The relationship between current(I), voltage(V or E), and resistance(R) was discovered by a German scientist named Georg Ohm. This relationship is named Ohms Law in his honor. Ohm found that the current in a circuit varies directly with the voltage when the resistance is constant. Ohm varied the voltage across the resistance and measured the current through it. In each case, when he divided voltage by the current, the result was the same. In short, Ohms Law, which can be stated as, "The current is directly proportional to the voltage and inversly proportional to the resistance".
Written as a mathematical expression, Ohms Law is
Resistance(R)=voltage(V)/current(I) or R=V/I
The above equation allows you to determine the value of the resistance when the voltage and the current are known.
Of course, Ohms Law can be rearranged to solve for either current or voltage. The rearranged relationships are
current(I)=voltage(V)/resistance(R) or I=V/R
voltage(V)=current(I)*resistance(R) or V=IR
Here is a list of equations for power, voltage, current, and resistance
Power(W)= EI, I(squared)*R, E(squared)/R
Voltage(V)= IR, P/I, (sqrt)PR
Current(I)=E/R, P/E, (sqrt)P/R
Resistance(R)= E/I, E(squared)/P, P/I(squared)