# 2nd PUC Physics Question Bank Chapter 8 Electromagnetic Waves

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## Karnataka 2nd PUC Physics Question Bank Chapter 8 Electromagnetic Waves

### 2nd PUC Physics Electromagnetic Waves One Marks Questions and Answers

Question 1.
What field is generated by a time-varying electric field?
Answer:
Magnetic field.

Question 2.
Say whether time varying magnetic field generates electric field or not.
Answer:
Yes. Time varying magnetic field generates electric field. Question 3.
Who propounded the theory of electromagnetic waves? (March 2014)
Answer:
James Clerk Maxwell.

Question 4.
What led to the conclusion that light is an electromagnetic wave?
Answer:
The speed of electromagnetic wave is equal to 3 x 108 ms-1 which is the same experimental value of speed of light obtained from optical measurement. This fact led to the conclusion that light is an electromagnetic wave.

Question 5.
Give the mathematical form of Ampere’s circuital law.
Answer:
$$\oint \overrightarrow{\mathrm{B}} \cdot \overrightarrow{d l}$$ = µ0i(t)

Question 6.
Write the expression for displacement current or Maxwell’s displacement current.
Answer: Question 7.
Write the corrected form of Ampere’s circuital law.
Answer: Question 8.
What is the source of electromagnetic waves?
Answer:
An accelerating charged particle produces electromagnetic waves.
Note : It is interesting to note that varying electric field produces varying magnetic field and vice-versa. Question 9.
Who disproved the existence of luminiferous, all pervading, highly elastic and inertial ether?
Answer:
Michelson and Morley in the year 1887, disproved the presence of ether.

Question 10.
Express velocity of electromagnetic wave in a material medium in terms of µ and ε.
Answer:
v = $$\frac{1}{\sqrt{\mu \varepsilon}} m s^{-1}$$

Question 11.
Give the expression for velocity of electromagnetic waves in vacuum.
Answer:
c = $$\frac{1}{\sqrt{\mu_{0} \varepsilon_{0}}}$$
where, c = velocity of light in vacuum
ε0 = electrical permittivity for free space
µ0 = magnetic permeability for free space

Question 12.
Name a polarised electromagnetic wave.
Answer:
Radio wave.

Question 13.
Give the expression for the energy density of electric field.
Answer:
Electric field energy density = $$\frac { 1 }{ 2 }$$ε0E2

Question 14.
Give the expression for the energy density of magnetic field.
Answer:
Magnetic field energy density $$\frac{1}{2} \frac{B^{2}}{\mu_{0}}$$

Question 15.
Say whether electromagnetic waves transfer momentum and energy or not.
Answer:
Yes. Electromagnetic waves transfer momentum and energy. Question 16.
What is meant by radiation pressure?
Answer:
The force per unit area exerted by electromagnetic waves is called radiation pressure which is about 7 × 10-6 Nm-1

Question 17.
If ‘IP is the total energy transferred to the surface.by an electromagnetic wave then relate total momentum (p) of electromagnetic wave delivered to the surface in terms ‘IP.
Answer:
p = $$\frac{\mathrm{U}}{c}$$, where‘c’is the speed of light.

Question 18.
Why is that we do not feel the momentum of light and radiant heat from the sun?
Answer:
We note that momentum p = $$\frac{\mathrm{U}}{c}$$ and ‘c’ is very large, we do not feel the momentum inspite of the fact that our body becomes warm due to incident radiations from the sun.

Question 19.
Name the American scientists who succeeded in measuring the radiation pressure of visible light (1903).
Answer:
Nicols and Hull.

Question 20.
What is an electromagnetic wave?
Answer:
An electromagnetic wave consists of oscillating electric and magnetic field vectors produced by an accelerating charged particle.

Question 21.
Who discovered γ radiation?
Answer:
P.U. Willard discovered γ rays.

Question 22.
Who discovered X-rays?
Answer:
K.W. Roentgen discovered X rays. Question 23.
Who discovered UV rays?
Answer:
Ritter discovered UV rays.

Question 24.
Who discovered IR-radiation?
Answer:
W. Herschel discovered IR radiation.

Question 25.
What are microwaves?
Answer:
Microwaves are very short radiowaves whose frequency lies between 109 Hz and 1011 Hz.

Question 26.
What is the wavelength range of γ rays?
Answer:
Wavelength of γ rays varies from 0.01 Å to 0.1 Å.

