Q24 of 31 Page 1

An inductor L of inductance XL is connected in series with a bulb B and an ac source. How would brightness of the bulb change when (i) number of turn in the inductor is reduced, (ii) an iron rod is inserted in the inductor and (iii) a capacitor of reactance = is inserted in series in the circuit. Justify your answer in each case.

(i) When the number of turns in the inductor is reduced, its reactance XL decreases. The current in the circuit increases andhence brightness of the bulb increases.


(ii) When an iron rod is inserted in the inductor, the self-inductance increases. Consequently, the inductive reactance XL = ωL increases. This decreases the current in the circuit and the bulb glows dimmer.


(iii) With capacitor of reactance XC=XL, the impedance becomes minimum, the current in circuit becomes maximum. Hence the bulb glows with maximum brightness.


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(a) State Ampere’s circuital law. Use this law to obtain the expression for the magnetic field inside an air cored toroid of average radius ‘r’, having ‘n’ turns per unit length and carrying a steady current I.

(b) An observer to the left of a solenoid of N turns each of cross section area ‘A’ observes that a steady current I in it flows in the clockwise direction. Depict the magnetic field lines due to the solenoid specifying its polarity and show that it acts as a bar magnet of magnetic moment m = NIA.