Find any of voltage, current or resistance from the other two, and know what series and parallel circuits share.
Syllabus: Ohm's Law; Series and parallel circuits
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 14 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
Voltage (E) pushes. Resistance (R) resists. The result is current (I) flowing.
E = I × RRearranged, it gives the other two:
| Find | Formula | In words |
|---|---|---|
| Voltage E | E = I × R | current times resistance |
| Current I | I = E ÷ R | voltage over resistance |
| Resistance R | R = E ÷ I | voltage over current |
RememberVoltage is on top. To find anything else, divide E by the other one.
Every calculation question is the same three steps.
| Question | Formula | Answer |
|---|---|---|
| 90 V, 3 A → R? | R = E ÷ I = 90 ÷ 3 | 30 Ω |
| 120 V, 80 Ω → I? | I = E ÷ R = 120 ÷ 80 | 1.5 A |
| 0.5 A through 2 Ω → E? | E = I × R = 0.5 × 2 | 1 V |
Memory hookSanity check: more resistance means less current. If your current came out huge (9,600 A), you multiplied when you should have divided.
Memory hookParallel = Potential (voltage) is the same. Series = Stream (current) is the same.