Question 27.
What is the wavelength range of X- rays?
Answer:
Wavelength of X rays varies from 0 .1 Å to 100 Å.

Question 28.
What is the wavelength range of UV rays?
Answer:
Wavelength of UV rays varies from 100 Å to 4000 Å.

Question 29.
Give the frequency range of IR rays.
Answer:
Frequency of IR rays varies from 8 × 1010 to 4 × 1014 Hz.

Question 30.
What is the wavelength range of visible light?
Answer:
Wavelength of visible light varies from 4000 Å to 8000 Å.

Question 31.
What is the frequency range of radiowaves?
Answer:
Frequency of radiowaves varies from 8 × 1010 Hz to 3 × 104 Hz. Question 32.
Given below are some famous numbers associated with electromagnetic radiations in different contexts in physics. State the parts of the electromagnetic spectrum to which each belongs.
(i) 21 cm (Wavelength emitted by atomic hydrogen in interstellar space)
Answer:
Radio waves of short wavelength end.

(ii) 1057 MHz (frequency of radiation arising from the two close energy levels in hydrogen known as Lamb shift).
Answer:
Radio waves (Short wavelength end).

(iii) 2.7K (temperature associated with the isotropic radiation filling all space, thought to be relics of the Big-bang origin of the universe).
Answer:
Microwave.

(iv) 5890A – 5896A (double lines of sodium).
Answer:
Visible spectrum of yellow light.

(v) 14.4 keV (energy of a particular transition) in 57Fe nucleus associated with a famous high resolution spectroscopic method – Mossbauer spectroscopy)
Answer:
X-rays (or soft y-rays) range.

Question 33.
Give the range of wavelengths of electromagnetic waves. (March 2014)
Answer:
The range of wavelengths of electromagnetic waves ranges from 10-14 m for cosmic radiations to 107m for radio waves.

Question 34.
Mention any one Greenhouse gas.
Answer:
Carbondioxide.

### 2nd PUC Physics Electromagnetic Waves Two Marks Questions and Answers

Question 1.
Represent electric and magnetic fields of an electromagnetic wave by suitable wave equations.
Answer:
Ex = E0 sin(kz – ωt) and By = B0 sin (kz – ωt) where k = $$\frac{2 \pi}{\lambda}$$ rad m-1 is known as the propagation constant and $$\left(\frac{\omega}{k}\right)$$ = v, represents speed of propagation of the electro magnetic wave.  Question 2.
Explain the need for displacement current in Ampere’s circuital law.
Answer:
If we consider a parallel plate capacitor, the conduction current between the plates is zero and hence Ampere’s circuital law is not applicable. Therefore the concept of displacement current is introduced to explain the inconsistency in the circuital law.

Question 3.
Long-distance radio broadcasts use short wavebands. Why?
Answer:
The ionosphere reflects waves in these bands. Hence long-distance radio broadcasts use short wavebands.

Question 4.
It is necessary to use satellites for long-distance TV transmission. Why?
Answer:
Ionosphere does not reflect the high-frequency TV signals. In a way, the ionosphere will be transparent to TV signals. Thus satellites are used for TV transmission.

Question 5.
Optical and radio telescopes are built on the ground but X-ray astronomy is possible only from satellites orbiting the earth. Why?
Answer:
The atmosphere absorbs X-rays while visible and radio waves can penetrate the atmosphere. Therefore X-ray astronomy is possible only from satellites orbiting the earth.

Question 6.
The small ozone layer on top of the stratosphere is crucial for human survival. Why?
Answer:
Ozone absorbs higher wavelengths of UV radiation from the sun. Thus ozone prevents plants, animals, humans, and rest of the nature from being affected by UV rays. Exposure to UV radiation causes skin cancer in humans and low yields in plants.

Question 7.
If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now?
Answer:
The temperature of the earth would be lower because of the absence of green house effect.

