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Physics M.C.Q (1 Marks)

Electromagnetic Waves · MP Board (MPBSE) STD 12 Science (Madhya Pradesh - English Medium). 168 questions.

Questions · Page 4 of 4

M.C.Q (1 Marks)

MCQ 1511 Mark
The amplitude of the sinusoidally oscillating electric field of a plane wave is $60v/m$. Then the amplitude of the magnetic field is:
  • A
    $12 \times 10^{-7} T$
  • B
    $6 \times 10^{-7} T$
  • C
    $6 \times 10^{-8} T$
  • ✓
    $2 \times 10^{-7} T$
Answer
Correct option: D.
$2 \times 10^{-7} T$

Given: Electric field $=60 v / m$
$C =3 \times 10^8 m / s ^2$
To find: Magnetic field
Solution:
We know, $C =\frac{ E _0}{B_0}$
Therefore, $B _0=\frac{ E _0}{ C }$
$\frac{60}{3 \times 10^8}=2 \times 10^7 T$

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MCQ 1521 Mark
An electron oscillating with a frequency of $3 \times 10^6 Hz$, would generate:
  • A
    $X-$rays
  • B
    Ultraviolet rays
  • ✓
    Radio waves
  • D
    Microwaves
Answer
Correct option: C.
Radio waves
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MCQ 1531 Mark
Which one of the following is the primary effect of $UV$ radiation caused due to depletion of ozone layer?
  • A
    Heart attack
  • ✓
    Skin cancer
  • C
    High fever
  • D
    All of these
Answer
Correct option: B.
Skin cancer

The ozone layer prevents most harmful $UV$ wavelengths of ultraviolet light $(UV$ light$)$ from passing through the Earth's atmosphere. These wavelengths cause skin cancer, sunburn and cataracts, which were projected to increase dramatically as a result of thinning ozone, as well as harming plants and animals.

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MCQ 1541 Mark
Displacement current is continuous:
  • ✓
    when electric field is changing in the circuit
  • B
    when magnetic field is changing in the circuit
  • C
    in both types of fields
  • D
    through wires and resistance only
Answer
Correct option: A.
when electric field is changing in the circuit

Displacement current is a quantity appearing in Maxwell's equations that is defined in terms of the rate of change of electric displacement field.
When electric field is changing with time continuously, the displacement current is constant.

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MCQ 1551 Mark
In Maxwell's velocity distribution curve area under the graph:
  • A
    Increases when temperature is increased.
  • B
    Deccreases when temperature is increased.
  • ✓
    Remains same at all temperature.
  • D
    Depends on the pressure of the gas.
Answer
Correct option: C.
Remains same at all temperature.

Area under the Maxwell's velocity distribution curve gives the number of particles. Since number of particles remains the same at all the temperatures, so the area under the curve also remains the same at all temperature.

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MCQ 1561 Mark
In an electromagnetic wave, the electric magnetising fields are $\frac{100\text{V}}{\text{m}}$ and $\frac{0.265\text{A}}{\text{m}}$ The maximum energy flow is?
  • ✓
    $\frac{26.5\text{W}}{\text{m}^2}$
  • B
    $\frac{36.5\text{W}}{\text{m}^2}$
  • C
    $\frac{46.7\text{W}}{\text{m}^2}$
  • D
    $\frac{765\text{W}}{\text{m}^2}$
Answer
Correct option: A.
$\frac{26.5\text{W}}{\text{m}^2}$

Maximum rate of energy flow, $S = E _0 \times H _0$
Given, $\text{E}_0=\frac{100\text{V}}{\text{m}},\text{H}_0=\frac{0.265\text{A}}{\text{m}}$
$\therefore\text{S}=100\times0.265=\frac{26.5\text{W}}{\text{m}^2}$​

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MCQ 1571 Mark
The figure here gives the electric field of an $EM$ wave at a certain poiunt and a certain instrant. the wave is transporting energy in the negative $z$ direction. what is tha direction of the magnetic field of the wave at that point and instant.
  • ✓
    Towards $+X$ direction
  • B
    Towards $-X$ direction
  • C
    Towards $+Z$ direction
  • D
    Towards $-Z$ direction
Answer
Correct option: A.
Towards $+X$ direction

The direction of $EM$ wave is given by the direction of $\overrightarrow{\text{E}}\times\overrightarrow{\text{B}}$

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MCQ 1581 Mark
Electromagnetic waves are transverse in nature is evident by.
  • ✓
    polarization
  • B
    interference
  • C
    reflection
  • D
    diffraction
Answer
Correct option: A.
polarization
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MCQ 1591 Mark
The conduction current is same as displacement current when source is:
  • A
    $Ac$ only
  • B
    $Dc$ only
  • ✓
    Either $ac$ or $dc$
  • D
    Neither $dc$ nor $ac$
Answer
Correct option: C.
Either $ac$ or $dc$
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MCQ 1601 Mark
The speeds of microwaves, infrared waves, and ultraviolet waves are $V_m, V_i$, and $V_u$ respectively. Identify the correct combination showing the different waves in vacuum.
  • A
    $V_m > V_i > V_u$
  • ✓
    $V_m=V_i=V_u$
  • C
    $V _{ m } < V _{ i } < V _{ u }$
  • D
    $V _{ m } > V _{ i } < V _{ u }$
Answer
Correct option: B.
$V_m=V_i=V_u$

The correct combination is $\rightarrow Vm = Vi = Vu$. This is because, in vacuum, all the electromagnetic waves in question will travel at the same speed. The speed with which they travel in vacuum is the speed of light. $( c =3 \times 108 m / s ).$

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MCQ 1611 Mark
Maxwell's equation describe the fundamental laws of.
  • A
    electricity
  • B
    magnetism
  • C
    mechanics
  • ✓
    both $(A)$ and $(B)$
Answer
Correct option: D.
both $(A)$ and $(B)$

Maxwell's equation describe the fundamental laws of electricity and magnetism. His equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents.

