Ohm's Law — The Foundation

Three values govern every electrical circuit. Voltage. Current. Resistance. Ohm's Law describes the relationship between them. V = I × R. Voltage equals Current multiplied by Resistance. Know any two values and you can calculate the third. This is the foundation of every electrical diagnosis you will ever make.

Picture that garden hose again. Voltage is the water pressure — the force pushing current through the circuit. Current is the flow rate — how much electricity actually moves through the wire per second. Resistance is anything that restricts that flow — a narrow section of hose, a kink, a partial blockage. Add resistance anywhere in the circuit and current drops. The component at the end gets less current, less power, less of what it needs to operate correctly.

A motor that should spin at full speed runs slowly. A fuel pump that should deliver full pressure delivers less. A starter that should crank strongly cranks sluggishly. In every case the explanation is the same: resistance is reducing current. Ohm's Law tells you this before you even put a test lead on the vehicle.

If a circuit has 12 volts and 4 ohms of resistance, current is 3 amps. If a bad connection adds 2 more ohms, current drops to 2 amps — a 33% reduction in power to the component. If you measure 0.4 volts of voltage drop across a section that should have 0.1 volts or less, you know that section has excessive resistance. You do not need to guess. The math tells you.