What is Ohm's Law?

• A. I = VR
• B. V = IR
• C. P = VI
• D. R = V/I

Answer: (B)

Ohm's Law shows how voltage, current, and resistance relate in a conductor: the voltage across a component equals the current through it times its resistance. This direct form, V = IR, is the most practical way to use the law because it immediately tells you how changing one quantity affects the others. For example, if a resistor of 5 ohms carries 2 amperes, the voltage across it is V = IR = 2 × 5 = 10 volts. If you know voltage and resistance you can find current as I = V/R, and if you know voltage and current you can find resistance as R = V/I; these are just rearrangements of the same relationship. A related, but distinct, equation is P = VI, which gives power, and it follows from multiplying voltage and current (and would agree with Ohm’s Law when you substitute voltage from V = IR). The standard, most direct expression to use for relating V, I, and R is V = IR.