Question 8.
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?
Answer:
The clouds produced by global war would cover substantial parts of the sky preventing solar light from reaching many parts of the globe. In the absence of sunshine it would cause a permanent winter. Question 9.
A plane electromagnetic wave travels in vacuum along Z-direction. What can you say about the direction of its electric and magnetic field vectors? If the frequency of the wave is 30 MHz, what is its wavelength?
Answer: Question 10.
State Ampere-Maxwell law. Express the law in the mathematical form.
Answer:
The total current passing through any surface of which the closed loop is the perimeter, is the sum of the conduction current (ic) and the displacement current (ij).
i = ic + ij
i.e., μ = μ + μ Question 11.
Based on the symmetrical nature of electricity and magnetism, give Maxwell’s form of Faraday’s law.
Answer:
From Faraday’s law of E.M.I, e = $$-\frac{d \phi_{\mathrm{B}}}{d t}$$ Question 12.
Give any two examples for electromagnetic waves.
Answer:
Polarised radio and T.V. signal waves and Light waves. Question 13.
Write any two characteristic properties of electromagnetic waves.
Answer:

• Electromagnetic waves are produced by accelerated charged particles.
• At any point in vacuum, the electric and the magnetic field vectors are mutually at right angles to each other and have the same variation.
Note : The other properties are:
• The ratio $$\frac{E_{0}}{B_{0}}$$ = c where‘c’is speed of electromagnetic wave.
• The entire spectrum of radiation belonging to e.m.w travels with the same speed in vacuum and is equal to the speed of light.
• In a material medium, the speed of e.m wave will be ν = $$\frac{1}{\sqrt{\mu \varepsilon}}$$ where ‘μ’ is absolute magnetic permeability of the medium and ‘ε’ is absolute electrical permittivity of the medium.
• As the electromagnetic wave propagates through a material medium, there will be changes in the electric and magnetic field intensities.
• Electromagnetic waves transport momentum and energy as they travel through space
and exert radiation pressure on the surface of the medium. ‘
• It is the electric vector which plays the role of light vector.
• Electromagnetic waves exhibit the properties such as rectilinear propagation, reflection, refraction, interference, diffraction and polarisation.

Question 14.
What are radio waves? Give the frequency range of the radio waves.
Answer:
Electromagnetic waves of wavelength longer than lm are called radio waves.
Range of frequencies : 3kHz to 1000 MHz
Note: AM band : 530 kHz to 1.71 MHz
Short wave band : Upto 54 MHz
FM band : 88 MHz to 108 MHz
TV: 54 MHz to 890 MHz
Cellular phones : 840 MHz to 935 MHz Question 15.
Mention any two methods of producing microwaves?
Answer:
Microwaves are produced by Klystrons, magnetrons and gunn diodes (which are special vacuum tubes).

Question 16.
Give any two uses of microwaves. (July 2014)
Answer:
Microwaves are used in aircraft navigation, detection of speeds of fast balls, tennis serves and automobiles. Microwaves are used in micro oven in which water molecules resonate resulting in the raise of temperature of food containing water.

Question 17.
Mention any two sources for IR waves.
Answer:
Pointolite lamps, Globar and Nernst filament produce IR waves. IR waves are produced by hot bodies and molecules.

Question 18.
Mention any two uses of IR waves. (March 2015)
Answer:

1. IR waves help in maintaining warmth of the earth at night.
2. IR waves are used in light photography, remote controls of TV, VCD & High-fidelity systems.
The other uses are: IR waves are used in molecular spectroscopy, identifying forgery in the documents and help in local heating of affected human tissues.

Question 19.
Give any two sources for the production of UV rays.
Answer:
UV rays are produced by electric arc lamps, oxy-acetylene gas and vapours of metals under low pressure.

Question 20.
Mention any two applications of UV rays.
Answer:

1. Highly focussed UV radiations are used in (LASIK – Laser Assisted in Situ Keratomileusis)
2. UV rays are used in sterilization of air, water, surgical instruments.
The other uses are: UV rays are used in molecular spectroscopy, identification of artificial gems from real gems.
Note: Ordinary glass absorbs UV radiations.

Question 21.
Give any two uses of X-rays.
Answer:

1. X-rays are used in treatment of cancer.
2. X-rays are used*in computer tomography scanning. Question 22.
A radio can tune into any station in the 7.5 MHz to 12 MHz band. What is the corresponding wavelength band?
Answer:
We know that wave length, λ = $$\frac{c}{v}$$ The band of wavelengths will be 25 m to 40m.

Question 23.
About 5% of the power of a 100W light bulb is converted to visible radiation. What is the average intensity of visible radiation.
(i) at a distance 1 m from the bulb?
(ii) at a distance 10m?
Answer:  ### 2nd PUC Physics Electromagnetic Waves List of Formulae   