27 + Interview Questions in ELECTROMAGNETIC-WAVE IN GENERAL AWARENESS Page 1 InterviewSolution

1.	Suppose that the electric field part of an electromagnetic wave in vacuum is E={(3.1 N/C) cos [(1.8 rad/m)y+{5.4`xx10^(6)` rad/s} t]} i. What is the direction of propagation ?
Answer» From the given question it is clear that direction of motion of e.m. wave is along negative y direction i.e. along -`hatj`

Discussion

2.	Suppose that the electric field part of an electromagnetic wave in vacuum is E={(3.1 N/C) cos [(1.8 rad/m)y+{5.4`xx10^(6)` rad/s} t]} i. What is the wavelength `lambda` ?
Answer» Comparing the given question with equation `E=E_(0)cos (ky+omegat)` We have , `K=1.8` rad/m, `omega=5.4xx10^(8)` rad/s , `E_(0)=3.1` N/C `lambda=(2pi)/(k)=(2xx(22//7))/(1.8)=3.492~~3.5m`

Discussion

3.	Suppose that the electric field part of an electromagnetic wave in vacuum is E={(3.1 N/C) cos [(1.8 rad/m)y+{5.4`xx10^(6)` rad/s} t]} i. What is the frequency v ?
Answer» `V=(omega)/(2pi)=(5.4xx10^(8))/(2xx((22)/(7)))=85.9xx10^(6)~~86` MHz

Discussion

4.	The charging current for a capacitor is 0.6 A. What is the displacement current across its plates ?
Answer» i=change current for a capacitor=0.6 A `i=i_(d)=epsi_(0)(d phi_(B))/(dt)` `therefore i=i_(d)=0.6A` `therefore` Displacement current `(i_(d))` =0.6A

Discussion

5.	The figure shows a capacitor made of two circular plates each of radius 12 cm, and separated by 5.0 cm . The capacitor is beging charged by an external source (not shown in the figure. ) The charing is constant and equal to 0.15 A Obtain the displacement current across the plates.
Answer» Displacement current is equal to conduction current i.e. , 0.15 A.

Discussion

6.	The figure shows a capacitor made of two circular plates each of radius 12 cm, and separated by 5.0 cm . The capacitor is beging charged by an external source (not shown in the figure. ) The charing is constant and equal to 0.15 A Calculate the capacitance and the rate of charge of potential difference between the plates.
Answer» Given `epsi_(0)=8.85xx10^(-12) C^(2)N^(-1) m^(-2)` Here , R=12 cm =0.12 m, d=5.0 mm=`5xx10^(-3)m `, I=0.15 A Area , A=`pi R^(2)=3.14xx(0.12)^(2) m^(2)` We know that capacity of a parallel plate capacitor is given by `C=(epsi_(0)A)/(d)=(8.85xx10^(-12)xx3.14xx(0.12)^(2))/(5xx10^(-3))=80.1 xx10^(-12) F` `=80.1 ` PF Now `q=CV` or `(dq)/(dt)=Cxx(dv)/(dt)` or `I=C.(dv)/(dt)` Or `(dv)/(dt)=(I)/(C)=(0.15)/(80.1xx10^(-12))=1.87xx10^(9) Vs^(-1)`

Discussion

7.	A parallel plate capacitor in the figure made of circular plates each of radius R=6.0 cm has a capacitance C=100 pF. The capacitor is connected to a 230 V ac supply with a (angular) frequency of 300 rad `s^(-1)` . What is the rms value of the conduction current ?
Answer» `I_("rms")=(E_("rms"))/(X_(c))=(E_("rms"))/((1)/(omegaC))=E_("rms")xx omega C` `therefore I_("rms")=230xx300xx100xx10^(-12)=6.9xx10^(-6)A=6.9 muA`

Discussion

8.	A charged particle oscillates about its mean equilibrium position with a frequency of `10^(9)` Hz. What is the frequency of the electromagnetic waves produced by the oscillator ?
Answer» According to Maxwell, a charged particles oscillating with a frequency produces electromagnetic waves of same frequency waves of same frequency . Hence frequency of EM wavs produced is, `10^(9)` Hz.

Discussion

9.	In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of `2.0xx10^(10) Hz` and amplitude 48 V `m^(-1)`. What is the wavelength of the wave ?
Answer» Here, v=`2.0xx10^(10) Hz, E_(0)=48 Vm^(-1),C=3xx10^(8) m//s` wavelength of the wave , `lambda=(c)/(v)=(3xx10^(8))/(2.0xx10^(10))=1.5xx10^(-2)`m

Discussion

10.	The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is `B_(0) = 510` nT. What is the amplitude of the electric field part of the wave ?
Answer» Here, `B_(0)=510xx10^(-9) T` `E_(0)=CB_(0)=3xx10^(8)xx510xx10^(-9)=153 NC^(-1)`.

