EMW & Communication

Displacement Current -

In Ampere's law, according to Maxwell opinion –

z  
B . dl = 0i + ?

Maxwell called this missing term as displacement current.

Electric field between plates of capacitor

 Q
E =    A  Q = 0(EA)
0 0

E = EA = Electrix flux , Q = 0E

dQ d E dE
I=  ID = 0 = 0A
dt dt dt

This current is called displacement current.

Ampere-Maxwell law –
z  
B . dl = (ic + iD)

ic = Conduction current (flows in conducting wire)

iD = Displacement current (between plates of capacitor)

Maxwell's equations -

1. z  
E. dA 
q
0 (Gauss's law for electricity) 2. z  
B. dA = 0 (Gauss's law for magnetism)

3. z   d
E. dl   B (Faraday's law)
dt
4. z  
B. dl =  (i + i ) (Ampere-Maxwell law)
0 c D

Charactristics of EMW's -
A variable electric field produces variable magnetic field (Ampere–Maxwell law) and variable magnetic field produces
variable electric field (Faraday's law). In LC osillator charge oscilates between plates of capacitor which produces time
variable sinusoidal electric and magnetic field and their frequency is given by –
1
f = (for L – C oscillator)
2 LC
1. The EMW's consists of sinusoidally time varying electric field and magnetic field at right angle to each other as well as at
right angle to the direction of propagation.

Eg = E0 sin (t – kx), along y-axis

B = B0 sin (t – kx), along z-axis

Propagation of ware is along x-axis.

2 2
= = 2f and K =
T 
 
2. Direction of propagation of EMW is given by direction of E  B

3. At large distance variation of electric field and magnetific field are in phase.

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 EMW & Communication
  E
4. For EMW's relation between magnitude of E and B – C=
B

5. Velocity of EMW's in vacuum Velocity in medium

1 1 c c
C=  3  108 m/sec Cm   
r  0 r  0  r r n
 0 0

where n =  r  r = Retrative index of medium.

6. EMW's are tranverse wave and non mechanical waves
7. EMW's possess energy and they can carry energy from one place to other. EMW's carry energy from sun to earth
8. EMW's exert pressure on a surface when they incident on it called radiation presure. If U is the energy of EMW incidenton
a surface then momentum deliverd to this surface

U U
P  (for complete absorption) P  2 (for complete reflection)
C C
9. For EMW's the electromagnetic energy per unit volume

1 B2
u = uE +uB =  0E 2 
2 2 0

1 E 1 1 B2
Now use c = c   0  2  2  u   0E 2 
0 0 B  0 c  0c 0

  Electromagnetic waves carry energy. The rate of flow of energy crossing a unit area is described by the Poynting vector S .
 1  
Where S   E  B
0

Hint to remember EMW – Rahul's Mother Is Visiting Uncle Xavior Garden.]

Type Wavelength Range Production Application

Rapid acceleration and decelerations In radio and T.V. communaction
system,Celluar phones
Radio > 0.1 m ofelectrons of elecrons in aerials
Klystron, magnetron valve, Gun diode In aircraft and satellite communication
Microwave 0.1 m to 1mm and to cook food (microwave oven)
Vibration of atoms and molecules In Physiotherapy, green house, military
Infra-red 1 mm to 700 mm purpose and in agriculture
Electrons in atoms emit light when the
move from one energy level to a lower
Visible Light 700 nm to 400 nm energy level
Inner shell electrons in atoms moving In welding , eye surgery and to kill
Ultraviolet 400 nm to 1nm from one energy level to a lower level germs
X-ray tubes or inner shell elecrons of To destroy living tissues and organisms
X-rays 1 nm to 1 pm atom
Gamma rays <1 pm Radioactive decay of the nucleus To destroy cancer cells

