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Ohm's law is one of the most useful electrical calculations you can ever know. The law states basically that the current through a conductor between two points is directly proportional to the potential difference (voltage drop) across the two points, and inversely proportional to the resistance between them.

The mathematical equation is I = E / R

Where I = Current in amps, E = Voltage in volts and R = Resistance in ohms

It is just a matter of transposing this equation to calculate the value of the other two known values.

I = E / R

E = I x R

R = E / I