What the formula calculates
Ohm's Law describes the relationship between voltage V (volts), current I (amperes), and resistance R (ohms) in an electrical conductor: V = I × R. The law states that voltage across a conductor is directly proportional to the current flowing through it, with resistance as the constant of proportionality. It can be rearranged: I = V/R and R = V/I.
Electrical power
Electrical power P (watts) relates to voltage and current: P = V × I. Combining with Ohm's Law gives two additional forms: P = I² × R and P = V² / R. These allow power to be calculated from any two of the three quantities. A 60W light bulb on a 120V circuit draws I = 60/120 = 0.5A and has resistance R = 120/0.5 = 240 ohms.
Worked examples
A 9V battery connected to a 330-ohm resistor: Current = 9 / 330 = 0.027A (27 mA). Power dissipated = 9 × 0.027 = 0.24W. A resistor carrying 2A with 15V across it: Resistance = 15 / 2 = 7.5 ohms. Power = 15 × 2 = 30W.
Limitations
Ohm's Law applies to ohmic (linear) conductors at constant temperature. It does not apply to semiconductor devices like diodes and transistors, which have non-linear current-voltage relationships. Temperature also affects resistance in most conductors: metals increase resistance at higher temperatures, while semiconductors decrease.