can change the value of their resistance.

This is done by changing their length (the longer length, the greater their resistance)

Series and parallel circuits

Series circuits consist of components linked in a single loop.

The current is the same in all parts of a series circuit (before and after components).

The voltage supplied by the cell or battery will equal the sum of the voltages used by components in a series circuit.

Doubling the resistance will half the current in a series circuit (assuming the voltage stays the same).

Doubling the voltage supplied will double the current in a series circuit (assuming that the resistance stays the same).

Parallel circuits contain junctions where the current splits.

Parallel circuits are useful as they allow components to be switched on and off individually, unlike series circuits where all components are either on or off.

The current into a junction of a parallel circuit, equals the current out of it.

When more bulbs are added in parallel their brightness is not affected as the voltage across each branch is the same as the cell voltage.