Ohm's Law and Power

Introduction

Voltage, current and resistance are linked by one beautifully simple rule discovered by Georg Ohm. With it, you can predict what a circuit will do before you build it.

Ohm's law

Voltage equals current times resistance.

V = I × R

Here V is voltage in volts, I is current in amps, and R is resistance in ohms. Rearranged, you can find any one of them from the other two:

The Ohm's law triangle. Cover the quantity you want to find, and the other two show how to calculate it.

Worked examples

Find the current. A 9 V battery is connected across a 300 Ω resistor.

I = V ÷ R = 9 ÷ 300 = 0.03 A  (30 mA)

Find the voltage. A current of 0.5 A flows through a 4 Ω resistor.

V = I × R = 0.5 × 4 = 2 V

Find the resistance. A 12 V supply drives 0.2 A through a load.

R = V ÷ I = 12 ÷ 0.2 = 60 Ω

Prefixes

Electronics uses small and large numbers, so prefixes help:

Always convert to amps, volts and ohms before you calculate.

Power

Power is how fast energy is used, measured in watts (W).

P = V × I

Combining it with Ohm's law gives P = I² × R and P = V² ÷ R.

Example. A 12 V lamp draws 2 A.

P = 12 × 2 = 24 W

Why power matters

Resistance turns electrical energy into heat. A resistor has a power rating, often 0.25 W (a quarter watt) for small parts. If the power in it is higher than its rating, it gets very hot and can fail or even cause a fire.

For the 9 V and 300 Ω example above, power is 9 × 0.03 = 0.27 W. That is slightly over a quarter-watt resistor's rating, so a larger rated resistor would be needed. Always check.

Try it yourself

A 5 V supply is connected across a 250 Ω resistor. Find the current in amps and milliamps, and then the power. Is a 0.25 W resistor safe?

Summary

• Ohm's law: V = I × R, which can be rearranged to find any one value.
• Power is P = V × I, measured in watts.
• Check a resistor's power rating, because too much power makes it overheat.