Discussion

11.	A plane electromagentic wave of frequency 25 MHz travels in free space along the x-direction. At a particular point in space and time , E=6.3 `hatj` V/m . What is B at the point ?
Answer» Using Eq. `B_(0)=[E_(0)//C]` the magnitude of B is `B=(E)/(c)` `=(6.3 V//m)/(3xx10^(8) m//s)=2.1xx10^(-8)T` To find the direction , we not that E is along y-direction and the wave propagates along x-axis. Therefore, B should be in a direction perpendicular to both x-and y-axes.Using vector algebra, `E xx B` should be along x-direction . Since `(+hatj)xx(+hatk)=i`, B is along the z-direction . Thus, B=`2.1xx10^(-8) hat kT`

Discussion

12.	What is the principle of production of electromagnetic waves ?
Answer» If the charge is accelerated both the magnetic field and electric field wila change with space and time, then electromagnetic waves are produced.An accelerated charge produces electric and magnetic fields which vary both in space and time. The two oscillating fields act as sources of each other and sustain each other. This results in the propagation of EM waves through space.

Discussion

13.	What is the relation between the amplitudes of the electric and magnetic fields in free space for an electromagnetic wave ?
Answer» `E_(0)=CB_(0)` Where `E_(0)`=Amplitude of electric field. `B_(0)=` Amplitude of magnetic field. C=velocity of light.

Discussion

14.	What are the applications of microwaves ?
Answer» i) Microwaves are used in Radars. ii) Microwaves are used for cooking purposes. iii) A radar using microwave can help in detecting the speed of automobile while In Motion.

Discussion

15.	The magnetic field in a plane electromagnetic wave is given by `B_(y)=2xx10^(-7)sin (0.5xx10^(3)x + 1.5 xx 10^(11) t)T`. What is the wavelength and frequency of the wave ?
Answer» Comparing the given equation with `B_(y)=B_(0)sin[2pi((x)/(lambda)+(t)/(T))]` We get , `lambda=(2pi)/(0.5xx10^(3))m=1.26 cm,` and `(1)/(T)=v=(1.5xx10^(11))//2pi=23.9` GHz

Discussion

16.	If the wavelength of electromagnetic radiation is doubled, what happens to the energy of photon ?
Answer» Photon energy (E) =hv=`(hc)/(lambda)` `E prop (1)/(lambda)` Given `lambda_(1)=lambda, lambda_(2)=2lambda,E_(1)=E` `(E_(1))/(E_(2))=(lambda_(2))/(lambda_(1))` `(E)/(E_(2))=(2lambda)/(lambda)` `E_(2)=E//2` `therefore ` The energy of photon reduces to half to its initial value.

Discussion

17.	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. 21 cm (wavelength emitted by atomic hydrogen in interstellar space).
Answer» This wavelength correspongs to radiowaves.

Discussion

18.	State six characteristics of electromagnetic waves. What is Greenhouse effect ?
Answer» Characteristics of electromagnetic waves : l) Electromagnetic waves are produced by accelerated charges. 2) Electromagnetic waves are transverse in nature. 3) Electromagnetic waves donot require material medium for their propagation. 4) Electromagnetic waves obey principle of superposition of waves. 5) Velocity of E.M waves in vaccum depends on permittivity and permeability of free space. 6) Electromagnetic waves carry energy and momentum. Electromagnetic waves exert pressure when they strike a surface.

Discussion

19.	What does an electromagnetic wave consists of ? On what factors does its velocity in vacuum depend ?
Answer» Maxwell concluded that the variation in electric and magnetic field vectors perpendicular to each other leads to the production of electromagnetic waves in space. They can travel in space even without any material medium. These waves are called electromagnetic waves. According to Maxwell, electromagnetic waves are those waves in which there are sinusoidal variation of electric and magnetic field vectors at right angles to each other as well as at right angles to the direction of wave propagatipn. Thus electomagnetic waves have transverse nature. Electric field `E_(x)=E_(0)` Sin (kz-`omegat`) Magnetic field `B_(y)=B_(0)sin(kz -omegat)` Where K is propagation constant `(K=(2pi)/(lambda))` The velocity of electromagnetic waves C=`(1)/(sqrt(mu_(0)epsi_(0))` Velocity of E.M waves depends on i) Permeability in free space `(mu_(0))`. ii) Permittivity in free space `(epsi_(0))` . Velocity of e.m waves is `3 xx 10^(8)` m/s.

Discussion

20.	Define displacement current.
Answer» Displacement current `(I_(d))` is equal to` epsi_(0)` times to the rate of change of electric flux. Displacement current is not the current produced due to charge carried. But it is due to varying electric flux. It is the current in,the sense that it produces a magnetic field. `I_(d)=epsi_(0)(dphi_(epsi))/(d t)`

Discussion

21.	What physical quantity is the same for X-rays of wavelength `10^(-10)`m, red light of wavelength 6800 Å and radiowaves of wavelength 500m ?
Answer» The speed in vacuum is same for all the given wavelengths, which is 3 `xx 10^(8)` m/s.

