Apprentice path · Stage 3: The theory you cannot fake later · Lesson 9

Ohm's law and the power wheel

Four quantities -- volts, amps, ohms and watts -- are locked together, so if you know any two of them you can work out the other two.

Why this matters on the job

Half the calculation questions on an exam are this and nothing else. On the job it is how you know whether a circuit can carry what you are about to put on it.

How to think about it

  1. Write down the two values you already have, with their units.
  2. Decide which of V, I, R or P you are being asked for.
  3. Pick the formula in that quadrant that uses the two values you have.
  4. Do the arithmetic, then write the unit next to the answer.

Worked through

Take 12 volts across 6 ohms. You want current, so go to the I quadrant and pick the formula that uses V and R: I = V / R. That is 12 / 6 = 2 amps. Now you want power, and you have V and I, so P = V x I, which is 12 x 2 = 24 watts. You never needed a new measurement -- the second answer came out of the first.

Check yourself: You have 24 volts across 8 ohms. What is the current, and what is the power?

Answer: Current is 24 / 8 = 3 amps. Power is 24 x 3 = 72 watts.

I = V / R gives you current from the two things you were handed. Once you have current, P = V x I gives you power without measuring anything else. Every formula on the wheel is the same three relationships rearranged.

Watch out for: Do not memorise twelve formulas. Memorise V = I x R and P = V x I, and rearrange. Also: this wheel is for DC resistive circuits -- AC power may also require power factor, which is a later lesson.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.

Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.