Electric current

What Is Ohm's Law? Definition, Formula and Example

What is Ohm's Law? Definition, formula and example

Ohm's law is a mathematical formula that describes the correlation of electrical parameters (resistance, current, voltage) with which they vary.

The name of the law is due to the German physicist Georg Ohm.

Definition of Ohm's Law

Ohm's law is a law in electricity that states that: "the force of a direct current is directly proportional to the potential difference and inversely proportional to the resistance of the circuit."

Ohm's Law Formula

In mathematical terms, Ohm's law is applied by the equation:

Ohm's law formula


  • R is the electrical resistance. By resistance we mean the obstacle that the current encounters in its path, the higher it is, the more difficult it will be for the current to cross. The unit of measurement for resistance is ohms, symbolized by the Greek letter omega (Ω).

  • I is the intensity of an electric current that passes through a conductor expelled in Amperes (A).

  • V is the voltage. By voltage instead we refer to the potential difference between one point with respect to another expressed in Volts (V).

The current intensity of electrical loads is directly proportional to the applied voltage and inversely proportional to the resistance.

For the voltage, on the other hand, the higher it is, the greater the attractive force it generates to move the charges, so for the same resistive value it will be directly proportional to the current.

What Is Ohm's Law Triangle?

Ohm's law triangle is a trick to remember the formula.

Ohm's law triangle

To obtain the triangle formula we have to cover the variable we want to obtain with our hand. If the two remaining elements are one on top of the other they are divided, if they remain in line they are multiplied.

The three possible combinations are:

  • I = V / R

  • V = IR

  • R = V / I

Ohm's Law and Electrical Power

Sometimes the formula for electrical power is used by applying Ohm's law.

The power formula is as follows (with unit of watts):

P = VI, (power = voltage x current)

and its variants: V = P / I and I = P / V,

The two variants can be substituted in Ohm's law formula. For example, if we start from the formula to calculate the voltage and we substitute we have the following formula:

V = (P / V) R

Isolating the power we have the following form:

V² R = P

Ohm's Law Example

To better understand Ohm's law we will turn to a simple hydraulic example:

Let's imagine a hose connected to the tank of a fire truck.

Example of Ohm's law with a hydraulic simileIn this example, the elements equivalent to an electrical circuit are the following:

  • Voltage (V). The voltage equals the power of the truck's water pump. The water pump “applies a voltage” to the circuit.

  • Resistance (R). The resistance of the electrical circuit is equivalent to the resistance offered by the hose (diameter). If the hose has a large diameter, the water will flow more easily than if the diameter is small. In the example of the electrical circuit, the concept of resistance is equivalent but is expressed in Ohms (Ω).

  • Current intensity (I). In our example, intensity is the number of water molecules that flow through a section of hose per unit of time. In an electrical circuit, the equivalent of molecules are electrical charges.

In this simile, it is easy to understand that if we increase the capacity of the pump (voltage) the water flow (intensity) will increase. In the same way, if we use a smaller hose (greater resistance), the flow will also decrease.


Published: September 13, 2021
Last review: September 13, 2021