Resistance (R) is the property of a material to oppose the flow of electric current. You can calculate it using three equations, depending on which quantities you know:
Power rating is the maximum power a resistor can safely dissipate without being damaged. The actual power dissipated is calculated using:
Always choose a resistor with a power rating at least twice the calculated dissipation for safety.
Exceeding a resistor's wattage can cause failure.
A circuit has a voltage of 12 V and a current of 0.02 A. What is the resistance?
Solution: R = 12 / 0.02 = 600 Ω
Power dissipated: P = 12 × 0.02 = 0.24 W
A resistor has 10 V across it and dissipates 2 W. What is the resistance?
Solution: R = 10² / 2 = 100 / 2 = 50 Ω
Power dissipated: P = 2 W (given)
A resistor dissipates 5 W with a current of 2 A. What is the resistance?
Solution: R = 5 / (2²) = 5 / 4 = 1.25 Ω
Power dissipated: P = 5 W (given)