Em Wave Hots

8/10/2019 Em Wave Hots

1/7

1.What is the highest frequency for which antennas can be made (approximately)

1012to 1013 Hz.

2. What is the condition of the electrons in the transmitting antenna when maximum

magnetic field is being transmitted?

A maximum current is flowing, so the electrons have maximum speed up and

down the antenna. The electric current produces the magnetic field.

3. How can the electric portion of the electro-magnetic wave be detected?

It can be detected by an antenna similar to the transmitting antenna except that a

detector of electric current replaces the voltage source.

4. How can the magnetic field portion of an electro-magnetic wave be detected?

The magnetic wave is best detected by placing a loop of wire (with its ends hooked

to a current detector) in the path of the wave. The changing magnetic field causes a

current in the loop.

5.The small ozone layer on the top of the atmosphere is crucial for human survival.

Why?

The ozone in the atmosphere is confined to the ozone layer, some 50-80 km above

the ground. The ozone layer blocks the passage of the ultra-violet radiations; x-rays and

y-rays from the solar and other extra-terrestrial sources and effectively protects us from

the dangerous and harmful portions of solar radiations as they cause genetic damages

to living cells. Practically all radiations of wave length less than 3 x 10 -7m are absorbed

by the ozone layer. This explains why ozone layer on the top of the atmosphere is

crucial for human survival.

6.How does "Green House Effect" affect the temperature of the earth's surface ?

Green house effect serves to keep the earth's surface warm at night.

7.How do we make television broadcasts for larger coverage and for long distance?

By using (i) tall antennas which is familiar landmark in many cities and (ii) using

artificial satellites called geostationary satellites. Since television signals are of high

frequency and are not reflected by ionosphere so we use satellites to get them reflected

& transmission of TV signals can be used for larger coverage as well as for long

distance.


2/7

8. Scientists put x-ray astronomical telescope on the artificial satellite orbiting above the

earth's atmosphere whereas they build optical and radio-telescopes on the surface of the

earth. Why ?

X-rays have very high frequency and much smaller wavelength. These rays get

absorbed by the earth's atmosphere. On the other hand, optical (visible) radiations and

radio-waves can pass through the atmosphere. That is why optical and radio telescopes

can be installed on the earth's surface.

9. For an e lec t romagnet ic wave, wr i te the re la t ionship between ampl i tude of

e lec t ric and m agne t ic f ie lds in f ree spa ce.

[H ints I f E o i s the ampl i tude o f an e lec t r i c f i e ld and B o i s the ampl i tude o f the

assoc ia ted magnet i c f i e ld in f ree space then0

0

B

Ec= whe re c i s the speed o f l igh t

i n f ree spa ce i .e. 3x 10 8 m / s.

10. The charging current for a capacitor is 0.25 A. What is the displacement current

across its plates?

Hints: Displacement current = Charging current = 0.25 A]

11. What is a ground wave? How does it differ from a sky wave?

Ans. A signal emitted by an antenna from a certain point can be received at another point of

the surface of the earth in two ways. The wave which travels directly following the surface of the

earth is called groundnave. The wave that can reach the same point after being reflected

from the ionosphere is called sky wave.

12. Why short wave communication over long distances is not possible via ground waves?

|Hints: Because the wave gets attenuated.]

13 . A re condu c t ion a nd d i sp laceme nt cu r ren ts the sam e?

[H in t: No; they are d i f fe rent but they are equa l .

14. I t is neces sary to use sate l li tes for long d is tance TV t ransmiss ion. W hy?

I t is so becau se te lev is ion s igna ls are no t proper ly re f lec ted b y the

ionosphe re. Therefore , fo r re f lec t ion o f s igna ls sate l li tes a re nee ded as re f lec t ion i s

effected by sa tel li tes.

15. Op t ica l and rad io te lescope s are bu i lt on groun d but X-ray ast ronom y is

poss ib le on ly f rom sate l li tes o rb i ting the e ar th . W hy?


3/7

A tmo sphere a bsorbs X-rays, wh i le v is ib le and rad iowaves can pene tra te i t .

Tha t is why o pt ica l and rad io te lescopes ca n wo rk on e ar th 's sur face b ut X-ray

astronom ica l te lescop es m ust be us ed on sate l li tes orb i t ing the e ar th .

16. I f the ear th d id not have an a tmos phere, would i ts average sur face tem perature

be h igher or lower than wh at it is now ?

Ans. The tem perature o f the ear th would be lower because the green h ouse e f fec t

o f the a tmosphere wou ld be absen t .

.

