First Condition When the current flows normally, the formula for the potential difference across the cell is: V=E− ir We are given that V=20V and i=2A. Plug these values into the formula: 20=E−2r Second Condition When the current direction is reversed, the formula for potential difference becomes: V=E+ir. Here, V=30V and i=2A, so: 30=E+2r Find the Internal Resistance Now, subtract equation (i) from equation (ii): ‌(30)−(20)=(E+2r)−(E−2r) ‌10=4r So, r=‌
