Ohm’s law and electric power, with worked examples
Three quantities describe a simple circuit: voltage, current and resistance. One relation links them, and a second gives the power. With those two you can handle most everyday calculations.
Ohm’s law
V = I × R
V is the voltage in volts, I the current in amperes and R the resistance in ohms. Rearranged, I = V ÷ R and R = V ÷ I.
Example: 12 V across 4 Ω
- I = V ÷ R
- I = 12 ÷ 4
I = 3 A
Electric power
P = V × I
Example: a device drawing 5 A at 230 V
- P = 230 × 5
P = 1,150 W
Series and parallel
series: R = R₁ + R₂
parallel: 1/R = 1/R₁ + 1/R₂
Example: 6 Ω and 3 Ω in parallel
- 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6
- R = 6 ÷ 3
R = 2 Ω
A parallel combination is always smaller than its smallest resistor. If your result is larger, check the inversion at the last step.
Common pitfalls
- Forgetting to convert milliamperes to amperes: 20 mA = 0.02 A.
- Adding resistors in parallel as if they were in series.
- Stopping at 1/R without inverting the final result.
Worked exercises
Try to find the answer first, then open the solution.
What current flows when 5 V is applied across 250 Ω?
Show the solution
- I = 5 ÷ 250 = 0.02 A
Answer: 0.02 A, or 20 mA
A current of 2 A flows through 6 Ω. What is the voltage?
Show the solution
- V = 2 × 6
Answer: 12 V
A 60 W device runs at 12 V. What current does it draw?
Show the solution
- I = P ÷ V = 60 ÷ 12
Answer: 5 A