Electromagnetic Waves MCQ Questions & Answers in Electrostatics and Magnetism | Physics
Learn Electromagnetic Waves MCQ questions & answers in Electrostatics and Magnetism are available for students perparing for IIT-JEE, NEET, Engineering and Medical Enternace exam.
71.
In a certain region of space electric field $$E$$ and magnetic field $$B$$ are perpendicular to each other and an electron enters in region perpendicular to the direction of $$B$$ and $$E$$ both and moves undeflected, then velocity of electron is
A
$$\frac{{\left| E \right|}}{{\left| B \right|}}$$
B
$$E \times B$$
C
$$\frac{{\left| B \right|}}{{\left| E \right|}}$$
D
$$E \cdot B$$
Answer :
$$\frac{{\left| E \right|}}{{\left| B \right|}}$$
For electron to pass undeflected, electric force on electron = magnetic force on electron
i.e. $$eB = evB$$
$$\eqalign{
& {\text{or}}\,\,v = \frac{E}{B}\,\, \cr
& {\text{or}}\,\,v = \frac{{\left| E \right|}}{{\left| B \right|}} \cr} $$
72.
All components of the electromagnetic spectrum in vacuum have the same
A
energy
B
velocity
C
wavelength
D
frequency
Answer :
velocity
All components of electromagnetic spectrum travel in vacuum with velocity $$3 \times {10^8}\,m/s.$$
73.
The average electric field of electromagnetic waves in certain region of free space is $$9 \times {10^{ - 4}}N{C^{ - 1}}.$$ Then the average magnetic field in the same region is of the order of
A
$$27 \times {10^{ - 4}}T$$
B
$$3 \times {10^{ - 12}}T$$
C
$$\left( {\frac{1}{3}} \right) \times {10^{ - 12}}T$$
D
$$3 \times {10^{12}}T$$
Answer :
$$3 \times {10^{ - 12}}T$$
For electromagnetic waves we know that,
$$\eqalign{
& \frac{E}{B} = c \cr
& \therefore \frac{{9 \times {{10}^{ - 4}}}}{B} = 3 \times {10^8}\,m{s^{ - 1}} \cr
& \Rightarrow B = 3 \times {10^{ - 12}}\,T \cr} $$
74.
Microwave oven acts on the principle of :
A
giving rotational energy to water molecules
B
giving translational energy to water molecules
C
giving vibrational energy to water molecules
D
transferring electrons from lower to higher energy levels in water molecule
Answer :
giving vibrational energy to water molecules
Microwave oven acts on the principle of giving vibrational energy to water molecules.
75.
The velocity of electromagnetic wave is along the direction of
A
$$B \times E$$
B
$$E \times B$$
C
$$E$$
D
$$B$$
Answer :
$$E \times B$$
An electromagnetic wave is the wave composed of the oscillations of electric and magnetic fields in mutually perpendicular planes and these oscillations are perpendicular to the direction of propagation of wave.
The direction of propagation of electromagnetic wave is given by poynting vector
$$S = E \times H = \frac{{E \times B}}{{{\mu _0}}}$$
This is parallel to $$E \times B.$$
76.
The waves which are electromagnetic in nature are
A
sound waves and light waves
B
water waves and radio waves
C
light waves and X-rays
D
sound waves and water waves
Answer :
light waves and X-rays
Light waves are electromagnetic waves.
77.
In a wave $${E_0} = 100\,V{m^{ - 1}}.$$ Find the magnitude of Poynting’s vector
A
$$13.25\,W{m^{ - 2}}$$
B
$$26.5\,W{m^{ - 2}}$$
C
$$18.25\,W{m^{ - 2}}$$
D
$$19.7\,W{m^{ - 2}}$$
Answer :
$$26.5\,W{m^{ - 2}}$$
$$\vec s = \frac{{{E^2}}}{{C{\mu _0}}} = 26.5\,W{m^{ - 2}}$$
78.
A plane electromagnetic wave is incident on a plane surface of area $$A,$$ normally and is perfectly reflected. If energy $$E$$ strikes the surface in time $$t$$ then force exerted on the surface is ($$c$$ = speed of light)
A
$$\frac{{2E}}{{Atc}}$$
B
$$\frac{E}{{2c}}$$
C
$$\frac{{2E}}{{ct}}$$
D
zero
Answer :
$$\frac{{2E}}{{ct}}$$
Incident momentum, $$p = \frac{E}{c}$$
For perfectly reflecting surface with normal incidence
$$\eqalign{
& \Delta p = 2p = \frac{{2E}}{c}; \cr
& F = \frac{{\Delta p}}{{\Delta t}} = \frac{{2E}}{{ct}}; \cr} $$
79.
An electromagnetic wave of frequency $$1 \times {10^{14}}hertz$$ is propagating along $$z$$-axis. The amplitude of electric field is $$4\,V/m.$$ If $${\varepsilon _0} = 8.8 \times {10^{ - 12}}{C^2}/N - {m^2},$$ then average energy density of electric field will be :
A
$$35.2 \times {10^{ - 10}}J/{m^3}$$
B
$$35.2 \times {10^{ - 11}}J/{m^3}$$
C
$$35.2 \times {10^{ - 12}}J/{m^3}$$
D
$$35.2 \times {10^{ - 13}}J/{m^3}$$
Answer :
$$35.2 \times {10^{ - 12}}J/{m^3}$$
Given:
Amplitude of electric field,
$${E_0} = 4\,v/m$$
Absolute permitivity,
$${\varepsilon _0} = 8.8 \times {10^{ - 12}}{c^2}/N - {m^2}$$
Average energy density $${u_E} = ?$$
Applying formula,
Average energy density $${u_E} = \frac{1}{4}{\varepsilon _0}{E^2}$$
$$ \Rightarrow {u_E} = \frac{1}{4} \times 8.8 \times {10^{ - 12}} \times {\left( 4 \right)^2} = 35.2 \times {10^{ - 12}}J/{m^3}$$
80.
Which of the following is the infrared ray wavelength?
A
$${10^{ - 4}}cm$$
B
$${10^{ - 5}}cm$$
C
$${10^{ - 6}}cm$$
D
$${10^{ - 7}}cm$$
Answer :
$${10^{ - 4}}cm$$
The wavelength of infrared region is $$8 \times {10^{ - 5}}cm$$ to $$3 \times {10^{ - 3}}cm.$$ So maximum wavelength of infrared region
$$\eqalign{
& = 8 \times {10^{ - 5}}cm \cr
& \approx {10^{ - 4}}cm. \cr} $$