OHM's Law

Quick Reference

Ohm's Law Calculations

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

Power Triangle

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:

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

• Ohm's Law article on Wikipedia
• Resistors
• Ohm's Law Calculator