EM Waves Flashcards
Displacement Current
Rate of change of electric displacement field
dΦ/dt =
i(d)/ε0
What is EMW?
Accelerated motion of charge
i(c) = 0 ____ the conductor
inside
i(d) = 0 _______ the conductor
outside
Ampere maxwell law
The total current passing through any surface of which the closed loop is the perimeter is the sum of conduction & displacement current
E0 is called
amplitude of Electric Field
Relation btn EF and MF
C = E0/B0 = 1/(ε0µ0)^1/2
Speed of EM wave in medium
V = 1/(µε)^1/2
Speed of EM wave in vacuum is
C = w/K = nyu lamda
Energy density with EF
U(e) = 1/2 ε E^2
Energy denstiy with MF
U(b) = ½B²/µ⁰
Intensity of Em wave
the energy crossing per unti area per unti time in direction perp to direction of propagation
Intensity I =
UC = 1/2 B0^2C/µ0
Momentum of EM wave P =
U/C
Radiation pressure
When EM wave falls on a surface, it exerts pressure on it
P(r) =
I/C
Properties of Em waves
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
Radio waves
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
Microwaves
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
Infra red waves
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
Visible rays
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
UV rays
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
X rays
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
Gamma rays
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
Long distance radio broadcasts use short wavebands?
They are reflected by the ionosphere
Is it necessary to use satellites for long distance TV transmission. Why?
Yes, because TV signals are of high frequency
Optical & radio telescopes are built on the ground but X ray astronomy is possible only for satellites orbiting the earth. Why?
They use optical & radiowaves which can penetrate the atmosphere but X rays are of smaller wavelength & are absorbed by the atmosphere.
Small ozone layer on top of stratosphere is crucial for human survival. why?
It absorbs UV radiations from the sum which are dangerous & cause genetic damage to the living cells.
If earth didn’t have atmosphere, would its average surface temp be higher or lower than it is now?
Yes lower due to green house effect of atmosphere would be absent
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?
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.
