Calculating electric charge
- Current can also be defined as the charge passing through a circuit per unit time
- Electric charge is measured in units of coulombs (C)
- Charge, current and time are related by the following equation:
Q = It
- Where:
-
- Q = charge (C)
- I = current (A)
- t = time (s)
Worked example
When will 8 mA of current pass through an electrical circuit?
A. When 1 J of energy is used by 1 C of charge
B. When a charge of 4 C passes in 500 s
C. When a charge of 8 C passes in 100 s
D. When a charge of 1 C passes in 8 s
Answer: B
Step 1: Write out the equation relating current, charge and time
Step 2: Rule out any obviously incorrect options
- Option A does not contain time, so can be ruled out
Step 3: Try the rest of the options to determine the correct answer
- Consider option B:
I = 4 ÷ 500 = 8 × 10–3 = 8 mA
- Consider option C:
I = 8 ÷ 100 = 80 × 10–3 = 80 mA
- Consider option D:
I = 1 ÷ 8 = 125 × 10–3 = 125 mA
- Therefore, the correct answer is B
Examiner Tip
Although electric charge can be positive or negative, since the conventional direction of current is the flow of positive charge the current should always be a positive value for your exam answers.