Assume two copper rods A and B are connected to each other at one end. Lengths of the rods ar equal.
Radii of cross section of A and B are 2 mm and 4 mm respectively. If the current density in A is `9xx10^5 A//m^2` then calculate the current density in B.

Explanation

To solve the problem, we need to find the current density in rod B (denoted as \( J_B \)) given the current density in rod A (\( J_A \)) and the dimensions of both rods.

### Step-by-step Solution:

1. **Understand the relationship between current densities and areas**:
Since rods A and B are connected, the current flowing through both rods must be the same. Therefore, we can use the relationship:
\[
I_A = I_B
...