Consider two resistors R1 and R2 are connected in series. the charge which moves out from resistor R1
be entering R2. so the same value of current flows through R1 and R2.
potential difference across R1 =V1 =IR1 -------(i)
potential difference across R2 = V2 = IR2 (ii)
the potential difference across the combination V = V1+V2.-----(iii)
By putting the values of (i), (ii) in equations (iii) we get
IReq =IR1 +IR2 = I(R1+R2)
Req = R1 +R2-----------------(iV)
consider now the parallel combination of two resistors. the charge that flows in A at from the left
flows out partly through R1 and partly flow through R2. so the value of currents are the rates of flow of charge at the point I = I1+I2
the potential difference between A and B is given by ohm's law applied to R1
V =I1R1,
I1 = V/R1
I2= V/R2
let Req be the equivalent resistance of combination