1 Marks Question

Question 511 Mark

Identify the part of the electromagnetic spectrum to which the following wavelengths belong:

1mm
$10^{-11}m$

Answer

Wavelength 1mm belongs to the microwaves.
Wave length $10^{-11}m= 0.1A$ belongs to gamma rays.

Question 521 Mark

A plane electromagnetic wave of frequency $25 MHz$ travels in free space along the $x$-direction. At a particular point in space and time, $E =6.3 \hat{ j } V / m$. What is $B$ at this point?

Answer

Using Eq. (8.10), the magnitude of $B$ is
$
\begin{aligned}
B & =\frac{E}{c} \\
& =\frac{6.3 V / m }{3 \times 10^8 m / s }=2.1 \times 10^{-8} T
\end{aligned}
$
To find the direction, we note that $E$ is along $y$-direction and the wave propagates along $x$-axis. Therefore, B should be in a direction perpendicular to both $x$ - and $y$-axes. Using vector algebra, $E \times B$ should be along $x$-direction. Since, $(+\hat{ j }) \times(+\hat{ k })=\hat{ i }, B$ is along the $z$-direction. Thus, $\quad B =2.1 \times 10^{-8} k T$

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Physics 1 Marks Question