Ohm's law links voltage, current, and resistance in an idealized electrical relationship. Knowing which quantity is missing makes the calculation a quick rearrangement.
The three forms
Voltage is V = IR. Current is I = V/R. Resistance is R = V/I. Use volts, amps, and ohms together when working in SI units.
A quick example
If a 12-volt source is connected to a 6-ohm resistance in an ideal model, the current is 12/6 = 2 amps. Actual components may heat, vary, or behave nonlinearly.
Know the limits
Ohm's law is appropriate for an ohmic relationship under the relevant conditions. Batteries, diodes, lamps, temperature-sensitive elements, and complex circuits may need a fuller model.
A worked example
Worked example: a 12-volt source across 6 ohms gives an ideal current of 2 amps. The power relationship then gives 24 watts, but the component must be rated for the actual conditions.
Measure or verify the circuit values before relying on the result. Heating, wiring, batteries, and nonlinear components can invalidate a simple ideal model.
A circuit calculation should also check current capacity, heat, insulation, and the source's limits. The numerical answer is not permission to connect components without a suitable design.
For a real design, treat the calculated current as an input to component selection and protection rather than as the final engineering decision.
Use the rearranged form that matches the missing value, then confirm that the component and conditions fit the model.
COMMON QUESTIONS
Frequently asked questions
What happens if resistance is zero?
The ideal equation would imply an undefined or extremely high current for nonzero voltage; real circuits require current limits and safety analysis.
Is power part of Ohm's law?
Power is related through P = VI and can be combined with Ohm's law, but it is a separate quantity.