What are Electromagnetic Waves?
Electromagnetic waves are defined as:” Waves which requires no medium for transmission and which rapidly propagates through vacuum.”radio waves,television waves,X-rays light rays etc,are the examples of electromagnetic waves.
In 1864 British physicist James Clark Maxwell formulated a set of equations known as Maxwell’s equations which explained the various electromagnetic phenomena. According to these equations, a changing magnetic flux creates an electric field and a changing electric flux creates a magnetic field. Suppose a change of magnetic flux is taking place through it. This changing magnetic flux will set up a changing electric flux in the surrounding region. The creation of electric field in the region, CD will cause a change of electric flux through it due to which a magnetic field would be set up in the space surrounding CD and so on. Thus each field generates the other and the whole package of electric and magnetic field will move along propelling itself through space. Such moving electric and magnetic fields are known as electromagnetic waves. The electric field, magnetic field and the direction of their propagation are mutually orthogonal. It can be seen in the figure that the electromagnetic waves are periodic, hence very have a wavelength λ which is given by the relation c=fλ where f is the frequency and c is the speed of the wave. In free space the speed of electromagnetic waves 3×108 ms-1.
Depending upon the value of wavelength of frequency, the electromagnetic waves have been classified into different types of waves as radio waves, microwaves, infrared rays, visible light etc. The complete spectrum of electromagnetic waves from the low radio waves to high frequency gamma rays.
Electromagnetic waves from the low radio waves to a high frequency gamma rays.
Principle of Generation,Transmission and Reception of Electromagnetic Waves:
We have seen that electromagnetic waves are generated when electric or magnetic flux is changing through a certain region of space. An electric charge at rest gives rise to a Coulomb’s field which does not radiate in space because no change of flux takes place in this type of field. A charge moving with constant velocity is equivalent to a steady current which generates a constant magnetic field in the surrounding space, but such a field also does not radiate out because no changes of magnetic flux are involved. Thus only chance to generate a wave by moving field is when we accelerate the electrical charges.
A radio transmitting antenna provides a good example of generating electromagnetic waves by acceleration of charges. The piece of wire along which charges are made to accelerate is known as transmitting antenna. It is charged by an alternating source of potential of frequency f and time period T. As the charging potential alternates, the charge on the antenna also constantly reverses. For example if the top has + q charges at any instant, then after time T/2 the charge on it will be – q. Such regular reversal of charges on the antenna gives rise to an electric flux that constantly changes with frequency f. This changing electric flux sets up an electromagnetic wave which propagates out in space away from the antenna. The frequency with which the fields alternate is always equal to the frequency of the source generating them. Theses electromagnetic waves which are propagated out in space from antenna of a transmitter are known as radio waves. In free space these waves travel with the speed of light.
Suppose these waves impinge on a piece of wire. The electrons in the wire move under the action of the oscillating electric field which give rise to an alternating voltage across the wire. The frequency of this voltage is the same as that of the wave intercepting the wire. This wire receiving the wave is known as receiving antenna. As the electric field of the wave is very weak at a distance of many kilometers from the transmitter, the voltage that appears across the receiving antenna is very small. Each transmitter propagates radio waves of one particular frequency. So when a number of transmitting stations operate simultaneously, we have a number of radio waves of different frequencies in the space. Thus the voltage that appears across a receiving antenna placed in space in usually due to radio waves of large number of frequencies. The voltage of one particular frequency can be picked up by connecting an inductance L and a variable capacitor C in parallel with one end of the receiving antenna.
If one adjusts the value of the capacitor so that the nature frequency of L – C circuit is the same as that of the transmitting station to be picked up, the circuit will be resonate under the driving action of the antenna. Consequently, the C circuit build up a large response of the action of only at radio wave to which it is tuned. In their radio receiver set when you change stations you actually adjust the value of C.