Definition · Plain-language
Ohm’s law
Ohm’s law states that the voltage across a conductor equals the current through it multiplied by its resistance, written V = IR.
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Voltage, current and resistance
Ohm’s law links the three core quantities of an electric circuit. Voltage (V), measured in volts, is the electrical “push” driving charge around the circuit. Current (I), measured in amperes, is the rate at which charge flows. Resistance (R), measured in ohms, is how strongly the conductor opposes that flow. The law states they are tied together by V = IR. A useful analogy is water in a pipe: voltage is the pressure, current is the flow rate, and resistance is how narrow the pipe is — more pressure or a wider pipe means more flow.
Using the equation
Because V = IR contains three quantities, knowing any two gives the third. Rearranged, I = V ÷ R finds the current from a known voltage and resistance, and R = V ÷ I finds the resistance. Many learners use the “Ohm’s law triangle” — V on top, I and R below — to recall these forms quickly. The law is the everyday tool of electronics: it tells an engineer what value of resistor is needed to limit current through an LED, why a thinner wire heats up more, and how much current a given appliance will draw from the mains.
Ohmic and non-ohmic materials
Ohm’s law is strictly a property of certain materials, called ohmic conductors, for which resistance stays constant regardless of the voltage applied — most metals at a steady temperature behave this way, so their current is exactly proportional to voltage. Many components are non-ohmic: a filament lamp’s resistance rises as it heats up, and a diode conducts in only one direction, so their voltage–current graphs are not straight lines. For these, V = IR still defines the resistance at any single moment, but resistance is no longer a fixed number. Ohm’s law is therefore a precise rule for ohmic materials and a useful instantaneous definition for the rest.
Key facts
At a glance
- Definition: current is proportional to voltage, with resistance as the constant
- Equation: V = IR (voltage = current × resistance)
- Rearranged: I = V ÷ R and R = V ÷ I
- Units: volts (V), amperes (A), ohms (Ω)
- Applies to: ohmic conductors, where resistance is constant
- Exceptions: non-ohmic components such as filament lamps and diodes
Common misconceptions
What people often get wrong
Often heard: Ohm’s law holds for every electrical component.
Actually: It holds only for ohmic conductors with constant resistance. Filament lamps, diodes and many other components are non-ohmic, so current is not simply proportional to voltage.
Often heard: Current is what pushes electricity through a circuit.
Actually: Voltage is the push (the potential difference); current is the resulting flow of charge. Voltage causes current, not the other way around.
Often heard: A higher voltage always means a higher current.
Actually: Only if resistance stays the same. Because I = V ÷ R, raising the resistance can keep current low even at high voltage.
Going deeper
