5 Marks Questions

Question 15 Marks

The intensity of the sunlight reaching Earth is 1380Wm^-2. Assume this light to be a plane, monochromatic wave. Find the amplitudes of electric and magnetic fields in this wave.

Answer

Intensity of wave $=\frac{1}{2}\in_0\text{E}^2_0\text{C}$

$\in_0=8.85\times10^{-12},\text{ E}_0=?,\text{ C}=3\times10^{8},\text{ I}=1380\text{W/m}^2$

$1380=\frac{1}{2\times8.85\times10^{-12}\times\text{E}^2_0\times3\times10^8}$

$\text{E}^2_0=\frac{2\times1380}{8.85\times3\times10^{-4}}=103.95\times10^4$

$\text{E}_0=10.195\times10^{2}=1.02\times10^3$

$\text{E}_0=\text{B}_0\text{C}$

$\text{B}_0=\frac{\text{E}_0}{\text{C}}$

$\text{B}_0=\frac{1.02\times10^3}{3\times10^8}$

$\text{B}_0=3.398\times10^{-5}$

$\text{B}_0=3.4\times10^{-5}\text{T}$

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Question 25 Marks

A plane electromagnetic wave is passing through a region. Consider (a) electric field (b) magnetic field (c) electrical energy in a small volume and (d) magnetic energy in a small volume. Construct the pairs of the quantities that oscillate with equal frequencies.

Answer

Let the electromagnetic wave be propagating in the z-direction. The vibrations of the electric and magnetic fields are given by,

$\text{E}_\text{x}=\text{E}_0\sin(\text{kz}-\omega\text{t})$

$\text{B}_\text{y}=\text{B}_0\sin(\text{kz}-\omega\text{t})$

Let the volume of the region be V.

The angular frequency of the vibrations of the electric and magnetic fields are same and are equal to $\omega$

Therefore, their frequency, $\text{f}=\frac{\omega}{2\pi},$ is same.

The electrical energy in the region,

$\text{U}_\text{E}=\Big(\frac{1}{2}\in_0\text{E}^2\Big)\times\text{V}$

It can be written as,

$\text{U}_\text{E}=\Big(\frac{1}{2}\in_0\big(\text{E}^2_0\sin^2(\text{kz}-\omega\text{t})\big)\Big)\times\text{V}$

$\text{U}_\text{E}=\Bigg(\frac{1}{2}\in_0\text{E}_0^2\times\frac{\big(1-\cos2(\text{kz}-\omega\text{t})\big)}{2}\Bigg)\times\text{V}$

$\text{U}_\text{E}=\Big(\frac{1}{4}\in_0\text{E}_0^2\times(1-\cos2(\text{kz}-\omega\text{t}))\Big)\times\text{V}$

The magnetic energy in the region,

$\text{U}_\text{B}=\Big(\frac{\text{B}^2}{2\mu_0}\Big)\times\text{V}$

$\text{U}_\text{B}=\bigg(\frac{\text{B}^2_0\sin^2(\text{kz}-\omega\text{t})}{2\mu_0}\bigg)\times\text{V}$

$\text{U}_\text{B}=\Bigg(\frac{\text{B}^2_0\big(1-\cos(2\text{kz}-2\omega\text{t})\big)}{4\mu_0}\Bigg)\times\text{V}$

The angular frequency of the electric and magnetic is same and is equal to $2\omega$

Therefore, their frequency,

$\text{f}'=\frac{2\omega}{2\pi}=2\text{f}$

Will be same.

Thus, the electric and magnetic fields have same frequencies and the electrical and magnetic energies will have same frequencies.

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