(i) State Biot-Savart.s law.
(ii) Find magnetic flux density at a point on the axis of a long solenoid having 5000 turns m when it carrying a current of 2 A.

Explanation

### Step-by-Step Solution

**(i) State Biot-Savart's Law:**

Biot-Savart's Law describes the magnetic field generated by a current-carrying conductor. It states that the magnetic field \( \mathbf{B} \) at a point in space due to a small segment of current-carrying wire is directly proportional to the current \( I \), the length of the current segment \( d\mathbf{l} \), and the sine of the angle \( \theta \) between the current element and the line connecting the current element to the point where the magnetic field is being calculated. The law can be mathematically expressed as:

\[
d\mathbf{B} = \frac{\mu_0}{4\pi} \frac{I \, d\mathbf{l} \times \mathbf{r}}{r^3}
...