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MCQ 1621 Mark
One requires $11eV$ of energy to dissociate a carbon monoxide molecule into carbon and oxygen atoms. The minimum frequency of the appropriate electromagnetic radiation to achieve the dissociation lies in:
  • A
    Visible region.
  • B
    Infrared region.
  • ✓
    Ultraviolet region.
  • D
    Microwave region.
Answer
Correct option: C.
Ultraviolet region.
Here it is given, the energy required to dissociate a carbon monoxide molecule into catbon and oxygon atoms is $E = 11 eV$
We know that, $E=h f=6.62 \times 10^{-34} J- s$
$f =$ frequency
$\Rightarrow 11 eV = hf$
$f =\frac{11 \times 1.6 \times 10^{-19}}{6.62 \times 10^{-34}}=2.65 \times 10^{15} Hz$
This frequency radiation belongs to ultraviolet region.
Important point:
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MCQ 1631 Mark
Which of the following waves have a maximum frequency?
  • A
    Infrared waves
  • ✓
    Gamma rays
  • C
    Microwaves
  • D
    Radio waves
Answer
Correct option: B.
Gamma rays

Gamma rays have a maximum frequency.

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MCQ 1641 Mark
A free electron is placed in the path of a plane electromagnetic wave. The electron will start moving:
  • ✓
    Along the electric field.
  • B
    Along the magnetic field.
  • C
    Along the direction of propagation of the wave.
  • D
    In a plane containing the magnetic field and the direction of propagation.
Answer
Correct option: A.
Along the electric field.

As the electron is at rest initially, only the electric field will exert force on it. There will be no magnetic force on the electron in the stating. Hence, the electron will start moving along the electric field.

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MCQ 1651 Mark
Pick out the electromagnetic wave which is highly harmful to humans.
  • A
    Radio waves
  • ✓
    Ultraviolet Rays
  • C
    Microwaves
  • D
    Infrared Waves
Answer
Correct option: B.
Ultraviolet Rays

Ultraviolet $(UV)$ radiations in large quantities are highly harmful to humans. These rays in solar radiation on reaching earth are absorbed by the ozone layer in the atmosphere. $UV$ rays are produced by special lamps such as mercury and from arc lamps and by very hot bodies like the sun.

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MCQ 1661 Mark
Choose the correct answer from the alternatives given. Electromagnetic wave consists of periodically oscillating electric and magnetic vectors:
  • A
    In mutually perpendicular planes but vibrating with a phase difference of $\pi$
  • B
    In mutually perpendicular planes but vibrating with a phase difference of $2\pi$
  • C
    In randomly oriented planes but vibrating in phase
  • ✓
    In mutually perpendicular planes but vibrating in phase
Answer
Correct option: D.
In mutually perpendicular planes but vibrating in phase
The Electromagnetic wave consists of the two mutually perpendicular electric and magnetic fields present in the medium and also both being perpendicular to the direction of propagation of the wave. The two fields are in same phase as they obtain their peaks at the same instant.
An electromagnetic wave consists of periodically oscillating electric and the magnetic vector in mutually perpendicular planes but vibrating in phase.

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MCQ 1671 Mark
What is the full form of $\text{UHF?}$
  • A
    Under heavy frequency
  • ✓
    Ultra-high frequency
  • C
    Ultra heavy frequency
  • D
    Under high frequency
Answer
Correct option: B.
Ultra-high frequency

$\text{UHF}$ stands for Ultra high frequency. Cellular phones used radio waves to transmit voice communication in the Ultra high$-$frequency Band. The $\text{UHF}$ band extends from $900 MHz$ to $5 \times 109 Hz$ or $5000 MHz$. Radio waves are produced by oscillating circuits having an inductor and capacitor.

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MCQ 1681 Mark
The pressure exerted by an electromagnetic wave of intensity I$\Big(\frac{\text{watt}}{\text{m}^2}\Big)$ on a non-reflecting surface is : $[c$ is the velocity of light$].$
  • A
    $\text{Ic}$
  • B
    $\text{Ic}^2$
  • ✓
    $\frac{\text{I}}{\text{c}}$
  • D
    $\frac{\text{I}}{\text{c}^2}$
Answer
Correct option: C.
$\frac{\text{I}}{\text{c}}$

Momentum of a photon
$=\frac{\text{h}}{\lambda}=\frac{\text{h}}{\frac{\text{c}}{\text{v}}}=\frac{\text{hv}}{\text{c}}=\frac{\text{E}}{\text{c}}$
Momentum over unit area
$=\frac{\text{E}}{\text{Ac}}=\frac{\text{I}}{\text{c}}\Big[\text{I}=\frac{\text{E}}{\text{A}}\text{ For wave}\Big]$
Since surface is non reflecting, final momentum of photon $= 0,$ change in momentum $=\frac{\text{I}}{\text{c}}$
So, force per unit area $=\frac{\text{I}}{\text{c}}$
Pressure of radiation $=\frac{\text{I}}{\text{c}}$

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M.C.Q (1 Marks) - Page 4 - Physics STD 12 Science Questions - Vidyadip