Current is the rate of flow of electric charge

Imagine current in a wire like water flowing through a pipe. The flow rate is similar to the volume of water passing through per unit of time. Electric current (I) is similarly the rate at which charge (Q) flows over time (t):

$I=\frac{Q}{t}$

Where:

I = current (A)

Q = charge (C)

t = time (s)

A coulomb (C) is the standard unit for electric charge. One coulomb equals the amount of charge that flows in one second when the current is one ampere.

Worked example 1 - Calculating current

Calculate the current in a wire when 30 coulombs of charge flows through it in 15 seconds.

Step 1: Equation

$\text{I}=\frac{\text{Q}}{\text{t}}$

Step 2: Substitution and Calculation

$\text{I}=\frac{30}{15}=2\text{ A}$

Worked example 2 - Calculating the number of electrons passing a point

A current of 2 A flows through a point in 20 s. Calculate the Charge that passes the point and hence determine the number of electrons flow in 20 s.

Step 1: Equation

$\text{Q = I t}$

Step 2: Substitution and correct evaluation

$\text{Q =}2\times 20\text{= 40 C}$

Step 3: Calculate number of electrons

$\text{number of electrons = }\frac{\text{Q}}{\text{e}}\text{ = }\frac{40}{1.6\times {10}^{-19}}{\text{ = 2.5 x 10}}^{20}$