NCERT Class 12 Part 1 - Physics

Peyyeti Rajakumar, Shiv Kumar, Shveta Uppal

Chapter 8

Electromagnetic Waves - all with Video Answers

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Chapter Questions

Problem 1

Figure $8.6$ shows a capacitor made of two circular plates each of radius $12 \mathrm{~cm}$, and separated by $5.0 \mathrm{~cm} .$ The capacitor is being charged by an external source (not shown in the figure). The charging current is constant and equal to $0.15 \mathrm{~A}$.
(a) Calculate the capacitance and the rate of change of potential difference between the plates.
(b) Obtain the displacement current across the plates.
(c) Is Kirchhoff's first rule (junction rule) valid at each plate of the capacitor? Explain.

Ajay Singhal

Ajay Singhal

Numerade Educator

Problem 2

A parallel plate capacitor (Fig. 8.7) made of circular plates each of radius $R=6.0 \mathrm{~cm}$ has a capacitance $C=100 \mathrm{pF}$. The capacitor is connected to a $230 \mathrm{~V}$ ac supply with a (angular) frequency of $300 \mathrm{rad} \mathrm{s}^{-1}$.
(a) What is the rms value of the conduction current?
(b) Is the conduction current equal to the displacement current?
(c) Determine the amplitude of $\mathbf{B}$ at a point $3.0 \mathrm{~cm}$ from the axis between the plates.

Ajay Singhal

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Problem 3

What physical quantity is the same for X-rays of wavelength $10^{-10} \mathrm{~m}$, red light of wavelength $6800 \AA$ and radiowaves of wavelength $500 \mathrm{~m} ?$

Ajay Singhal

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Problem 4

A plane electromagnetic wave travels in vacuum along $z$ -direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave is $30 \mathrm{MHz}$, what is its wavelength?

Ajay Singhal

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Problem 5

A radio can tune in to any station in the $7.5 \mathrm{MHz}$ to $12 \mathrm{MHz}$ band. What is the corresponding wavelength band?

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Problem 6

A charged particle oscillates about its mean equilibrium position with a frequency of $10^{9} \mathrm{~Hz}$. What is the frequency of the electromagnetic waves produced by the oscillator?

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Problem 7

The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is $B_{0}=510 \mathrm{nT}$. What is the amplitude of the electric field part of the wave?

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Problem 8

Suppose that the electric field amplitude of an electromagnetic wave is $E_{0}=120 \mathrm{~N} / \mathrm{C}$ and that its frequency is $v=50.0 \mathrm{MHz}$. (a) Determine, $B_{0}, \omega, k$, and $\lambda$. (b) Find expressions for $\mathbf{E}$ and $\mathbf{B}$.

Ajay Singhal

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Problem 9

The terminology of different parts of the electromagnetic spectrum is given in the text. Use the formula $E=h v$ (for energy of a quantum of radiation: photon) and obtain the photon energy in units of $\mathrm{eV}$ for different parts of the electromagnetic spectrum. In what way are the different scales of photon energies that you obtain related to the sources of electromagnetic radiation?

Ajay Singhal

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Problem 10

In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of $2.0 \times 10^{10} \mathrm{~Hz}$ and amplitude $48 \mathrm{~V} \mathrm{~m}^{-1}$.
(a) What is the wavelength of the wave?
(b) What is the amplitude of the oscillating magnetic field?
(c) Show that the average energy density of the $\mathbf{E}$ field equals the average energy density of the $\mathbf{B}$ field. $\left[c=3 \times 10^{8} \mathrm{~m} \mathrm{~s}^{-1}\right.$.]

Ajay Singhal

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Problem 11

Suppose that the electric field part of an electromagnetic wave in vacuum is $\mathbf{E}=\left\{(3.1 \mathrm{~N} / \mathrm{C}) \cos \left[(1.8 \mathrm{rad} / \mathrm{m}) y+\left(5.4 \times 10^{6} \mathrm{rad} / \mathrm{s}\right) t\right]\right\} \hat{\mathbf{i}}$.
(a) What is the direction of propagation?
(b) What is the wavelength $\lambda$ ?
(c) What is the frequency $v ?$
(d) What is the amplitude of the magnetic field part of the wave?
(e) Write an expression for the magnetic field part of the wave.

Ajay Singhal

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Problem 12

About $5 \%$ of the power of a $100 \mathrm{~W}$ light bulb is converted to visible radiation. What is the average intensity of visible radiation
(a) at a distance of $1 \mathrm{~m}$ from the bulb?
(b) at a distance of $10 \mathrm{~m}$ ? Assume that the radiation is emitted isotropically and neglect reflection.

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Problem 13

Use the formula $\lambda_{m} T=0.29 \mathrm{~cm} \mathrm{~K}$ to obtain the characteristic temperature ranges for different parts of the electromagnetic spectrum. What do the numbers that you obtain tell you?

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Problem 14

Given below are some famous numbers associated with electromagnetic radiations in different contexts in physics. State the part of the electromagnetic spectrum to which each belongs.
(a) $21 \mathrm{~cm}$ (wavelength emitted by atomic hydrogen in interstellar space).
(b) $1057 \mathrm{MHz}$ (frequency of radiation arising from two close energy levels in hydrogen; known as Lamb shift).
(c) $2.7 \mathrm{~K}$ [temperature associated with the isotropic radiation filling all space-thought to be a relic of the 'big-bang' origin of the universe].
(d) $5890 \AA$ \AÅ $-5896$ \AÅ [double lines of sodium]
(e) $14.4 \mathrm{keV}$ [energy of a particular transition in ${ }^{57} \mathrm{Fe}$ nucleus associated with a famous high resolution spectroscopic method (Mössbauer spectroscopy)].

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Problem 15

Answer the following questions:
(a) Long distance radio broadcasts use short-wave bands. Why?
(b) It is necessary to use satellites for long distance TV transmission. Why?
(c) Optical and radiotelescopes are built on the ground but X-ray astronomy is possible only from satellites orbiting the earth. Why?
(d) The small ozone layer on top of the stratosphere is crucial for human survival. Why?
(e) If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now?
(f) Some scientists have predicted that a global nuclear war on the earth would be followed by a severe 'nuclear winter' with a devastating effect on life on earth. What might be the basis of this prediction?

Mirza Aslam Beig

Mirza Aslam Beig

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