Ohm’s law Formulas

Ohm’s law formula helps to calculate voltage, current and resistance. By lumping Ohm’s law with joules law, one can easily achieve the formula for power. Let’s take a look at formulas:

Voltage calculation formula

When current and resistance are given use V = IR to calculate voltage.

Example #1: Find the voltage applied across 10 kΩ resistors when  5 mA current flows through it

Solution: V = 10 kΩ * 5 mA = 50 V

When current and power are given use V = P/I to find the volts.

Example #2: Find the voltage applied across an unknown resistor which dissipates 20 watts power while  5 A current is flowing through it.

Solution: V = 20 watts / 5 A = 4 V

When power and resistance are given use V = √P*R to find the volts.

Example #3: Find the voltage applied across the 200 Ω resistor which dissipates 20 watts power.

Solution: V = √200Ω * 20 W = 63.24 V

Current calculation formula

When voltage and resistance are given use R = P/I to calculate current.

Example #4: Find the current flowing through a 2.5 MΩ resistor when a potential difference of 20 V is applied across it.

Solution: I = V/R = 20 V / 2.5 MΩ = 8 μA

Here is another example of current calculation through a 47 ohms resistor.

When power and voltage are given use I = P/V to calculate the amperes.

Example #5: Find the current flowing through an unknown resistor which dissipates 20 watts power while dropping 10 volts across it.

Solution: I = P/V = 20 watts / 10 volts = 2 A

When power and resistance are known use I = √P/R to calculate the amperes.

Example #6: A 15 kΩ resistor dissipates 5 watts power. Find the value of current flowing through it.

Solution: I = √P/R = √5/15 = 0.577 mA

Resistance calculation formula

When power and current are known use R = P/I² to calculate the resistance.

Example #7: Find the value of unknown resistor which dissipates 30 watts when 5 mA current flows through it.

Solution: R = 30 / (5mA)²  = 1.2 MΩ

When voltage and current are known use R = V/I to calculate the value of the resistor.

Example #8: Find the value of unknown resistor which drops 10 V when 25 mA current is flowing through it.

Solution: R = 10 V / 25mA  = 400 Ω

When voltage and power are known use the formula R = V²/P for finding the value of the unknown resistor.

Example #9: Find the value of unknown resistor which drops 10 V while dissipating 200 watts.

Solution: R = 10 V / 25mA  = 400 Ω

Power calculation formula

When voltage and current are known use P = VI to calculate the value of the power.

Example #10: The application of 10 V potential drop across a resistor results in 20 mA current flowing through it. Find the power dissipated across it.

Solution: P = VI = 10 V. 20 mA = 0.2 W

When current and resistance are known use P = I²R to calculate the value of the power.

Example #11: 50 mA current is flowing through a 10 kΩ resistor. Find the power dissipated across it.

Solution: P = (50 mA)²*10 kΩ = 25 W

When voltage and resistance are known use P = V²/R to calculate the value of the power.

Example #12: A potential difference of 20 V is applied across 10 kΩ resistors. Find the dissipated power.

Solution: P = (20)²/ 10 kΩ = 40 mW