Ohm’s Law & Power

Voltage, current, resistance and power are linked by two simple laws. Fill in any two and get the other two — plus the energy use and running cost.

ElectricityCircuitsPower

Inputs

Click Use on exactly two values you know and fill them in. The other two are calculated.

Input examples

Voltage V

Current I

Resistance R

Power P

Electricity price — optional, for running cost

€ per kWh

What is Ohm’s Law?

Think of a wire as a water pipe: voltage is the pressure pushing the charge, current is how much flows per second, and resistance is how narrow the pipe is. Ohm’s law says the current is proportional to the voltage: double the voltage, double the current.

The power is the energy converted per second — into light, heat or motion. Combining P = VI with V = IR gives the full set of formulas, so any two quantities determine the other two.

V × 2current doubles

I × 2heat (I²R) × 4

1 kWh1 kW for 1 hour

R → 0short circuit

Live Graph

V–I

V = I·R for your resistance

curve of equal power

operating point

Result

Enter two values above

All Twelve Formulas

To find	Formulas

Worked Techniques

Current from Power

Appliance labels show power and voltage. Dividing them gives the current — useful for choosing the right fuse or cable.

Example:

LED Series Resistor

An LED needs a resistor to limit its current. The resistor takes the voltage the LED does not use.

Example:

Energy and Cost

Energy suppliers charge per kilowatt-hour. Multiply power by time of use to find the energy and the cost.

Example:

Sources & Verification

References

A. R. Hambley, Electrical Engineering: Principles and Applications, 7th ed., Pearson, 2018 — Sec. 1.3 (Power and Energy).
D. Halliday, R. Resnick & J. Walker, Fundamentals of Physics, 10th ed., Wiley, 2014 — Ch. 26 (Current and Resistance).