Ohm's Law links voltage, current, and resistance — the three fundamental quantities in any electrical circuit. Here's the formula, Watt's Law extension, and practical applications.
Ohm's Law is to electronics what Newton's Laws are to mechanics — a fundamental relationship that makes a huge range of calculations possible. Whether you're troubleshooting a home circuit or designing electronics, this is where you start.
Voltage (V) — measured in Volts (V). The electrical pressure driving current through a circuit. Mains electricity in the UK is 230V. A standard AA battery is 1.5V.
Current (I) — measured in Amperes, or Amps (A). The flow of electrons through the circuit. A phone charger draws about 1–2A. A kettle draws about 10–13A.
Resistance (R) — measured in Ohms (Ω). The opposition to current flow. Insulators have very high resistance; conductors have very low resistance.
The triangle memory aid: draw a triangle with V at the top, I bottom-left and R bottom-right. Cover the one you want — the remaining two show whether to multiply or divide (side by side = multiply; stacked = divide).
Example 1 — Finding current: A 12V car battery powers a device with 6Ω resistance. How much current flows?
Example 2 — Finding resistance: A 230V circuit carries 10A. What is the circuit's resistance?
Example 3 — Finding voltage: A circuit with 5Ω resistance carries 3A. What is the voltage?
Watt's Law connects Ohm's Law to power (measured in Watts). Combined, the two laws let you calculate any quantity from any other two.
This explains why high-voltage power transmission is efficient: to transmit the same power (P) at higher voltage (V), you need less current (I). Since power lost in transmission lines = I²R, less current means dramatically less power loss.
Series: Resistances add together. Current is the same everywhere. Voltage divides.
Parallel: Resistance decreases. Voltage is the same across each. Current divides.
Fuse sizing: A circuit running an 1,800W (1.8kW) appliance on 230V: I = P/V = 1,800/230 = 7.8A. Use a 10A fuse, not a 5A (which would blow) or 30A (which is unsafe).
LED resistor calculation: Connecting a 2V LED to a 9V battery with 20mA (0.02A) target current: R = (9−2)/0.02 = 350Ω. Use a 360Ω resistor (nearest standard value above).