The voltage drop across a resistor is equal the product of the current passing through it by the resistance.

**Formula 1 **

V = R x I

**Where: **

R is the resistance of the resistor in ohm (Ω)

V is the voltage drop in volts (V)

I is the corrent flowing through the resistor in amperes (A)

**Derivated Formulas **

**Formula 2 **

I = V/R

**Formula 3 **

R = V x I

**Application Example **

Calculate the current flowing through a 100-ohm resistor connected to a 12 V power supply.

Data:

R = 100 Ω

V = 12 V

I = ?

**Using formula 2:**

I = V/R = 12/100 = 0,12 = 120 mA

**The Magic circle of Ohm´s Law**

Putting the three quantities involved in Ohm´s Law in a circle, as shown by the figure Bellow, voltage, current and resistance we can determine the formula to find the value of the quantity in a problem.

You only have to put your finger on the quantity you want to know. The other quantities will be placed in the position corresponding to the formula.

For instance, IF we put our finger on the current (I), by Ohm´s Law you may divide V by R.