Ohm's law relates voltage, current, and resistance. Which equation expresses Ohm's law?

• A. V = I × R
• B. I = V × R
• C. V = R ÷ I
• D. P = V × I

Answer: (A)

Ohm's law shows how voltage, current, and resistance are connected: the voltage across a conductor equals the current flowing through it times its resistance. The best way to express this relationship is V = I × R. This form directly ties the three quantities together and lets you solve for any one if you know the other two. For example, with a known voltage and resistance, current is I = V / R; with a known current and resistance, voltage is V = I × R; with a known voltage and current, resistance is R = V / I. A related but different concept is power, given by P = V × I, which describes electrical power, not the fundamental voltage–current–resistance relationship. The other forms—I = V × R and V = R ÷ I—don’t match the units and definitions of current and resistance, so they don’t describe Ohm’s law correctly.