Electromagnetic Waves Test

Result

Electromagnetic Waves Test - 5

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Weekly Quiz Competition

Question 1
1 / -0

Which of the following is the correct expression for velocity of light?

A

B

C

D

Solution
Question 2
1 / -0

When a plane electromagnetic wave travels in vacuum, the average electric energy density is given by
(Here, E₀ is the amplitude of the electric field of the wave)

A

B

C

D

Solution

Energy density of the electromagnetic wave corresponding to the electric field can be represented as:

Here, 'Ad' is the volume covered by the electromagnetic wave, and .
E is the rms value of the electric field.

Substituting the values, we get
Question 3
1 / -0

Visible light is due to

A
electric field oscillations

B
magnetic field oscillations

C
oscillations of electric and magnetic fields in the same direction

D
oscillations of electric and magnetic fields in mutually perpendicular directions

Solution

Visible light is the electromagnetic radiations in which oscillating electric field and the magnetic field are perpendicular to each other; both are also perpendicular to the direction of propagation of the wave.
Question 4
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Which of the following pairs of space-time varying E = () and B = () would generate a plane electromagnetic wave travelling in the z-direction?

A
E_x and B_z

B
E_y and B_z

C
E_z and B_x

D
E_x and B_y

Solution

E_x and B_ywould generate a plane electromagnetic wave travelling in the z-direction because the components of mutually perpendicular electric field and magnetic field are perpendicular to the direction of propagation of the wave.
Question 5
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Radio waves received by a radio telescope from distant stars may have a wavelength of about 0.20 m. If the speed of the waves is km s^–1, then the frequency of the waves will be

A

B

C

D

Solution

c = 3 x 10⁸ m/s
Wavelength = 0.2 m
= 1.5 x 10⁹ Hz
Question 6
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If B is the magnetic field, E is the electric field and c is the speed of light in an electromagnetic wave, then which of the following relations is correct?

A
c = E/B

B
c = BE

C
c = B/E

D
c = 1/EB

Solution

The speed of electromagnetic wave is given by the ratio of electric field to the magnetic field, i.e. c = E/B.
Question 7
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A new system of units is evolved in which the values of ₀ and ₀ are 2 and 8, respectively. The speed of light in this system will be

A
0.25

B
0.5

C
0.75

D
1

Solution

A new system of units is evolved in which the values of o and are 2 and 8, respectively. The speed of light in this system will be:

c = =
Question 8
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Which of the following statements is false?

A
Sun is a natural source of UV radiations.

B
UV radiations have wavelengths extending from 200 nm to 400 nm.

C
UV radiations are useful in astronomy.

D
UV radiations are used for sterilisation of water.

Solution

Ultraviolet astronomy is the observation of electromagnetic radiation at ultraviolet wavelengths between approximately 10 and 320 nanometres, shorter wavelengths (higher energy photons) are studied by X-ray astronomy and gamma ray astronomy. Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space.
Question 9
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The average energy flux of sunlight is 1.0 kWm^-2. This energy (light) is falling normally on a plate surface of area 10 cm², which completely absorbs the energy. How much force (in Newtons) is exerted on the plate if it is exposed to sunlight for 20 minutes?

A
5.9 x 10^-8

B
2.9 x 10^-9

C
3.3 x 10^-9

D
3.8 x 10^-8

Solution

Here, Energy flux = 1.0 kWm^-2
= 1.0 x 10³ Wm^-2
Area of plate,
A = 10 cm² = 10 x 10^-4 m²
= 10^-3 m²
Time, t = 20 min = 20 x 60 s = 1200 s
Total energy falling on the plate,
U = Energy flux x Area x time
= 1 x 10³ Wm^-2 x 10^-3 m² x 1200 s
= 1200 J
Momentum of the radiation, p =
Or p =
= 4 x 10^-6 kg m s^-1
Since plate absorbs the entire energy, so the momentum delivered to the plate
= 4 x 10^-6 kg m s^-1 (= change in momentum)
Force exerted on the plate, F =
=
Question 10
1 / -0

If and ^_thenequals (symbols have their usual meanings)

A

B
-

C

D

Solution

By dividing both the given equations, we will get , which is equal to c, which is further equal to .
Question 11
1 / -0

A parallel plate capacitor of area 60 cm² and separation 3 mm is charged initially to 90 C. If the medium between the plate gets slightly conducting and the plate loses the charge initially at the rate of 2.5 x 10^-8 C/s, what is the magnetic field between the plates?

A
5.56 x 10^-4T

B
0

C
9.56 x 10^-4T

D
6.56 x 10^-4T

Solution

Displacement current is

I_d = A

= A

Using the given value of charge lost by plate
I_d = 2.5 x 10^-8 C/s
= 2.5 x 10^-8 A
Thus, I_d has same magnitude as the conduction current but opposite direction. The total current
I = I_c + I_d = 0
From Ampere's law,

Magnetic field within the plates is zero at all points.
Question 12
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The wavelength of light received from a galaxy is 5% greater than that received from an identical source on the earth. The velocity of the galaxy relative to earth is:

A
20 10⁶ ms^-1

B
18 10⁶ ms^-1

C
15 10⁶ ms^-1

D
12 10⁶ ms^-1

Solution
Question 13
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The potential difference between the plates of a parallel plate capacitor is changing at the rate of 10⁶ Vs^-1. If the capacitance is 2 F, then the displacement current in the dielectric of the capacitor will be

A
1 A

B
2 A

C
3 A

D
4 A

Solution

The Displacement current is given by

Hence the correct choice is (2).
Question 14
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Displacement current flows in a dielectric of a capacitor when the potential difference across its plates

A
becomes zero

B
is changing with time

C
does not depend upon the potential difference

D
has assumed a constant value

Solution
Question 15
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Which of the following statements is correct in relation to electromagnetic waves in an isotropic medium?

A
Energy due to electric field is equal to that due to magnetic field.

B
Electric vector E and magnetic vector B are in phase.

C
For a given amplitude of E vector, the intensity increases as the first power of frequency f.

D
For cylindrical wavefronts, the amplitude of the wave varies in proportion to 1/r², where r is the radius of the wavefront.

Solution