The flow of electric charges constitute an electric current. For an electrical appliance to work, electric current must flow through it. An electric current is measured by the amount of electric charge moving per unit time at any point in the circuit. The conventional symbol for current is ‘I’.
Unit of Electric Current
The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of one coulomb per second.
I = q / t
Where I ⇒ current (in Ampere - A)
q ⇒ charge (in coulomb - c)
t ⇒ time taken (in seconds - s)
Worked example 2.1
If 30 coulomb of electric charge flows through a wire in two minutes, calculate the current in the wire?
Charge (q) = 30 coulomb
Time (t) = 2 min x 60s
= 120 s
Current I = q/t = 30C/120s = 0.25 A
1. Conventional Current and Electron Flow
Before the discovery of electrons , scientists believed that an electric current consisted of moving positive charges.
This movement of positive charges is called conventional current.
After the electrons were discovered, it was known that electron flow actually takes place from the negative terminal to the positive terminal of the battery. This movement is known as electron flow.
Conventional current is in the direction opposite to electron flow.
2. Measurement of electric current
Electric current is measured using a device called ammeter. The terminals of an ammeter are marked with + and - sign. An ammeter must be connected in series in a circuit.
Instruments used to measure smaller currents, in the milli ampere or micro ampere range, are designated as milli ammeters or micro ammeters.
1 milliampere (mA) = 10-3 ampere.
= 1/1000 ampere
1 microampere (µA) = 10-6 ampere
= 1/1000000 ampere
Worked Examples 2.2
If 0.002A current flows through a circuit, then convert the current in terms of micro ampere?
Given that the current flows through the circuit is 0.002A
We know that
0.002A = 0.002 × 106 µA
= 2 × 10-3 × 106 µA
0.002A = 2000 µA