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providing the stical prehe ther. Such a wave is called an electromagnetic ave

Maxwell theoretically predicted the existance of such wave in 1865 but had to wait for more

existence.

than twenty years before Hertz in 1888 succeeded in experimentally confirming their electromagnetic radiation.

We have already seen. that an oscillating electric charge radiates out

The radiation carries energy and this energy is being supplied at the cost of kinetic energy of the oscillating charge. Detailed calculation, (beyond the scope of our present level of discussion) however tell us that the radiation is appreciable only if the extent to which the-charge oscillates is comparable to the wavelength of the radiation. Thus, a charge oscillating with a frequency of 1000 c/s (which certainly can be produced by mechanical means) would radiate e.m. waves of wavelength (3 × 10%103 = 300 km.

We would thus require the charges oscillating over a distance 300 km in order to radiate sufficient amount of energy. This realization came to Hertz, who then designed a system of

## Given Figures are:

oscillating charges of much higher frequency, so that the system could be used in a laboratory.

connected to a

a schematic representation of Hertz’s set-up. The two metal sheets are source of high voltage. The voltage is high enough such that the air in the

small gap between the plates gets ionized and provides a path for discharge of the plates.

The plate arrangement clearly gives a very low value of L, so that according to our result w = 1//LC. Very high frequency oscillations of charges on the plates will result. With the kind of arrangement just outlined, Hertz was able to produce electromagnetic waves of wavelength around 6 m. The detectors also shown in our schematic diagram. It is held in a position such that the magnetic field produced by the oscillating current is perpendicular to the plane of the coil. The resultant electric field, induced by the oscillating magnetic field causes sparks to appear at the narrow gap.

The successful demonstration of electromagnetic waves created a sensation and sparked of other important achievements.

deserve mention.

## Gap

Seven years after Hertz, Jagadish Chandra Bose,

To junction core

working at Calcutta, succeeded in producing and observing electromagnetic waves of much shorter*

## Director

wavelength (25mm to 5mm). His experiments, like Hertz’s, were confined to the laboratory. At around the same time Guglielmo Marconi, in Italy, followed Hertz’s work but succeeded in transmitting

Figere Schematic diagram of Hertz’s experimental

electromagnetic waves over distances of many

set up. The metal plates are charged to a high voltage by an induction coil. When the voltage is

miles. The oscillator used by Marconi was similar sufficiently. high, the plates get discharged by but different in detail compared to Hertz’s; one

sparking across the narrow gap.

metal plate was atop a pole connected to a similar metal plate on the surface of the earth. Marconis experiments mark the beginning of the field of

communications using electromagnetic

Figure shows the variation of B with applied field H when the specimen is taken through a complete cycle of B and H. After the specimen has become saturated, and the field is reduced to zero, the iron is still quite strongly magnetised, setting up a flux-density Br. This flux-density is called the remanence; it is due to the tendency of groups of molecules, or domains, to stay, once they have been aligned.

## When the field is reversed,

the residual magnetism is opposed. Each increase of magnetising

field now causes a decrease of flux-density, as the domains are twisted farther out of alignment. Eventually, the flux-density is reduced to zero; when the opposing field H has the value He. This value of H is called the coercive force of the iron; it is a measure of the difficulty of breaking up the alignment of the domains.

We now see that, when once the iron has been magnetised, its magnetisation curve never passes through the origin again. Instead, it forms the closed loop PQRS, which is called a hysteresis-loop. Hysteresis, which comes from a Greek word meaning ‘delayed’, can be defined as the lagging of B behind the magfletising field, H. when the specimen is taken through a magnetic cycle. The area of the hysteresis loop is proportional to the thermal energy developed per unit volume of the material as it goes through the hysteresis cycle.

horizontal wire, of length 5 cm and carying a curent of 2 A, is placed in the middle of a long solenoid at right angles to its axis. the solenoid has 1000 turns per metre and carrying a steady crrrent I. Calculate I if the nore has he w turns vertically downwards and equal to 10-4 N.

Two vertical parallel conductors X and Y are 0.12 m apart and carry currents of 2A and 4A respectively in a downward direction (Given figure). (a) Ignoring the earth’s magnetic find, fad the distance Ba of a te where me i and du

and show their direction in a sketch.

A horizontal straight wire 5 cm long weighing 1.2 g m-1 is placed perpendicular to a uniform horizontal magnetic field of flux density 0.6 T. If the resistance of the wire is 3.8 02 m-1

, calculate the p.d, that has to be applied between the ends of the

wire to make it just self-supporting;

DIC

A straight horizontal rod X, of mass 50 g ad length 0.5 m, is placed in a uniform horizontal magnetic field of 0.2 t perpendicular to. X. Calculate the current inb X if the force acting on it just balances its weight.

A narrow vertical rectangular coil is suspended from the middle of its upper side with its plane parallel to a uniform horizontal magnetic field of 0.02 I. The coil has 10 turns, and the lengths of its vertical and horizontal sides are 0.1 m and m respectively. Calculate the torque on the coil when a current of 5A is

passed into it.

A horizontal rod PQ, of mass 10 g and length 0.10 m, is placed on a smooth plane inclined at 60° to the horizontal, as shown in figure.

A uniform vertical magnetic field of value B is applied in the region of PQ. Calculate

magnetic induction at the centre due to the current in the ring is

- proportional to 2 (180 – 0)
- inversely proportional to r
- zero only if e = 180°
- zero for all values of e

Two wires of same length are shaped into a square and a circle. If they carry same current, the ratio of the magnetic moments is

(a) 2: T (b)r: 2 (c) : 4 (d) 4: T

An electron is projected along the axis of a circular conductor carrying some current. Electron will experience force

- along the axis
- perpendicular to the axis

(C) at an angle of 4° with axis

(d) no force experienced

A current I is flowing through a circular coil of radius r and a current of 21 is flowing through a circular coil of radius 2r, then the ratio of their magnetic fields at the centre of the coil is

A current i ampere flows along an infinitely long straight thin walled tube, then the magnetic induction at any point inside the tube

## INTRODUCTION :

The discussion in this chapter is based on the rectilinear propagation of light which appears to be in apparent contraction with the fact that light is an electromagnetic chappene answer to this question is — light waves have very small wavelength (~ 10-7 m) in comparison to the size of the obstcles they face (~ 10-3 m). In this situation light wave can be considered to travel from one point to another along a straight line path joining them. This path is called a ray of light and a bundle of such rays constitutes a beam of light.

## REFLECTION OF LIGHT BY SPHERICAL SURFACES :

The laws of reflection for plane surfaces hold good equally in case of reflection on spherical surfaces. The laws are

The angle of incidence (angle between incident ray and normal to the reflecting surface at the ponit of incidence) is equal to the angle of reflection (angle between reflected ray and the normal)

The incident ray, the reflected ray and the normal to the reflecting surface at the point of incidence, all lie in the same plane.

The normal to the reflecting surface is along the radius of the spherical surface, the line joining the centre of curvature of the surface to the point of incidence.

## 3. SOME DEFINITIONS:

Image: A point is said to be the image of a ponit object, if the rays coming from the pont object, after reflection or refraction, actually converge to or appear to diverge from the point. The image is real if the rays actually converge to the point and virtual if they diverge from that point.

A spherical mirror is a part of a glass sphere silvered on the convex side (concave mirror) or concave side (convex mirror).

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