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 EMW & Communication
Radio waves -
Radio waves are produced by the accelerated motion of charges in conducting wires. They are used in radio and television
communication systems. They are generally in the frequency range from 500 kHz to about 1000 MHz. The AM (amplitude
modulated) band is from 530 kHz to 1710 kHz. Higher frequencies upto 54 MHz are used for short wave bands. TV waves
range from 54 MHz to 890 MHz. The FM (frequency modulated) radio band extends from 88 MHz to 108 MHz. Cellular
phones use radio waves to transmit voice communication in the ultrahigh frequency (UHF) band.
Microwaves-
Microwaves (short-wavelength radio waves), with frequencies in the gigahertz (GHz) range, are produced by special
vacuum tubes (called klystrons, magnetrons and Gunn diodes). Due to their short wavelengths, they are suitable for the
radar systems used in aircraft navigation. Radar also provides the basis for the speed guns used to time fast balls, tennisserves,
and automobiles. Microwave ovens are an interesting domestic application of these waves. In such ovens, the frequency of
the microwaves is selected to match the resonant frequency of water molecules so that energy from the waves is transferred
efficiently to the kinetic energy of the molecules. This raises the temperature of any food containing water.
Infrared waves-
Infrared waves are produced by hot bodies and molecules. This band lies adjacent to the low-frequency or long-wave
length end of the visible spectrum. Infrared waves are sometimes referred to as heat waves. This is because water
molecules present in most materials readily absorb infrared waves (many other molecules, for example, CO 2, NH3, also
absorb infrared waves). After absorption, their thermal motion increases, that is, they heat up and heat their surroundings.
Infrared lamps are used in physical therapy. Infrared radiation also plays an important role in maintaining the earth’s
warmth or average temperature through the greenhouse effect. Incoming visible light (which passes relatively easily
through the atmosphere) is absorbed by the earth’s surface and reradiated as infrared (longer wavelength) radiations. This
radiation is trapped by greenhouse gases such as carbon dioxide and water vapour. Infrared detectors are used in Earth
satellites, both for military purposes and to observe growth of crops. Electronic devices (for example semiconductor light
emitting diodes) also emit infrared and are widely used in the remote switches of household electronic systems such as TV
sets, video recorders and hi-fi systems.
Visible rays-
It is the most familiar form of electromagnetic waves. It is the part of the spectrum that is detected by the human eye. It
runs from about 4 × 1014 Hz to about 7 × 1014 Hz or a wavelength range of about 700 –400 nm. Visible light emitted or
reflected from objects around us provides us information about the world. Our eyes are sensitive to this range of wavelengths.
Different animals are sensitive to different range of wavelengths. For example, snakes can detect infrared waves, and the
‘visible’ range of many insects extends well into the utraviolet.
Ultraviolet rays-
It covers wavelengths ranging from about 4 × 10–7 m (400 nm) down to 6 × 10–10m (0.6 nm). Ultraviolet (UV) radiation is
produced by special lamps and very hot bodies. The sun is an important source of ultraviolet light. But fortunately, most of
it is absorbed in the ozone layer in the atmosphere at an altitude of about 40 – 50 km. UV light in large quantities has
harmful effects on humans. Exposure to UV radiation induces the production of more melanin, causing tanning of the skin.
UV radiation is absorbed by ordinary glass. Hence, one cannot get tanned or sunburn through glass windows. Welders wear
special glass goggles or face masks with glass windows to protect their eyes from large amount of UV produced by welding
arcs. Due to its shorter wavelengths, UV radiations can be focussed into very narrow beams for high precision applications

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 EMW & Communication
such as LASIK (Laserassisted in situ keratomileusis) eye surgery. UV lamps are used to kill germs in water purifiers. Ozone
layer in the atmosphere plays a protective role, and hence its depletion by chlorofluorocarbons (CFCs) gas (such as freon)
is a matter of international concern.
X-rays-
Beyond the UV region of the electromagnetic spectrum lies the X-ray region. We are familiar with X-rays because of its
medical applications. It covers wavelengths from about 10–8 m (10 nm) down to 10–13 m (10–4 nm). One common way to
generate X-rays is to bombard a metal target by high energy electrons. X-rays are used as a diagnostic tool in medicine and
as a treatment for certain forms of cancer. Because X-rays damage or destroy living tissues and organisms, care must be
taken to avoid unnecessary or over exposure.
Gamma rays-
They lie in the upper frequency range of the electromagnetic spectrum and have wavelengths of from about 10–10m to less
than 10–14m. This high frequency radiation is produced in nuclear reactions and also emitted by radioactive nuclei. They are
used in medicine to destroy cancer cells. As mentioned earlier, the demarcation between different region is not sharp and
there are over laps.

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