Electromagnetic Waves One Shot
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INTRODUCTION
A changing electric field produces a changing magnetic field and vice versa which gives rise to a transverse
wave known as electromagnetic waves. The time varying electric field and magnetic field mutually perpendicualr
to each other also perpendicular to the direction of propagation.
Thus the electromagnetic waves consist of sinusoidally
time varying electric and magnetic field acting at right
angles to each other as well as at right angles to the
direction of propagation
HISTORY OF ELECTROMAGNETIC WAVES
* In the year 1865, Maxwell predicted the electromagnetic waves theoretically. According to him, an accelerated
charge sets up a magnetic field in its neighborhood.
* In 1887, Hertz produced and detected electromagnetic waves experimentally at wavelength of about 6m.
* Seven year later, J.C. Bose became successful in producing electromagnetic waves of wavelength in the range
5mm to 25mm.
* In 1896, Marconi discovered that if one of the spark gap terminals is connected to an antenna and the other
terminal is earthed, the electromagnetic waves radiated could go upto several kilometers.
* The antenna and the earth wiresfrom the two plates of a capacitor which radiates radio frequency waves.
These waves could be received at a large distance by making use of an antenna earth system as detector.
* Using these arrangements, in 1899 Marconi first established wireless communication across the English channel
i.e., across a distance of about 50 km.
CONCEPT OF DISPLACEMENT CURRENT
When a capacitor is allowed to charge in an electric circuit, the current flows through connecting wires. As
capacitor charges, charge accumulates on the two plates of capacitor and as a result, a changing electric field is
produced across between the two plate of the capacitor.
According to maxwell changing electric field intensity is equivalent to a current through capacitor that current is known as displacement
current (Id). If + q and – q be the charge on the left and right plates of
the capacitor respectively at any instant if s be the surface charge
density of plate of capacitor the electric field between the plates
MAXWELL'S EQUATION
HERTZ EXPERIMENT
(Practical production of EM waves)
* In 1888, Hertz demonstrated the production of electromagnetic
waves by oscillating charge. His experimental apparatus is shown
schematically in fig.
* An induction coil is connected to two spherical electrodes with a
narrow gap between them. It acts as a transmitter. The coil provides
short voltage surges to the spheres making one positive and the
other negative. A spark is generated between the spheres when
the voltage between them reaches the breakdown voltage for air.
As the air in the gap is ionised, it conducts more rapidly and the
discharge between the spheres becomes oscillatory.
* The above experimental arrangement is equivalent to an LC circuit,
where the inductance is that of the loop and the capacitance is
due to the spherical electrodes.
* Electromagnetic waves are radiated at very high frequency (» 100 MHz) as a result of oscillation of free charges
in the loop.
* Hertz was able to detect these waves using a single loop of wire with its own spark gap (the receiver).
* Sparks were induced across the gap of the receiving electrodes when the frequency of the receiver was adjusted to match that of the transmitter.
PROPERTIES OF ELECTROMAGNETIC WAVES
* The electric and magnetic fields satisfy the following wave equations, which can be obtained from Maxwell's third and fourth equation
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