Formula for Series
Using V = I*R, we see that in series the current must move through both resistors.
(Think of water flowing down two water falls in series.) Thus Itotal = I1 = I2 .
Also, the voltage drop across the two resistors add to give the total voltage drop:
Vtotal = V1 + V2 . Thus, Reff = Vtotal / Itotal =
(V1 + V2)/Itotal, or V1/I1 + V2/I2 =