Homework Help: Internal Resistance in a Battery

1. Nov 23, 2004

joshanders_84

Hey I am confused as to the answer to this question; not sure exactly how to figure out what the answer is because it is not a constant resistance. Any help appreciated:

The potential difference across the terminals of a battery is V_1 when there is a current of I_1 in the battery from the negative to the positive terminal. When the current is I_2 in the reverse direction, the potential difference becomes V_2. What is the internal resistance of the battery?

I tried adding and subtracting the two resistances (R_1 = V_1/I_1 and R_2 = V_2/I_2), but that isn't correct, and I'm not sure of what to do. Thanks
Josh

2. Nov 23, 2004

ceptimus

Call the real voltage of the battery (when it's delivering no current) V, and its internal resistance R.

In the first case V = V_1 + I_1 * R

(Here, some of the battery's real voltage is wasted overcoming the internal resistance, so the measured voltage V_1 is less).

In the second case V2 - V = I_2 * R

(Here, the current is in the opposite direction, so both the battery's real voltage and it's internal resistance oppose the applied current).

So you have a pair of equations and a pair of unknowns. Should be easy...

3. Nov 23, 2004

marlon

correct, i get the same answer...
marlon