If we use Thevenin's Theorem for understanding this theorem then we will have Vth, Rth, and Rl at the end of Thevenin's Theorem. Statement: In a linear time-invariant network, if the load resistance Rl carrying current i is changed by delta R (dR). Then the current in all branches will change and the new current from new load resistance is obtained by attaching a new ideal voltage source Vc (i.e Vc=i.dR) in series with the new load resistance branch, and by replacing Vth by its internal resistance. Explanation: The formula of load current in Thevenin's Theorem is given by
[i = (Vth) / (Rth + Rl)]. Now if Rl is changed into Rl+dR, then the formula for new current would be [i' = (Vth) / (Rth + Rl + dR)]. And the difference in current is given by [di = i' - i]. Now, put the values of i' and i in the equation of di. So now we will get
[di = (-i.dR) / (Rth + Rl + dR)]. And as we know that Vc=i.dR, so the final equation will be
[dI = (-Vc) / (Rth + Rl + dR)].
And Vth will be replaced by its internal resistance.
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