Electrician · Ohm's Law
Ohm's Law Practice Problems
Free, unlimited Ohm's Law and power law practice for the Journeyman & Master Electrician exam. Every click generates a new random circuit — solve for whichever value is missing.
How to calculate it
Ohm's Law, worked by hand
Every problem the calculator generates reduces to one of these four relationships. Here's a full example working from voltage and current to find resistance and power.
Problem: A circuit has a voltage of 120V and draws a current of 8A. Find the resistance and the power consumed.
1. Solve for resistance using Ohm's Law:
2. Substitute:
3. Solve for power:
4. Substitute:
FAQ
Ohm's Law — frequently asked questions
What's the difference between Ohm's Law and the power law?
Ohm's Law (V = I × R) relates voltage, current, and resistance. The power law (P = V × I) brings power into the picture. Combining the two gives the two other forms used on the exam: P = I²R and P = V²/R — this calculator drills all four relationships, not just V = IR.
How do I know which formula to use for a given problem?
Identify which two values you're given, then pick the equation that isolates the one you need. Given V and I, find R with R = V/I. Given P and R, find V with V = √(P × R) or I with I = √(P ÷ R). Writing down what you're given before picking a formula is the single biggest time-saver on this section of the exam.
Does this apply the same way to AC circuits?
For a purely resistive load, yes — the same V = IR and power relationships hold. Once reactance (inductance/capacitance) enters the picture, AC circuits need impedance (Z) in place of R and power-factor corrections, which is a separate exam topic from this basic Ohm's Law drill.
What units does the exam expect for each variable?
Voltage in volts (V), current in amps (A), resistance in ohms (Ω), and power in watts (W). Answer choices on the real exam are usually unit-consistent, but double-checking units before you commit to an answer catches a surprising number of avoidable mistakes.