Ohm's law (named after the German physicist Georg Ohm) defines the relationship between Voltage, Current and Resistance.

*E = I x R*

Where:

- E is the electrical potential (voltage), measured in volts (V),
- I is the current, measured in Amperes (Amps/A), and
- R is the resistance, measured in Ohms (Ω).

Joule's law states that:

*P = E x I*

where:

- P is Power, measured in Watts.

The combination of Ohm's law and Joule's law gives us 12 formulas where 2 of the 4 variables are known. The wheel below is a handy tool and memory jogger. To use it, simply choose the quadrant corresponding to the variable you want to calculate, then select the segment corresponding to the variables that you know the values of.