Combination of Batteries: Practice Problem & Solution

Solution Explained:

To solve this problem, we apply the core principles of Combination of Batteries. Understanding the underlying formula is key to arriving at the correct answer below:

Using the relation $I = \frac{E}{R + r}$, we substitute the given values: $0.2 = \frac{2.1}{10 + r}$. Solving for $r$, we get $10 + r = \frac{2.1}{0.2} = 10.5$, which yields $r = 0.5 \Omega$.