Ohm's law says that the voltage across a resistance equals the current through it multiplied by the resistance:

V = I × R, which rearranges to I = V / R and R = V / I.

Know any two of the three and you can work out the third. It applies to resistors, to copper traces and to anything that behaves roughly like a resistance over the range you are interested in.

Everyday uses at the bench

How much current will a short draw?

A rail shorted to ground through 0.5 Ω, fed from a 3.3 V regulator, would try to draw 3.3 / 0.5 = 6.6 A. In practice the regulator's overcurrent protection shuts down long before that — which is why a shorted rail usually reads 0 V rather than burning something. On the system rail, fed by the adapter, a short can draw far more current, which is why the input fuse or the adapter's own protection trips.

What voltage drop does a current cause?

A current-sense resistor of 0.01 Ω carrying 3 A drops 0.01 × 3 = 0.03 V, or 30 mV. Charger ICs measure exactly that drop to know the charging current. If you measure 300 mV across the same resistor, either the current is ten times higher than you thought or the resistor has failed high.

What value does a pull-up need?

A 3.3 V signal pulled up through 10 kΩ and pulled low by a chip draws 3.3 / 10,000 = 0.33 mA while low. Designers pick values to keep that current small but still strong enough to define the level against noise and leakage.

Ohm's law with voltage injection

When you inject a voltage into a shorted rail from a bench supply to find the short, Ohm's law tells you what to expect. A supply set to 1 V into a 0.2 Ω short will push up to 5 A if its current limit allows; the shorted part then dissipates about 5 W, which warms it enough to show on a thermal camera. Start low and raise the current gradually — see voltage injection.

Where it does not apply simply

Ohm's law describes resistive behaviour. Diodes, transistors and integrated circuits do not follow it in a straight line: their current changes in a non-linear way with voltage. Capacitors and inductors behave differently with changing voltages, which is why AC circuits use impedance instead of plain resistance. For DC rails and resistors on a laptop board, though, Ohm's law covers almost every quick calculation you need.

A memory aid

Draw a triangle with V at the top and I and R side by side underneath. Cover the quantity you want: V leaves I × R, I leaves V over R, R leaves V over I.

Common mistakes

  • Mixing units — milliamps with ohms and volts. Convert to amperes first, or keep everything in mA and kΩ, which also works out.
  • Applying it to a regulator output under fault conditions; the regulator's protection, not the resistance, decides what happens.
  • Forgetting that resistance in circuit includes everything in parallel.

Related: electrical power adds the heat side of the same picture.