Circuit Calculations

Overview

This lesson introduces the first circuit calculations in the topic. The important part is not just substituting numbers into equations, but understanding what charge, current, and voltage mean physically.

What You Need to Know

• Charge is measured in coulombs.
• Current tells you how much charge passes a point each second.
• Voltage tells you how much energy is transferred per unit charge.
• Circuit equations are most useful when you connect them to the motion of charge and the transfer of energy in the circuit.
• Units matter because they help you check whether your calculation is sensible.

How to Work Through It

1. Start by reviewing the meanings of charge, current, and voltage.
2. Write down the relevant equations and identify the unit for each quantity.
3. Work through short calculations and explain what the answers mean physically.
4. Finish by checking answers for both numerical accuracy and sensible units.

Check Your Understanding

• What does current tell you about charge in a circuit?
• What does voltage tell you about energy transfer?
• Which unit belongs to each quantity?
• How can you tell if your final answer is physically sensible?

Common Mistakes

• Mixing up current and voltage.
• Writing the equation correctly but substituting the wrong value in the wrong place.
• Forgetting the unit in the final answer.
• Treating the equation as a maths exercise only, without linking it to the circuit.

Next Steps

• Practise short equation questions until the units and meanings feel secure.
• Keep the physical idea of current flow clear because the next lesson looks at models of circuits and resistance.