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www.GOALias.blogspot.comPhysicsJAMES CLERK MAXWELL (1831–1879)270James Clerk Maxwell(1831 – 1879) Born inEdinburgh, Scotland,was among the greatestphysicists of thenineteenth century. Hederived the thermalvelocity distribution ofmolecules in a gas andwas among the first toobtain reliableestimates of molecularparameters frommeasurable quantitieslike viscosity, etc.Maxwell’s greatestacheivement was theunification of the laws ofelectricity andmagnetism (discoveredby Coulomb, Oersted,Ampere and Faraday)into a consistent set ofequations now calledMaxwell’s equations.From these he arrived atthe most importantconclusion that light isan electromagneticwave. Interestingly,Maxwell did not agreewith the idea (stronglysuggested by theFaraday’s laws ofelectrolysis) thatelectricity wasparticulate in nature.the speed of light( 3 ×10 8 m/s), obtained from opticalmeasurements. This led to the remarkable conclusionthat light is an electromagnetic wave. Maxwell’s workthus unified the domain of electricity, magnetism andlight. Hertz, in 1885, experimentally demonstrated theexistence of electromagnetic waves. Its technological useby Marconi and others led in due course to therevolution in communication that we are witnessingtoday.In this chapter, we first discuss the need fordisplacement current and its consequences. Then wepresent a descriptive account of electromagnetic waves.The broad spectrum of electromagnetic waves,stretching from γ rays (wavelength ~10 –12 m) to longradio waves (wavelength ~10 6 m) is described. How theelectromagnetic waves are sent and received forcommunication is discussed in Chapter 15.8.2 DISPLACEMENT CURRENTWe have seen in Chapter 4 that an electrical currentproduces a magnetic field around it. Maxwell showedthat for logical consistency, a changing electric field mustalso produce a magnetic field. This effect is of greatimportance because it explains the existence of radiowaves, gamma rays and visible light, as well as all otherforms of electromagnetic waves.To see how a changing electric field gives rise toa magnetic field, let us consider the process ofcharging of a capacitor and apply Ampere’s circuitallaw given by (Chapter 4)“B . dl = μ 0i (t) (8.1)to find magnetic field at a point outside the capacitor.Figure 8.1(a) shows a parallel plate capacitor C whichis a part of circuit through which a time-dependentcurrent i (t) flows . Let us find the magnetic field at apoint such as P, in a region outside the parallel platecapacitor. For this, we consider a plane circular loop ofradius r whose plane is perpendicular to the directionof the current-carrying wire, and which is centredsymmetrically with respect to the wire [Fig. 8.1(a)]. Fromsymmetry, the magnetic field is directed along thecircumference of the circular loop and is the same inmagnitude at all points on the loop so that if B is themagnitude of the field, the left side of Eq. (8.1) is B (2π r).So we haveB (2πr) = μ 0i (t) (8 .2)