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www.GOALias.blogspot.comPhysicsFIGURE 7.20 Two arrangements for winding of primary and secondary coil in a transformer:(a) two coils on top of each other, (b) two coils on separate limbs of the core.When an alternating voltage is applied to the primary, the resultingcurrent produces an alternating magnetic flux which links the secondaryand induces an emf in it. The value of this emf depends on the number ofturns in the secondary. We consider an ideal transformer in which theprimary has negligible resistance and all the flux in the core links bothprimary and secondary windings. Let φ be the flux in each turn in thecore at time t due to current in the primary when a voltage v pis appliedto it.Then the induced emf or voltage ε s, in the secondary with N sturns isdφεs=− Ns(7.45)dtThe alternating flux φ also induces an emf, called back emf in theprimary. This isdφεp=− Np(7.46)dtBut ε p= v p. If this were not so, the primary current would be infinitesince the primary has zero resistance(as assumed). If the secondary isan open circuit or the current taken from it is small, then to a goodapproximationε s= v swhere v sis the voltage across the secondary. Therefore, Eqs. (7.45) and(7.46) can be written asdφ=− [7.45(a)]vsNsdtdφv N dtp=−p[7.46(a)]260From Eqs. [7.45 (a)] and [7.45 (a)], we havevvspN=Nsp(7.47)