EM Waves

Question 1

Displacement Current

Answer 1

Rate of change of electric displacement field

Question 2

dΦ/dt =

Answer 2

i(d)/ε0

Question 3

What is EMW?

Answer 3

Accelerated motion of charge

Question 3

i(c) = 0 ____ the conductor

Answer 3

inside

Question 4

i(d) = 0 _______ the conductor

Answer 4

outside

Question 4

Ampere maxwell law

Answer 4

The total current passing through any surface of which the closed loop is the perimeter is the sum of conduction & displacement current

Question 4

E0 is called

Answer 4

amplitude of Electric Field

Question 5

Relation btn EF and MF

Answer 5

C = E0/B0 = 1/(ε0µ0)^1/2

Question 6

Speed of EM wave in medium

Answer 6

V = 1/(µε)^1/2

Question 7

Speed of EM wave in vacuum is

Answer 7

C = w/K = nyu lamda

Question 8

Energy density with EF

Answer 8

U(e) = 1/2 ε E^2

Question 9

Energy denstiy with MF

Answer 9

U(b) = ½B²/µ⁰

Question 10

Intensity of Em wave

Answer 10

the energy crossing per unti area per unti time in direction perp to direction of propagation

Question 11

Intensity I =

Answer 11

UC = 1/2 B0^2C/µ0

Question 12

Momentum of EM wave P =

Answer 12

U/C

Question 13

Radiation pressure

Answer 13

When EM wave falls on a surface, it exerts pressure on it

Question 14

P(r) =

Answer 14

I/C

Question 15

Properties of Em waves

Answer 15

Produced by accelerated charges and don’t require medium for propagation
Directions of oscillations of E & B are perp to each other as well as direction of propagation
Transverse in nature
Oscillations of E & B are in same phase
Travel in free space with same speed
Carry energy as they travel through space and it is shared equally by E & B
Transport linear momentum as they travel
Exert radiation pressure
Not deflection by E or B
Their E.F is responsible for its optical effects because E0»B0
Direction of propagation is E X B

Question 16

Radio waves

Answer 16

Longest wavelength & least freq
Produced by accelerated motion of charges
600m to 0.1m
500kHz to 1000MHz
Used in radio & television communication systems, radio astronomy

Question 17

Microwaves

Answer 17

Wavelength next smaller to radiowaves
Produced by special vacuum tubes like klystrons, magnetrons & Gunn diodes
0.1m to 1mm
10⁹ to 10¹²Hz
Used in radar systems for aircraft navigation, long distance communication via geostationary satellites

Question 18

Infra red waves

Answer 18

Lie close to low freq & long wave
Produced by hot bodies, sun
1mm to 700nm
10¹¹ to 5×10¹⁴Hz
Used in remote control of TV or VCR, treatment for muscular complaints, earth satellites, observe growth of crops, haze photography

Question 19

Visible rays

Answer 19

Small part of the spectrum
Produced by sun, candle, filament lamp
700 to 400nm
4×10¹⁴ to 7×10¹⁴Hz
Used to provide info of the world to us, causes chemical rns

Question 20

UV rays

Answer 20

Wavelength shorter than V.L
Produced by sun, high voltage gas discharge tubes
400 to 1nm
10¹⁶ to 10¹⁷Hz
Used for food preservation, study of invisible writings, forged docs, fingerprints, study of molecular structure, LASIK eye surgery, water purifiers to kill germs

Question 21

X rays

Answer 21

Shorter wavelength than UV
Produced by sudden deceleration of fast moving e’s by a metal
1 to 1/10³nm
10¹⁸ to 10²⁰Hz
Used in medical fields for cancer, check fractures, study crystal structure, detect explosives & holes in the finished metal products

Question 22

Gamma rays

Answer 22

Highest freq & lowest wave
Produced by radioactive nuclei, nuclear rns
1/10¹⁴ to 1/10¹⁰m
10¹⁸ to 10²²Hz
Used in radiotherapy for cancer, initiate nuclear rns, preserve food stuffs, study structure of atomic nuclei

Question 23

Long distance radio broadcasts use short wavebands?

Answer 23

They are reflected by the ionosphere

Question 24

Is it necessary to use satellites for long distance TV transmission. Why?

Answer 24

Yes, because TV signals are of high frequency

Question 25

Optical & radio telescopes are built on the ground but X ray astronomy is possible only for satellites orbiting the earth. Why?

Answer 25

They use optical & radiowaves which can penetrate the atmosphere but X rays are of smaller wavelength & are absorbed by the atmosphere.

Question 26

Small ozone layer on top of stratosphere is crucial for human survival. why?

Answer 26

It absorbs UV radiations from the sum which are dangerous & cause genetic damage to the living cells.

Question 27

If earth didn’t have atmosphere, would its average surface temp be higher or lower than it is now?

Answer 27

Yes lower due to green house effect of atmosphere would be absent

Question 28

Some scientists have predicted that a global nuclear war on the earth would be followed by a severe nuclear winter with devastating effect on life on earth. what might be the basis of this prediction?

Answer 28

Clouds produced by a global nuclear war would perhaps cover most parts of the sky preventing solar light from reaching many parts of globe. This causes winter.