Glossary term
Ohm Law
The relation between voltage, current, and resistance in an ideal linear conductor.
Definition
lawThe relation between voltage, current, and resistance in an ideal linear conductor.
Ohm's law states that voltage across an ideal ohmic element is proportional to current through it, with resistance as the proportionality constant. It is a foundation of circuit analysis, electrical testing, power calculation, and equivalent-circuit modelling, but it applies directly only when the voltage-current relationship is linear and operating conditions remain within range.
Ohm’s law relates voltage, current, and resistance for an ohmic element:
where V is voltage, I is current, and R is resistance. The same relation can be rearranged as I=V/R or R=V/I, but the physical interpretation should not be lost: resistance is the slope of a linear voltage-current relationship under the stated conditions.
Circuit meaning
In lumped circuit models, Ohm’s law combines with Kirchhoff’s laws to solve resistor networks, bias circuits, sensor dividers, load currents, cable voltage drops, and fault-current approximations. It also leads directly to power relations:
Those equations are essential for checking resistor ratings, conductor heating, battery loading, protection sizing, and test measurements. In AC steady-state analysis, the idea generalizes to impedance: voltage and current are related by a complex quantity that includes resistance and reactance.
Physical limits
Ohm’s law is not a universal law for every electrical device. Diodes, transistors, lamps, thermistors, batteries, arcs, electrolytes, varistors, magnetic components, and many sensors have nonlinear voltage-current curves. Even metallic conductors deviate when temperature, self-heating, strain, frequency, or material state changes enough to alter resistance.
At field level, current density is related to electric field through material conductivity for a homogeneous ohmic medium. At circuit level, the lumped resistor model assumes geometry, temperature, and frequency effects can be represented by one resistance value.
Common mistakes
A common mistake is to infer resistance from one voltage-current point on a nonlinear device and then apply it as if constant. Another is to ignore meter burden, lead resistance, contact resistance, temperature coefficient, or power dissipation during measurement. A reliable calculation states whether values are DC, RMS AC, peak, phasor, hot, cold, nominal, or worst case.