Quick answer: start with V = IR. Then use P = VI, P = I²R or P = V²/R depending on the values given.
Ohm’s law triangle
V is potential difference in volts, I is current in amperes and R is resistance in ohms. Rearrange as I = V/R or R = V/I. Write the unit beside each value before substituting.
Worked example
A 12 V supply is connected to a 6 Ω resistor. Current I = 12/6 = 2 A. Power P = VI = 12×2 = 24 W. If it runs for 5 minutes, t = 300 s and energy E = Pt = 24×300 = 7,200 J.
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Which power formula?
- Use P = VI when voltage and current are known.
- Use P = I²R when current and resistance are known.
- Use P = V²/R when voltage and resistance are known.
Common mistakes
- Using minutes instead of seconds for joules.
- Writing current in mA without converting to A.
- Adding resistances without checking series or parallel arrangement.
Practice
A 10 Ω resistor carries 0.5 A. Answers: V = 5 V and P = 2.5 W.
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