21.Two coils are placed close to each other. The mutual inductance of the pair of coils depends upon:

22.Air cored chokes are used for reducing high frequency ac because

23.In a transformer, number of turns in the primary are 140 and that in the secondary are 280. If current in primary is 4A, then that in the secondary is:

24.For maximum output power in D.C. motor, the induced back emf (E) should be

25.A resistor and an inductor are connected in series to a 220 V A.C. supply. When measured with an A.C. voltmeter, the P.D. across resistor is 140 V. The P.D. across the terminals of the inductor will be nearly