Charge and current

Electric current is the rate of flow of electric charge. No current can flow if the circuit is broken - for example, when a switch is open.

An electric current flows when electrons move through a conductor, such as a metal wire. Metals are good conductors of electricity.

Electricity passes through metallic conductors as a flow of negatively charged electrons. The electrons are free to move from one atom to another. We call them a sea of delocalised electrons.

Loose electrons form a sea of delocalised electrons.

Current was originally defined as the flow of charges from positive to negative. Scientists later discovered that current is actually the flow of negatively charged electrons, from negative to positive. They termed the original definition ‘conventional current’ so as not to confuse it with the newer definition of current.

Calculating current

The size of an electric current shows the rate of flow of electric charge. You can calculate the size of a current using this equation:

current~in~amps = \frac{charge~in~coulombs}{time~in~seconds}

or:

I = \frac{Q}{t}

where:

I is the current in amperes (amps), A

Q is the charge in coulombs, C

t is the time in seconds, s

Question

What is the current if 20 C of charge passes in 5 s?

Current = 20 ÷ 5 = 4 A