91Ó°ÊÓ

The distance covered by the charge,$L=30\text{m}$

At ,$t=0$ a force act on charge for the $1\text{ns}$.

The expression to calculate the time for radiation pulse to reach at point$A$ is given as follows.

role="math" localid="1668588905933" $\mathit{t}\mathbf{=}\frac{\mathbf{L}}{\mathbf{c}}$ …(¾±)

Here, c is the speed of the light.

Calculate the time to reach at the point A.

Substitute $3×{10}^{8}m/s$for $\mathrm{c}$cand$30m$for L into equation (i).

$$\begin{gathered} t=\frac{30 \text{m} }{3×{10}^{8}m/s} \\ =10×{10}^{-8} \text{s} \\ =100×{10}^{-9} \text{s} \\ =100\text{ns} \end{gathered}$$

Let assume observer is at position $A$. The propagation of wave, direction of magnetic field and electric field are shown below.

The direction of acceleration from $t=0\text{nsto}t=1\text{ns}$is downward and the direction of radiative field from $100\text{nsto}101\text{ns}$upward. Since the direction of the propagation is $v=E×B$the right therefore the direction of the magnetic field is out of the page.

From role="math" localid="1668589871682" $101\text{nsto}102\text{ns}$the radiative field is zero due to zero acceleration. From $t=102\text{nsto}103\text{ns}$, there is radiative electric field in the downward direction and propagation direction is $v=E×B$. Therefore, the direction of the magnetic field is out of the page.