Define electric current as the charge passing a point per unit time; recall and use the equation I = Q / t

4.2.2 Electric Current

Definition

Electric current is the rate at which electric charge passes a given point in a circuit.

Mathematically,

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

where

• \(I\) = electric current (amperes, A)
• \(Q\) = charge transferred (coulombs, C)
• \(t\) = time taken (seconds, s)

Symbol and Unit

The symbol for electric current is I. The SI unit is the ampere (A), which is defined as 1 C s⁻¹.

Using the Equation \(I = Q/t\)

The equation can be rearranged to find any of the three quantities:

• Current: \(I = \dfrac{Q}{t}\)
• Charge:  \(Q = I \times t\)
• Time:   \(t = \dfrac{Q}{I}\)

Example Calculation

Find the current when a charge of 12 C passes a point in 3 s.

\$ I = \frac{Q}{t} = \frac{12\ \text{C}}{3\ \text{s}} = 4\ \text{A} \$

Therefore, the current is 4 A.

Common Misconceptions

• Current is not the same as charge; it is the *rate* of charge flow.
• The direction of conventional current is defined as the direction positive charge would move, opposite to electron flow.
• Current is a scalar quantity; it has magnitude but no direction.

Quick Check Questions

1. If a current of 2 A flows for 5 s, how much charge passes a point?
2. A circuit carries a charge of 30 C in 10 s. What is the current?
3. How long will it take for a current of 0.5 A to transfer 9 C of charge?

Unit Conversion Table