A metallic wire A of radius 1 mm is joined at one end with another metallic wire B of radius 2 mm .
when a battery is connected between two free ends of this composite wire, a currents of 20 A flows through it.
Calculate the currents density in wire A and also calculate the drift speed of electrons in wire B , if material of B has `9xx 10^(29)` free electrons per unit volume.

Explanation

To solve the problem step-by-step, we will calculate the current density in wire A and the drift speed of electrons in wire B.

### Step 1: Calculate the Current Density in Wire A

**Given:**
- Radius of wire A, \( r_A = 1 \, \text{mm} = 1 \times 10^{-3} \, \text{m} \)
- Current through the composite wire, \( I = 20 \, \text{A} \)

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