Calculate current, voltage, and resistance.

Ohm's law ties together the three basic quantities in any circuit: voltage (E, volts), current (I, amperes) and resistance (R, ohms). Know any two and you can find the third. It shows up on its own on the exam and is a foundation for many calculation questions.

Procedure

1. Write down what you are given, with units. Convert first: milliamps to amps (divide by 1,000), kilohms to ohms (multiply by 1,000).
2. Pick the form that solves for the unknown: E = I × R, I = E ÷ R, or R = E ÷ I.
3. Substitute and calculate.
4. Sanity-check: more voltage across the same resistance means more current; more resistance on the same voltage means less current.

Worked example
A heating element measures 16 Ω and is connected to 120 V. What current does it draw?

• The unknown is current, so use I = E ÷ R.
• I = 120 V ÷ 16 Ω = 7.5 A.
• Check by going back the other way: 7.5 A × 16 Ω = 120 V, which matches the supply.

Most common mistake
Dividing the wrong way round, for example R ÷ E instead of E ÷ R. Picture the E-over-I×R triangle: E sits on top, so when you divide, voltage is always the number on top, never the divisor. A second trap is assuming the current stays the same when the voltage changes. For a fixed resistance it is the resistance that stays the same.