Ohm's Law

A German physicist named Georg Simon Ohm formulated a law stating the relationship of the electric potential to electric current. His law which is known as the Ohm's Law states that

"The potential difference between any two points in a conductor

varies directly as the current between the two points."

This law may be expressed as \(\frac VI = \text{constant}\).This constant depends on the properties of the resistor used, thus the equation can also be written as

\(\frac VI=R\)


\(V=\text{electric potential (Volts, V)}\\ I=\text{electric current (Ampere, A)}\\ R=\text{resistance (Ohm,}\;\Omega)\)

Example 1.

An electric fan is connected to a 120 V outlet and draws a current of 8 A. Calculate for the resistance of the fan.


\(V=120\;V\\ I=8\;A\)


\(R=\frac VI=\frac {120\;V}{8\;A}=15\;\Omega\)

Example 2.

How much current will an electric iron receive from a 120 V supply if its resistance is \(10\;\Omega\)?


\(V=120\;V\\ R=10\;\Omega\)


From the formula, \(R= \frac VI\), we derive the formula for electric current as

\(I=\frac VR=\frac {120\;V}{10\;\Omega}=12\;A\)

thus, the electric iron draws a current of 12 A from the 120-V power supply.

Example 3.

Solve for the electric potential difference across an electric toaster if its resistance is \(8.5\;\Omega\) and current of 20 A.


\(R=8.5\;\Omega\\ I=20\;A\)


The eqution for electric potential is expressed as


therefore, the toaster needed a 170-V supply for the current to move pass through it.