About Ohms Law

Ohm's law, named after the German physicist Georg Ohm, states that the current through a conductor between two points is directly proportional to the potential difference (voltage) across the two points, and inversely proportional to the resistance between them.

The mathematical equation that describes this relationship is I = V/R where I is the current in amperes, V is the potential difference in volts, and R is the resistance in ohms. The potential difference is sometimes denoted by E or emf (electromotive force) instead of V. The equation may alternatively be expressed as R = V/I or V = I*R

About Power

Power is the rate at which work is done, or energy expended. The SI unit of power is the watt (W), which is equal to one joule per second.

The relationship between voltage, current and power can be expressed by the equation P =V*I . where P is the power in watts, V is the voltage in volts, and I is the current in amperes.

This can be rewritten as I = P/V or V = P/I to find an unknown voltage or current, given a known power. For example, a 12v 60w car headlamp consumes 60w/12v = 5 amps.

Power / Resistance Relationships

Voltage, current, resistance and power are all inter-related, such that given two of the variables, the other two can always be calculated.

Since V=I*R, we can substitute V in the basic power equation to give P = I²*R or we can substitute I to give P = V²/R

These in turn can be re-written as:

V² = P*R
I² = P/R
R = V²/P = P/I²

Handy Calculators

Ohms Law

Enter 2 known variables, and press appropriate "calculate" button to find the third:

The following commonly used relationships apply to the above calculator:

Voltage, Current & Power

Enter 2 known variables, and press appropriate "calculate" button to find the third:

These common relationships apply to the above calculator: