Syllabus: Unit V Electromagnetic Waves: Basic idea of displacement current, Electromagnetic waves, their characteristics, their transverse nature (qualitative idea only). Electromagnetic spectrum (radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays) including elementary facts about their uses.
Qn. What is the need of displacement current?
Qn. Write an expression for displacement current.
Qn. Write generalised (and correct) Ampere’s circuital law Or Ampere-Maxwell law.
Qn. How are electromagnetic waves produced?
Answer: Consider a charge oscillating with some frequency. An oscillating charge is an example of accelerating charge. This produces an oscillating electric field in space, which produces an oscillating magnetic field, which in turn, is a source of oscillating electric field, and so on. The oscillating electric and magnetic fields thus regenerate each other as the wave propagates through space.
Qn. What decides the frequency of EM waves?
Answer: The frequency of the electromagnetic wave equals the frequency of oscillation of the charge.
Qn. Write the source of energy of EM waves.
Answer: The energy associated with the propagating wave comes at the expense of the energy of the source that is, the accelerated charge.
Qn. Write the nature of electromagnetic waves and its equation and give a schematic diagram of EM waves.
Answer: EM waves are transverse in nature. Electric and magnetic fields in an electromagnetic wave are perpendicular to each other, and to the direction of propagation.
Equation for the EM wave shown in the figure is:
Ex= E0 sin (kz–ωt)
By= B0 sin (kz–ωt)
Here k is related to the wavelength λ of the wave by the usual equation k=2π/λ and ω is the angular frequency. k is the magnitude of the wave vector (or propagation vector) and its direction describes the direction of propagation of the wave.
Qn. Write properties of EM waves.
Answer:
I) the velocity of light in vacuum c = 1/(√€0µ0 )
and in a medium v= 1/(√€0µ0 )
II) The ratio of amplitudes of fields Eo/Bo = c
III) no material medium is involved in the vibrations of the electric and magnetic fields.
IV) EM waves can be polarised.
V) EM waves obey laws of reflection and refraction.
VII) EM waves show interference and diffraction.
VIII) electromagnetic waves carry energy and momentum.
IX) an electromagnetic wave also exerts pressure, called radiation pressure.
ELECTROMAGNETIC SPECTRUM
Electromagnetic waves include visible light waves, X-rays, gamma rays, radio waves, microwaves, ultraviolet and infrared waves.
The classification of em waves according to frequency is called electromagnetic spectrum.
Radio waves:
Radio waves are produced by the accelerated motion of charges in conducting wires.
They are used in radio and television communication.
the frequency ranges from 500 kHz to 1000 MHz.
The AM (amplitude modulated) band is from 530 kHz to 1710 kHz.
Higher frequencies up to 54 MHz are used for short wave bands.
TV waves range from 54 MHz to 890 MHz.
The FM radio band extends from 88 MHz to 108 MHz.
Cellular phones use ultrahigh frequency (UHF) band
Microwaves:
Microwaves with frequencies in the gigahertz (GHz) range, are produced by klystrons, magnetrons and Gunn diodes.
they are suitable for the radar systems used in (a)aircraft navigation, (b)speed guns used to time fast balls, tennis serves, and automobiles.
They are used in satellite communication.
Microwave ovens where the frequency of the microwaves is selected to match the resonant frequency of water molecules.
Infrared or Heat waves:
Infrared waves are produced by hot bodies and molecules. 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 by the greenhouse effect.
Infrared detectors are used in earth satellites, both for military purposes and to observe growth of crops.
Infrared waves are widely used in the remote switches of tv sets, and video recorder.
Visible rays:
It is detected by the human eye.
wavelength range of about 700 –400 nm.
Visible light emitted or reflected from objects around us provides us information about the world.
Ultraviolet Rays:
wavelengths range from 400 nm down to 0.6 nm.
Ultraviolet (UV) radiation is produced by special lamps, welding arcs and very hot bodies.
The sun is an important source of ultraviolet light.
Ultraviolet rays are used in LASIK (Laser assisted in situ keratomileusis) eye surgery.
UV lamps are used to kill germs in water purifiers.
They are used in alarm circuits-burglar alarm
They are used to check fake currency.
X Rays:
It covers wavelengths from about 10 nm down to 4 nm).
One common way to generate X-rays is to bombard a metal target with high energy electrons.
X-rays are used as a diagnostic tool in medicine such as bone fracture and as a treatment for certain forms of cancer.
X-rays are used at airports to check luggage.
Gamma rays:
They have wavelengths 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.
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