EddyCurrents & Self & Mutual Induction

1 Mark Questions

1. Why is the core of a transformer laminated? [Delhi 2013 c] Ans.

The core of a transformer is laminated because of preventing eddy current being produced in the core. (1)

1. The motion of copper plate is damped, when it is allowed to oscillate between the two

poles of a magnet. What is the cause this damping? [All India 2013] AnsAs, the copper plates oscillate in the magnetic field between the two poles of the magnet, there is a continuous change of magnetic flux linked with the pendulum. Due to this eddy currents are set up in copper plate which try to oppose the motion of the plate (according to plate the Lenz's law) and finally bring it to rest.

3.A light metal disc on the top of an electromagnet is thrown up as the current is switched on. Why? Give reason. [All India 2013] Ans.

When the current begins to grow through the electromagnet, the magnetic flux through the disc begins to increase. This sets up eddy current in the disc in the same direction as that of the electromagnetic current. (1/2)

Thus, if the upper surface of electromagnetic acquires N-polarity, the lower surface of the

disc also acquires N-polarity. As, same magnetic poles repel each other, the light metallic disc is thrown up. (1/2)

1. How does the mutual inductance of a pair of coils change, when

(i)distance between the coils is increased and

(ii)number of turns in the coils is increased? [All India 2013] Ans.

1. A <math>\phi = MI</math>, with the increase in the distance between the coils the magnetic flux linked with the secondary coil decreases and hence, the mutual inductance of the two coils will decreases with the increase of

separation between them. (1/2)

1. Mutual inductance of two coils can be found out by <math>M = \mu_0 n_1 n_2 Al</math>, i.e. <math>M \propto n_1 n_2</math>, so, with the increase in number of turns mutual inductance increases. (1/2)
2. How can the self-inductance of a given coil having N number of turns, area of cross- section A and lengths / be increased?[Forign 2009; Delhi 2012]

Ans. The self-inductance can be increased by the help of electric fields. It does not depend on the current through circuit but depends upon the permeability of material from which the core is made up off.

6.Define self-inductance of a coil. Write its SI unit. [All India 2010] Ans.

Self-inductance of a coil is equal to the total magnetic flux linked with the coil, when unit current passes through it.

Also, self-inductance of a coil, is equal to the emf induced in coil, when rate of change of current in coil is 1 A/s .

SI unit of self inductance is 1 Henry (H)

<math>1H = 1V - s/A</math> (1) NOTE Any one definition is sufficient.

7.A plot of magnetic flux versus current (I) is shown in the figure for two inductions A and B Which of two has larger value of self-inductance? [Delhi 2010] Ans.

<math>\langle \rangle</math> Self-inductance of the inductor, <math>L = \phi / I</math>.

The slope of <math>I - \phi</math> graph gives self-inductance of the coil.

Inductor A have got greater slope than inductor B, therefore self-inductance of A is greater than self-inductance of B. (1)