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Charge and current

As you already know electric charge in a solid is carried by particles called electrons. One electron has a very tiny charge and so for practical measurement of electric charge we use units called COULOMBS.

A coulomb (C) is an AMOUNT of electric charge in just the same way that a litre is an AMOUNT of water.

One coulomb is the charge of roughly six million million million electrons!

The movement of this charge round a circuit is called the electric current.

Electric current is the rate of flow of charge round a circuit. The current at a point in the circuit is the amount of charge that passes that point in one second.

Electric current is measured in AMPERES (AMPS, symbol A).

A current of 1 A is flowing in a circuit if a charge of 1 coulomb passes any point in the circuit every second.

1 Amp = 1 Coulomb per second

We can write this formula as:

Current (I) = Charge (Q) / Time (t)
Charge (Q) = Current (I) x Time (t)

(Charge must be given in coulombs, current in amps and time in seconds)

You may meet smaller currents than one amp in school. For these currents we use milliamps and microamps.
1 A = 1000 milliamp (mA)
1 A = 1 000 000 microamps (mA)

Currents of millions of amps are used in fusion reactors, lightning is thousands of amps, currents of around an amp or so flow in light bulbs, currents in an electric clock will be milliamps and electronic circuits operate on currents of a few microamps.

Example problems
1. A charge of 12 C passes through the filament of a car headlamp bulb in 4 s. What is the current?
Current = Charge/time = 12/4 = 3 A

2. A current of 0,5 A flows for 20 s through a small electric motor. How much charge has passed?
Charge = Current x time = 0.5 x 20 = 10C

3. A current of 200 mA flows for 2 minutes. How much charge has passed?
Charge = 0.200 x 120 = 24 C
(current in amps, time in seconds)

1. What is the current flowing when:
(a) 20 C flow by in 4 s
(b) 120 C flow by in 2 minutes
(c) 2400 C flow by in 5 minutes
(d) 20 C flow by in 100 s

2. How much charge has passed when
(a) a current of 2A flows for 5 s
(b) a current of 25 mA flows for 150 s
(c) a current of 3A flows for 5 minutes
(d) a current of 5 MA flows for 5 ms

3. Copy and complete the following table:

  Charge Current Time
1 50C   5s
2   0.2 A 2 minutes
3 120 C 12 A  
4 0.2 C   300 s
5   50 mA 25 s

© Keith Gibbs 2020