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www.GOALias.blogspot.comPhysicsFIGURE 6.8(d) Predict the polarity of the capacitor in the situation described byFig. 6.9.EXAMPLE 6.5FIGURE 6.9Solution(a) No. However strong the magnet may be, current can be inducedonly by changing the magnetic flux through the loop.(b) No current is induced in either case. Current can not be inducedby changing the electric flux.(c) The induced emf is expected to be constant only in the case of therectangular loop. In the case of circular loop, the rate of change ofarea of the loop during its passage out of the field region is notconstant, hence induced emf will vary accordingly.(d) The polarity of plate ‘A’ will be positive with respect to plate ‘B’ inthe capacitor.2126.6 MOTIONAL ELECTROMOTIVE FORCELet us consider a straight conductor moving in a uniform and timeindependentmagnetic field. Figure 6.10 shows a rectangular conductorPQRS in which the conductor PQ is free to move. The rod PQ is movedtowards the left with a constant velocity v asshown in the figure. Assume that there is noloss of energy due to friction. PQRS forms aclosed circuit enclosing an area that changesas PQ moves. It is placed in a uniform magneticfield B which is perpendicular to the plane ofthis system. If the length RQ = x and RS = l, themagnetic flux Φ Benclosed by the loop PQRSwill beΦ B= BlxFIGURE 6.10 The arm PQ is moved to the leftside, thus decreasing the area of therectangular loop. This movementinduces a current I as shown.Since x is changing with time, the rate of changeof flux Φ Bwill induce an emf given by:–dΦBdε = = –dtdt=( Blx )dx– Bl Blvdt = (6.5)