EM Waves Flashcards
logical inconsistency in changing electric and magnetic field
We have seen in Chapter 4 that an electrical current
produces a magnetic field around it. Maxwell showed
that for logical consistency, a changing electric field must
also produce a magnetic field. This effect is of great
importance because it explains the existence of radio
waves, gamma rays and visible light, as well as all other
forms of electromagnetic waves.
types of current
conduction current is the current due to the flow of charges
displacement current is the missing term in ampere’s circuital law and it is due the changing electric flux.
both these currents act as source of magentic field
what is displacement current
Displacement current is a current which is produced due to the rate of
change of electric flux with respect to time.
does not exist where electric flux is not changing
satisfies the property of continuity
Maxwell’s equations in vaccum
(Gauss’s Law for electricity)
1. ∮E.dA = Q/ε
(Gauss’s Law for magnetism)
2. ∮B.dA = 0
(Faraday’s Law)
3. ∮E.dl = –dΦB/dt
(Ampere – Maxwell Law)
4. ∮B.dl =µI + µ ε dΦe/dt
source of em waves
It is an important result of Maxwell’s theory that accelerated charges
radiate electromagnetic waves.
An accelerating charge produces both electric and magnetic field which
varies with space and time that forms an Electromagnetic Wave.
An EM Wave will be produced during the deexcitation of electrons from
one orbit to another orbit.
An EM Wave will be produced during the deexcitation of nucleons in
radioactivity
em waves are those waves in whcih there are sinusoidal variations of magnetic and electric field perpendicular to each other as well as to the direction of propogation of the wave.
characters of EM waves
- electromagnetic waves are produced by accelerated ir oscillating charge
- do not require material medium for propogation
-travel in free space with the speed of light
c=1/√μ∘ε∘
-In these waves E and B vary sinusoidally in the direction perpendicular to
each other and also perpendicular to the direction of propagation.
-The phase difference between the two fields is zero.
- the waves are transverse in nature.
-waves in dielectric is less than speed of light
c=1/√με
- they are not charged
energy of an electromagnetic waves
The waves carry energy which is divided equally between the electric field and magnetic field vectors.
what is meant by radiation pressure
em waves carry energy and momentum.
force exerted by an em wave on a unit area of a surface is called radiation pressure.
em spectrum
the orderly distrubution of the electromagnetic radiations according to their wavelength or frequncy;
radio waves
i) production
ii) frequency
iii)wavelength
iv) uses
i) produced by rapid acceleration and decelerationof electrons in conducting wires ( aerials)
ii) 500kHz- 1000MHz
iii) > 0.1m
iv)radioastronomy
radar and television broadcasting
telephone commmunication
marine and navigation use
they are detected by receiver’s aerials
give the frequency and uses of the following radiowaves
i)AM
ii) SHort wave
iii)TV waves
iv)FM
v)UHF
i)The AM (amplitude modulated) band is from 530 kHz to 1710 kHz.( ground wave propogation)
(ii)Higher frequencies upto 54 MHz are used for short wave bands.(sky wave propogation)
iii)TV waves range from 54 MHz to 890 MHz.
iv)The FM (frequency modulated) radio band
extends from 88 MHz to 108 MHz. (commerical FM radio)
v)Cellular phones use radio waves to
transmit voice communication in the ultrahigh frequency (UHF) band( 300MHz-3000MHz)
microwaves:
i) frequency
ii)wavelength
iii)production
iv)detection
i) 10⁹ - 3x10¹¹ Hz (!GHz- 300GHz)
ii)0.1m to 1mm
iii)are produced by special vacuum tubes (called
klystrons, magnetrons and Gunn diodes).
iv)Point contact diodes
uses of microwaves
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.
explain the working of microwave ovens
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.
this is more efficient than normal heating because there is no significant loss of energy. such as conduction losses
