Electrical circuits & Ohm’s law

Follow one charge around a circuit

A current of one ampere means that one coulomb of charge passes a point each second. A potential difference of one volt transfers one joule of energy per coulomb. In a steady circuit charge keeps circulating. The energy it transfers to a resistor warms the resistor.

For an ohmic resistor at constant temperature, the voltage across it is proportional to the current through it. Use V = I × R, with current in amperes and resistance in ohms. For 9 V across 300 Ω, the current is 9 ÷ 300 = 0.03 A or 30 mA. Electrical power is P = V × I, giving 0.27 W in this example. The model assumes ideal wires and a constant DC supply.

Two resistors in series

Connect a 100 Ω resistor and a 200 Ω resistor in a single path across 6 V. Their equivalent resistance is 300 Ω. Both carry 0.02 A because the whole current must pass through each. The first voltage drop is 0.02 × 100 = 2 V. The second is 4 V. Their sum equals the supply voltage.

Two resistors in parallel

Connect the same resistors across the same two supply terminals. Both now have 6 V across them. The 100 Ω branch draws 0.06 A and the 200 Ω branch draws 0.03 A. Together they draw 0.09 A. Seen by the supply, they act like a single resistor of approximately 66.7 Ω. Adding a path has made the total resistance smaller than either resistor.

Check a prediction. If both resistors are 100 Ω, a 6 V supply drives 0.03 A through the series pair. In parallel it drives 0.12 A in total. Compare these arrangements in the workbench. Check whether doubling the voltage changes either resistance or just the current and power.

These calculations describe fixed resistors. A motor, a filament lamp heating up or a charging capacitor behaves differently. To practise reading units from an inspection sheet, open the revision workshop.