Lesson : Ohm’s Law

Ohm’s law is generally one of the first things that everyone learns in electronics. Ohm’s law is the mathematical relationship between the current, resistance and voltage in a circuit. It is one of the equation that you going to use the most when designing or studying circuits. Ohm’s law is actually a pretty simple mathematical formula. Ohm’s law states that the voltage across a resistor is proportional to the current flowing through it.

1) \ V = R\cdot I

where :

  • V is the voltage across the resistor in Volt, (V)
  • R is the resistance in Ohms, (Ω)
  • I is the electric current flowing through the resistor in ampere, (A)

We can manipulate the equation above to find the current in a resistor if we have the voltage across it.

2) \ I = \cfrac{V}{R}

Finally, we can manipulate the first equation again to find the resistance if we have the voltage across a resistor and the current flowing through it.

3) \ R = \cfrac{V}{I}

Below you will find a couple of very simple example on how to use Ohm’s law.


Example 1: We want to calculate the current flowing into the resistor.

Example 1 : Ohm’s Law

We need to use the second equation since we want to calculate the current flowing through the resistor and we have the value of the voltage across the resistor and the resistance of the resistor.

I = \cfrac{V}{R} = \cfrac{12V}{50\Omega} = 0.24A


Example 2: We want to calculate the value of the resistor in this circuit.

Example 2 : Ohm’s Law
  • Note: the m before the A means milli. Example: 1 milli = 0.001.

We need to use the third equation since we want to calculate the value of the resistor and we have the value of the voltage across the resistor and the current flowing through it.

R = \cfrac{V}{I} = \cfrac{12V}{2.5mA} = 4800\Omega

The examples above are simple since there is only one resistor in the circuit. Circuit can have multiple resistances and solving circuits with multiple resistors is a bit more complicated. If you want to continue learning electronics, I suggests reading about the power law, series circuits and parallel circuits.