Ohm's law

Before you rely on it

What to check

It holds for resistive loads.

The common error

Where people go wrong with ohm's law

Motors, capacitors and semiconductors do not obey it directly. The answer still looks reasonable, which is the dangerous part.

Worked through

Ohm's law in plain numbers

A 12-volt battery drives a current through a 6-ohm resistor. Current is voltage divided by resistance, so I = 12 ÷ 6 = 2 amperes. Power is voltage times current, 12 × 2 = 24 watts, and that resistor must be rated for at least that much heat. Now a practical case. An LED needs about 20 milliamperes and drops about 2 volts. Run it from a 5-volt supply and the resistor must drop the other 3 volts: R = 3 ÷ 0.02, or 150 ohms; it dissipates 3 × 0.02 = 0.06 watts, so a small resistor is fine. One relationship, three unknowns, any two give the third.

Easily confused

Ohm's law and its neighbours

Ohm's law is not the same as the power law, though they are used together. V = I × R connects voltage, current and resistance. P = V × I connects power to voltage and current, and combined with Ohm's law it gives P = I²R and P = V²/R. Ohm's law describes ohmic materials, where resistance stays constant. Diodes, transistors and light bulb filaments do not follow it over their full range. In alternating current circuits, resistance is replaced by impedance, which includes reactance.

In real decisions

Where ohm's law turns up

Electricians, hobbyists and engineers use it to choose resistors, size wires and fuses, and check that a supply can deliver enough current; students meet it in the first week of a circuits class. Automotive and battery calculations lean on it. Troubleshooting uses it to test whether a measured voltage and current fit the expected resistance; it is the simplest and most used relationship in electronics.

Know the edges

Where ohm's law stops being reliable

It applies to components whose resistance is constant, and resistance changes with temperature. Real wires and batteries have internal resistance that drops voltage under load. Alternating current, inductors and capacitors need more than this law. Electricity can be dangerous. This page is educational. For mains wiring or high-voltage work, follow codes and use a qualified electrician.

Quick answers

What people ask about ohm's law

Does Ohm's law work for all circuits? Only for components with constant resistance, such as ordinary resistors. Diodes, transistors and bulbs change resistance with voltage or temperature.

How do I calculate power? Multiply voltage by current. Combined with Ohm's law you can also use current squared times resistance, or voltage squared over resistance.

Why does a wire get hot? Current through resistance produces heat, equal to current squared times resistance. Thin or long wires have more resistance, so they warm more. Use wire sized for the current.

What are the units? Volts for voltage, amperes for current and ohms for resistance. One volt drives one ampere through one ohm.

What is a quick way to remember the three forms? Cover the quantity you want in a triangle with V on top and I and R below. Cover V and you see I times R. Cover I and you see V over R. Cover R and you see V over I.