Electric current

An electric current is the rate at which electric charge flows in a circuit. When a circuit is complete, an electric current flows.

An electric current always flows from the positive terminal of the supply to the negative terminal of the supply.

Effects of an electric current

The existence of an electric current can be identified by its effects on an electric circuit:

If there is a lamp in a circuit, it lightens up because an electric current has passed through it.  The electric current also heats up the filament of the lamp as it passes through it.
When a plotting compass is placed near a piece of wire carrying an electric current, its needle will get deflected as a magnetic field is produced in the wire carrying the electric current.
When two terminals of the wire in a circuit are dipped in an acid, bubbles of gas form and electrolysis takes place

Measuring electric currents

To measure an electric current, we use a device called an ammeter.

There are two types of ammeters:

1.    An analogue ammeter

2.    A digital ammeter

An analogue ammeter has needle that points the amount of current flowing over a scale. Thus an analogue ammeter needs to be manually read and may lead to experimental errors.

A digital ammeter doesn’t have a needle and gives a direct reading of the amount of current flowing through it.

As electric current flows through the positive to negative, it must be connected to a circuit only in series.

Amperes and coulombs

An ammeter measures the rate at which electric charge flows past a point in a circuit; the amount of charge that passes per second.

These ideas help us to build up an equation to demonstrate the relationship between electric current, charge and time:

Hence, an electric current of 100A passing at a point will have 100C of charge flowing per second.

Exam tip!

You can rearrange the above equation into a more easy to remember equation:

Good and bad conductors of electricity

For an electric current to pass through a wire, the circuit must be connected from end to end. The best material to connect an entire circuit is by using metal wires.

The reason why metal wires are used is because metals are good conductors of electricity.

Metal wires need to be insulated with substances that are insulators of electricity. This improves their electrical safety.

Examples of good insulators of electricity are: polymers, minerals and glass.

Electric current in series and parallel circuits

Electric current in series circuits

Electric current in parallel circuits

The electric current is the same at all points in a series circuit.

The sum of the currents in the branches of a parallel circuit equals the current entering or leaving the parallel section.


Direct and alternating electric currents

The difference between direct and alternating electric currents

Direct current

Alternating current

In a direct current (d.c.) the electrons flow in one direction only.

In an alternating current (a.c.) the direction of flow reverses at regular intervals.

The pointer of an ammeter for measuring d.c. is deflected one way by the direct current.

Alternating current makes the pointer move to and fro about the zero if the changes are slow enough; otherwise no deflection can be seen.


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