OHM's Law

The fundamental law of electricity.

Quick Reference

Ohm’s Law Calculations

V = I * R
R = V / I
I = V / R

Power Triangle:

Illustration of a triangle divided into three sections with volts in the top and the bottom divided again with amps on the left and ohms on the right

More Info

The amount of current flowing through a conductor is proportional to the voltage applied across the conductor. This is true of all conductors with the exception of super-conductors. Mathematically this is stated as:

V = k * I

Where:

  • V is the voltage (in volts) drop across the conductor.
  • I (for Intensity), the current (in amps) flowing through the conductor.
  • k is the constant of proportionality.

Ohm’s Law

This relation is codified in Ohm’s Law, where, k is replaced by resistance (in ohms) of the conductor. The equation becomes:

V = I * R

For historical reasons, amperage is often labeled I (for intensity), and sometimes voltage is labeled E (for electromotive force). So, while the most common labeling for Ohm’s law is typically uses V for volts, I for amps, and R for resistance, sometimes the following is seen:

E = I * R

Where E is voltage.

Other Forms of Ohm’s Law

Using algebra, Ohm’s law can also be solved for resistance (R) or (I):

Solved for Resistance (R)

R = V / I
ohms = volts / amps

Solved for Voltage (V)

I = V / R
amps = volts / ohms

Ohm’s Law Pyramid

The Ohm’s Law triangle, also known as the power triangle is helpful in remembering the various solutions of Ohm’s law:

Illustration of a triangle divided into three sections with volts in the top and the bottom divided again with amps on the left and ohms on the right

It roughly represents the mathematical relationship of Ohm’s law; it’s divided vertically by a line that indicates division (below V), and horizontally, with a line that stands for multiplication (between I and R).

To read it, choose any given unit as the solution, and then use the division/multiplication rules of the lines.

Therefore, starting with a solution for V, it’s read as:

V = I * R

Further Reading

For a more in-depth discussion about Ohm’s law and resistance, check out Part 4 of the Electronics Tutorial.

Other Resources