Electricity links charge, current, potential difference, resistance, energy and power in one circuit. This section is part of the SPM Physics guide and has five lessons, a practice set and a reasoning extension.
What does this section cover?
Begin with relating current, charge and time. Then apply potential difference, resistance and Ohm’s law.
Next come series and parallel circuits and calculating electrical energy and power. The fifth skill is interpreting e.m.f. and internal resistance.
For explanation questions that use diagrams and readings, see electrical reasoning from diagrams and measurements. The practice set mixes the calculation skills.
One circuit, four questions
Here is a worked example. A 12 V supply is connected across one 6 Ω resistor for 10 seconds.
| Question | Formula | Result |
|---|---|---|
| Current | I = V ÷ R = 12 ÷ 6 | 2 A |
| Charge in 10 s | Q = It = 2 × 10 | 20 C |
| Power | P = VI = 12 × 2 | 24 W |
| Energy in 10 s | E = Pt = 24 × 10 | 240 J |
One circuit gives four answers. Read what the question asks, then pick the formula that has the quantities you are given.
Who should start where?
If units keep spoiling your answers, begin with current, charge and time. If you pick the wrong formula in questions with a resistor, begin with Ohm’s law.
If a diagram with several branches worries you, begin with series and parallel circuits. If power and kWh questions slow you down, begin with energy and power.
For a teacher to go through your working question by question, see online one-to-one Physics tuition or try the one-hour trial class (from RM50).