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Electromagnetic induction |SSC, ESE & GATE

Q. 1 . What is the origin of the name electromagnetic induction?

Ans. So-called because electricity is produced from magnetism (i.e. electromagnetic) and there is no physical connection (induction) between the magnetic field and the conductor.


Q. 2. What do you mean by flux linking the conductor or coil?

Ans. Magnetic lines of force form closed loops. Flux linking the conductor means that the flux embraces it i.e. it encircles the conductor.


Q. 3. When does inductance make itself felt in a circuit?

Ans. Inductance makes itself felt in a circuit (or coil) only when there is a changing current. For example, if a steady direct current (d.c.) is flowing in a circuit, there will be no inductance. However, when an alternating current is flowing in the same circuit, the current is constantly changing and hence the circuit exhibits inductance.

Q. 4. Does inductance play any part in a d.c. circuit?

Ans. During the opening and closing of d.c. circuit. however, once the circuit attains a steady current, the circuit does not exhibit any inductance.


Q. 5. Why is the concept of inductance important?

Ans. The inductance concept is extremely useful because it enables us to express an induced e.m.f. directly in terms of changing current, rather than having to go through the intermediate step of calculating the flux caused by the current, and then applying Faraday’s laws.


Q. 6. When an inductive circuit is opened, an arc appears across the opening contacts. why does the arc not appear during the closing of the circuit?

Ans. When an inductive circuit is opened by a switch, the current tries to fall to zero very quickly. Consequently, a large e.m.f. is induced across the switch contacts due to the rapid change in current. This induced e.m.f. is large enough to cause an arc between switch contacts. When an inductive circuit is closed, again large e.m.f. is induced in the circuit due to rapid change in current. But now the circuit is complete, the induced is distributed around the entire circuit.


Q. 7. What is a choke? Why is it so called?

Ans. A highly inductive coil can be used to impede the flow of alternating current while permitting direct current to flow unhindered. When used for this purpose, the inductor is known as a choke coil since it chokes out variations of the amplitude of the pulsating current. A choke consists of a large number of tums of wire wound over an iron core. Chokes are extensively used to smooth out the variations in the direct current obtained from rectifiers.


Q. 8. Why is a small air gap left in The iron core of a choke coil?

Ans. To prevent the core of the choke from becoming magnetically saturated. The air gap introduces so much reluctance in the magnetic circuit that the core never gets saturated.


Q. 9. How will you make a coil non-inductive?

Ans. we know that L = NΦ / I
This relation indicates how we wind a coil that has no inductance. If, after winding any number of tums, we double back and wind an equal number in the opposite direction. the net m.m.f. caused by a current in the winding will be zero. Consequently, no flub will link the turns, and the coil will have no self-inductance.


Q.10. How will you reduce the inductance of a coil?

Ans. The inductance of a coil can be reduced by one of the following methods :
(i) By taking some turns off the coil.
(ii) By stretching out of the coil until it has a greater length.
(iii) By using a core of less relative permeability.
(iv) By shorting one or more turns of the coil.


Q.11. What happens when one (or more) turn of a coil is short-circuited?

Ans. If one (or more) turn of a coil is shorted, there is one less turn in the coil and the coil has a little less inductance.
(i) If d.c. is flowing through the coil, there will be relatively little change in the magnetic field around the coil.
(ii) When a.c. flows through the coil, e.m.f. will be induced in the shorted turn. This e.m.f. will induce a large current in the shorted turn. The current in the shorted trunwill be in opposite direction to that flowing in the coil. The result is that the inductance of the coil will be reduced appreciably. In fact. the inductance of the coil is reduced by much more than would result from cutting off one turn. Consequently. curr.. the coil•will increase, resulting in heating up of shorted turn.


Q. 12. What is a solenoid?

Ans. The solenoid is an iron bar wound with a number of turns of wire. A solenoid is Greek. word meaning “tube-like”. It is found in relays, inductors, small transformers, etc.


Q. 13. What is the effect of mutual inductance on the coils between which it exists?

Ans. Mutual inductance comes into the picture when two coils are placed close together in such a way that flux produced by one coil links the other. Each coil has its own inductance but in addition, there is further inductance M due to coupling between the coils. If the fluxes of the two coils aid each other, the inductance of each coil will increases by M. If the fluxes of the two coils oppose each other, the inductance of each coil will decrease by M.


Q. 14. The coefficient of coupling between two coils is 0.6. What does it mean?

Ans. When we say that co-efficient of coupling between two coils is 0.6, it means that 60% of the flux set up in one coil links to the other coil.


Q. 15. Why do we use air-cored transformers for high-frequency applications?

Ans. At high frequencies, iron-core transformers have the disadvantage of high hysteresis and eddy-current losses. Moreover, the iron core becomes less effective as a magnetic circuit as the frequency reaches high values because the flux penetrates to a progressively lesser extent into the core. Since air does not exhibit hysteresis and eddy-current effects, the use of air-core transformers is common at radio frequencies found in electronics and communication engineering.


Q. 16. What is the significance of the time constant of an R—L series circuit?

Ans. The time constant of an R-L series circuit (= L/R) is the time required for the current to reach its steady-state value if it continued to change at its initial rate of decay and growth. The larger the time constant, the longer the current take to reach its final steady-state value and vice-versa.

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