17. W hat is the equat ion for the spe ed o f e lec t rom agne t ic waves in f ree spa ce?

C =00

1

18 . The wave length o f e lec t ro mag net ic rad ia t ion i s doub led. W hat w i ll happen to

the energy o f the photon?

The energy w i ll be ha lved becau se f requen cy w i ll be ha lved.

19. Nam e the par t o f e lec t roma gnet ic spectrum to which waves o f wave length ( i )

1Aand ( i i) 10 -2 m b e long. Us ing the re la tion T = (0 .29 cm ) K, obta in the

character is ti c ke lv in tem perature correspon ding to these two w ave lengths.

( i ) X-rays, ( i i) M icrowav es

Ag ain, ( i ) Kcm

cmT 8

10 1029

101

29.0

=

= (i i) Kcm

cmT 29.0

1

29.0==

20 . G ive a s im ple argum ent to sugges t that an acce lera ted charge mu st em i t

e lec t rom agne t ic rad ia t ion

Ans. W hen ch arge m oves w i th constant ve loc i ty , the ma gnet ic fie ld doe s not

chang e w i th t im e. So, it cannot produc e an e lec tr ic f ie ld . W hen ch arge is

acce lera ted, both e lec t ri c and ma gnet ic f ie lds chang e w i th time an d space , one

becom ing a source o f ano ther . Th is g ives r i ses to e lec t romag net ic wave.

21. Show that the ave rage en ergy dens i ty o f the E f ie ld equa ls the average energy

den si ty of the B f ie ld.

Ans. E nergy de ns i ty in E f ie ld ,

2

02

1Eu

E =


4/7

En ergy d ens i ty in B f ie ld,--------------

2

02

1BuB

=

Us ing E = cB and c =

00

1

, uE = uB .

22. On wh at fac tors does its ve loc ity in vacuum depe nd?

Ans. E lec troma gnet ic waves cons is t o f s inuso ida l var ia t ion o f e lec tr i c and

m agne t ic f ie ld vectors . The f ie ld vectors v ibra te w i th the sam e f requen cy and a re

in the sa m e ph ase. The f ie ld vectors and the d i rec t ion o f propaga t ion a re a l l

m utua l ly perpend icu lar . The ve loc i ty o f e lec t rom agn et ic wave s in vacuum

depe nds upon ab so lu te perm eabi l ity oand absolute permi tt iv i ty 0.Note that c

=

00

1

23 . W hat is the nam e assoc ia ted w i th the fo llow ing equat ions?

0.)(.)(0

== SdBiiq

SdEi

+== ISdEdt

dSdBiiSdB

dt

dldEiii

000 ..)(..)(

( i) Ga uss 's law ( ii ) No par t icu lar nam e

(i ii ) Fara day's law ( iv) Am pe re's law.

24. E lec t rom agne t ic waves w i th wave length

(i) 1are use d to t reat muscu lar s tra in

(ii) 2a re used by a FM rad io s ta tion for broadcast ing

(iii) 3a re used to de tect frac ture in bon es

(iv) 4a re absorbe d by the ozone layer o f the a tmo sphere.

Ident ify and nam e the par t o f the e lec t rom agne t ic spectrum to w hich these

rad ia t ions be long. Arrang e these wave leng ths in decreas ing orde r o f magni tude.

Ans. ( i ) 1- infrared ( i i) 2- rad iowave s

(iii) 3 > X-rays ( in ) 4> ul tra-vio let rays 2> 1> 4> 3.

25 . I f you f ind c losed loops o f B in a reg ion in space , does i t necessar i ly mea n that

actua l charges are f low ing across the area bo unde d by the loops?


5/7

Ans. No t necessar i ly . A d isp lacem ent current such as that between the p la tes o f a

capac i tor that is be ing charge d can a lso prod uce loops o f B

26. A c losed loop o f B i s produ ced by a chang ing e lec t ri c f ie ld . Does i t necessar i ly

mea n tha t E anddt

dEare non -zero a t a ll po in ts on the loop and in the area

enc losed by the loop?

An s. Not nec essa ri ly. Al l that is ne ede d is that the total e lectr ic f lux throu gh the

area en c losed by the loop shou ld vary in t im e. The f lux change m ay ar ise f rom

any por t ion o f the area. E lsewh ere E ordt

dEm ay be ze ro . In par t i cu lar , there need

be n o e lec t r ic f ie ld a t the po in ts which m ake the loop.

27. W hy is it tha t induced e lec t ri c f ie lds due to chang ing ma gnet ic f lux are m ore

read i ly observab le than the induced m agne t ic f ie lds due to chang ing e lec t ri c

f ie lds?