Discussion

22.	How are infrared rays produced ? How they can be detected ?
Answer» Infrared rays can be produced by vibrations of atoms and molecules. These waves can be detected by Thermopile, Bolometer, IR photographic film.

Discussion

23.	Give two uses of infrared rays.
Answer» i) Infrared rays are used for producing dehydrated fruits. ii) They are used in the secret writings on the ancient walls. iii) They are used in green houses to keep the plants warm.

Discussion

24.	What is the ratio of speed of infrared rays and ultraviolet rays in vacuum ?
Answer» The ratio of speed of infrared rays ând ultraviolet rays in vacuum is 1 :1. All electromagnetic waves travel with same speed `3xx 10^(8)` m/s in vacuum.

Discussion

25.	What is the average wavelength of X-rays ?
Answer» Wavelength range of X-rays is from `10^(-8)` m(10nm) to `10^(-13) `m (`10^(-13)` nm). Average wavelength of X-rays `=(10+0.0001)/(2)=5.00005` nm.

Discussion

26.	It is neccessary to use satellites for long distance TV transmission. Why ?
Answer» Yes, television signals being of high frequency are not reflected by ionosphere . Therefore to reflect them satellites are needed. That is why, satellites are used for long distance TV transmission.

Discussion

27.	Suppose that the electric field amplitude of an electromagnetic wave is `E_(0)=120 ` N/C and that its frequency is v=50.0 MHz.Find expressions for E and B.
Answer» Expression for `vecE` is E`=E_(0) sin (kx-omegat)` `=120 N//c)` Sin [(1.05 rad/m)x=`(3.14xx10^(8)` rad/s `)` t]`hatj` Expression for `vecB` is B=`B_(0)` sin `(kx-omegat)` `=(4xx10^(-7)T)sin[(1.05 "rad/m")x-(3.14xx10^(8) "rad/s")t] hatk`.

Discussion

Explore topic-wise InterviewSolutions in .

Suppose that the electric field part of an electromagnetic wave in vacuum is E={(3.1 N/C) cos [(1.8 rad/m)y+{5.4`xx10^(6)` rad/s} t]} i. What is the direction of propagation ?

Suppose that the electric field part of an electromagnetic wave in vacuum is E={(3.1 N/C) cos [(1.8 rad/m)y+{5.4`xx10^(6)` rad/s} t]} i. What is the wavelength `lambda` ?

Suppose that the electric field part of an electromagnetic wave in vacuum is E={(3.1 N/C) cos [(1.8 rad/m)y+{5.4`xx10^(6)` rad/s} t]} i. What is the frequency v ?

The charging current for a capacitor is 0.6 A. What is the displacement current across its plates ?

The figure shows a capacitor made of two circular plates each of radius 12 cm, and separated by 5.0 cm . The capacitor is beging charged by an external source (not shown in the figure. ) The charing is constant and equal to 0.15 A Obtain the displacement current across the plates.

A parallel plate capacitor in the figure made of circular plates each of radius R=6.0 cm has a capacitance C=100 pF. The capacitor is connected to a 230 V ac supply with a (angular) frequency of 300 rad `s^(-1)` . What is the rms value of the conduction current ?

A charged particle oscillates about its mean equilibrium position with a frequency of `10^(9)` Hz. What is the frequency of the electromagnetic waves produced by the oscillator ?

In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of `2.0xx10^(10) Hz` and amplitude 48 V `m^(-1)`. What is the wavelength of the wave ?

The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is `B_(0) = 510` nT. What is the amplitude of the electric field part of the wave ?

A plane electromagentic wave of frequency 25 MHz travels in free space along the x-direction. At a particular point in space and time , E=6.3 `hatj` V/m . What is B at the point ?

What is the principle of production of electromagnetic waves ?

What is the relation between the amplitudes of the electric and magnetic fields in free space for an electromagnetic wave ?

What are the applications of microwaves ?

The magnetic field in a plane electromagnetic wave is given by `B_(y)=2xx10^(-7)sin (0.5xx10^(3)x + 1.5 xx 10^(11) t)T`. What is the wavelength and frequency of the wave ?

If the wavelength of electromagnetic radiation is doubled, what happens to the energy of photon ?

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. 21 cm (wavelength emitted by atomic hydrogen in interstellar space).

State six characteristics of electromagnetic waves. What is Greenhouse effect ?

What does an electromagnetic wave consists of ? On what factors does its velocity in vacuum depend ?

Define displacement current.

What physical quantity is the same for X-rays of wavelength `10^(-10)`m, red light of wavelength 6800 Å and radiowaves of wavelength 500m ?

How are infrared rays produced ? How they can be detected ?

Give two uses of infrared rays.

What is the ratio of speed of infrared rays and ultraviolet rays in vacuum ?

What is the average wavelength of X-rays ?

It is neccessary to use satellites for long distance TV transmission. Why ?

Suppose that the electric field amplitude of an electromagnetic wave is `E_(0)=120 ` N/C and that its frequency is v=50.0 MHz.Find expressions for E and B.