Introduction
- Electric current is the rate of flow of electric charges.
- The standard symbol of current is I.
- The electric charge is denoted by Q, and its SI unit is Coulomb (C).
- The SI unit of current is Ampere and denoted by A.
- The instrument used to measure electric current is called an ammeter.
- The symbol for the ammeter is as follows:
The other units for electric current are:
- Milliamperes (mA): 1mA = $$10^{-3}$$A
- Microamperes ($$\mu$$A): 1$$\mu$$A = $$10^{-6}$$A
- Kiloamperes (kA) : 1kA = $$10^{3}$$
Relationship between charge and electric current
The relationship between charge and electric current is given by the following equation:
$$Q = I \times t$$
where Q is the charge in Coulombs (C), I is the electric current in Amperes (A) and t is time in Seconds (s).
Examples
Example 1
If a charge of 180 C flows through a lamp every 2 minutes, calculate the electric current passing through the lamp.
Solution:
Q = 180 C t = 2 x 60 = 120 s
$$I = \frac{Q}{t} = \frac{180}{120} $$ = 1.5 A
Example 2
Calculate the quantity of charge passing through a lamp if a current of 5A passes through the lamp for 30s.
Solution:
I = 5 A t = 30s
Q = I x t = 5 x 30 = 150 C
Example 3
Calculate the time taken by a battery to drive 300C charge through a 15A bulb.
Solution:
Q = 300C I = 15A
$$t = \frac{Q}{I} = \frac{300}{15}$$ = 20 s
Example 4
Convert the following electric currents to amperes:
(a) 500mA
Solution:
1 A = 1000mA
1mA = $$\frac{1}{1000}A$$
500mA = $$\frac{500}{1000}A$$ = 0.5 A
(b) 25000 $$\mu$$A
Solution:
1 A = 1000000 $$\mu$$A
1 $$\mu$$A = $$\frac{1}{1000000}A$$
25000 $$\mu$$A = $$\frac{25000}{1000000}A$$ = 0.025 A