Ans. The m agni tude o f the m agne t ic f ie ld due to d isp laceme nt current is too sm al l

to be ea s i ly observab le . Th is e f fec t can o f course be increased by increas ing the

d isp lacem ent current . [In an A C c i rcu i t, th is can be done by increas ing.]

On the o ther hand , the e f fec t o f induce d e lec t r ic f ie ld due to chan ging m agnet ic

f lux can b e increased s imp ly by tak ing m ore and m ore num ber o f tu rns in the co i l .

The induced em fs in d i f fe rent tu rns o f the sam e co i l add up in ser ies .

28. A var iab le- frequen cy AC so urce i s connected to a cap ac i tor . W i ll the

d isp lacem ent current increase o r decrease w i th increase in f requency?

Ans. Increase in f requency causes de crease in imp edan ce o f the capa c i tor and

conse quen t increase in the current wh ich equals d isp lacem ent current between

the plates.

29. Som e sc ient is ts have pred ic ted that a g loba l nuc lear war on the ear th would

be fo l lowed b y a severe 'nuc lear w in ter ' w i th a devasta t ing e f fec t on l ife on ear th .

W hat m ight be the bas is o f th is pred ic tion?

Ans. The c louds produ ced by a g loba l nuc lear war would perhaps cover

substant ia l par ts o f the s ky prevent ing so lar l igh t f rom reach ing m any par ts o f the

g lobe. Th is wo uld cause a 'w in ter '.


6/7

30. What is the contribution of the Greenhouse effect towards the surface temperature of

the earth?

Ans. The infrared radiation emitted by the earth's surface keeps the earth warm. In the

absence of this effect, the surface temperature of earth would be lower.

31. Why the small ozone layer on top of the stratosphere is crucial for human survival?

Ans. The small ozone layer on the top of the stratosphere absorbs ultraviolet radiations, y-

rays etc. from the sun. It also absorbs cosmic radiations. So, these radiations, which can

cause genetic damage to the living cells, are prevented from reaching the earth. Thus, the

small ozone layer on top of the stratosphere is crucial for human survival.

32. A plane electromagnetic wave travels in vacuum along z-direction. What can you say

about the directions of its electric and magnetic field vectors? If the frequency of the wave

is 30 MHz, what is the wavelength?

Ans. E and B lie in the x-y plane and are mutually perpendicular. Wavelength,

mm 101030

3xlO6

8

=

=

33. Given below are some famous numbers associated with electromagnetic radiation in

different contexts in physics. State the part of the e m spectrum to which each belongs.

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

(ii) 1057 MHz [frequency of radiation arising from two close energy levels in hydrogen;

known as Lamb shift].

(Hi) 2.7 K temperature associated with the isotropic radiation filling all space-thought to be

a relic of the 'big-bang origin of the universe.

(iv) 5890 A - 5896 A [double lines of sodium].

(v) 14.4 keV [energy of a particular transition in 57Fe nucleus associated with a famous

high resolution spectroscopic method (Mossbauer spectroscopy).

Ans. (i) Radio (short wavelength end) (ii) Radio (short wavelength end) (Hi) Microwave (iv)Visible (Yellow) (v) X-rays (or soft y-ray) region.

34. Electromagnetic waves in a cavity with conducting walls can exist only in certain

modes i.e., they cannot exist, for example, with any arbitrary wavelength. Suggest a

simple reason why this should be so.

Ans. The waves must satisfy a boundary condition. The electric field should be zero on the

walls of the conductor. This restricts the possible modes. [It is something like the restricted

modes of a string fixed at two ends.]


7/7

7. Write the order of frequency range and one use of the following e. m radiations.i)Microwaves ii) Ultra-violet rays iii) gamma rays

a. i) Microwaves :- 109to 1012Hz approx used in radars/microwave ovensii) Ultra violet rays:-1015 to 10 18 Hz approx. sterilizing the medical instrumentsiii) gamma rays:- 10

18to 10

22Hz in radio therapy ,treating cancer and tumours.

8. How are the following e.m radiations produced, Mention one used of them.i) radio waves ii ) infra red rays iii) x rays

a. i) radio waves :produced by oscillating circuits having inductor andcapacitor. Used in radio and TV communication.

ii)infra red rays: produced by all hot bodies treating muscular strains/ solarappliances etc.,

iii) gamma rays: produced when high energy electrons bombards metals. . to

detect fracture in bones/ study crystal structure

Documents

Em Wave Hots