Electromagnetic Induction  Important Questions for CBSE Class 12 Physics Eddy Currents and Self and Mutual Induction

1.Eddy currents are induced in solid metallic sheets/cylinders, when magnetic flux linked with them changes. Direction of eddy currents can be given by Lenz’s law or by Fleming right hand rule.

2.Eddy current causes undesirable heating and wastage of power in transformer, rotating armature and dynamo.

3.Eddy current can be minimised by taking laminated core which consists of thin metallic sheets insulated from each other by varnish and placed normal to the direction of  magnetic field.

4.Applications of Eddy Currents Inspite of the undesirable effects, eddy currents are used in many ways, Some of them are given below:
(i) Speedometer                                       (ii) Induction meter
(iii)Induction furnace                             (iv) Electromagnetic shielding
(v) Electromagnetic damping                                 (vi) Energy meter  1 Mark Questions

1.Why is the core of a transformer  laminated?     [Delhi 2013 c]
Ans. 2.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.  4.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. 5.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. 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. 8.Define mutual inductance. Give its  SI unit.  [Foreign 2009]
Ans. 9.Mention any two useful applications of eddy currents.[Delhi 2009 C]
Ans.(i) Magnetic brake.
(ii) Magnetic furnace.

10The coils in certain galvanometers, have a fixed core made of a non-magnetic metallic material. Why does the oscillating coil come to rest so quickly in such a core?
Ans.Due to eddy currents produced in core whitih  opposes the cause (deflection of coil), that produces it.

2 Marks Questions

11.What are eddy currents? Write their  tWO applications.                     [All India 2012]
Ans.Eddy current Eddy currents are the currents  induced in the bulk pieces of conductors when the amount of magnetic flux linked with the conductor changes.Eddy currents can be minimised by taking laminated core, consists of thin metallic sheet insulated from each other by varnish instead of a single solid mass. The plane of the sheets should be kept perpendicular to the direction of the currents. The insulation provides high resistance hence, eddy current gets minimised
Applications (i) Electromagnetic damping (ii) Induction furnace.

12.Define mutual inductance between  two long coaxial solenoids. Find out the expression for the mutual inductance of inner solenoid of length / having the radius rx and the number of turns per unit length due to the second outer solenoid of same length and n2 number of turns per unit length.    [Delhi 2012]
Ans. 13.Current in e circuit fails steadily  bum AU A to 0.0 A in 10 ms. ii an average emf of 200 V is induced, then calculate the self-inductance of the Circuit.       [Foreign 2011]
Ans. 14.Two concentric circular coils, one of small radius r and the other of large radius R, such that R >>r, are placed coaxially with centres coinciding. Obtain the mutual inductance of the a rran element.
Ans.  15.Two concentric circular coils C1  and  C2, radius r1  and r2  (r1<<r2)  respectively are kept coaxially.  If  current is passed through C2, then find an expression  for  mutual  inductance between the two coils.
[All India 2009,2011 C; Foreign 2011]
Ans.  Ans.  17.Derive an       expression for the  self-inductance of a long air-cored solenoid of length / and number of turns N.   [Delhi 2008]
Ans. 18.Define mutual inductance and write  its SI unit. Write the expression for the mutual inductance between a pair of circular coil of radii r and R (.R > r).   [Foreign 2008]
Ans.  19.Derive an       expression  for the  self-inductance of a long solenoid of N number of turns containing a medium of relative permeability ur.[Foreign 2008]
Ans.   Ans. 21.A large circular coil of radius R and a small circular coil of radius r are put in vicinity of each other. If the coefficient of mutual induction for this pair equals 1 mH, what would be the flux linked with the larger coil when a current of 0.5 A flows through the smaller coil?When the current in the smaller coil falls to zero, what would be its effect in the larger coil? [Delhi 2008 C]
Ans. 3 Marks Questions

22.(i) Draw a schematic sketch of an  AC generator describing its basic elements. State briefly its working principle. Show a plot of variation of (a) magnetic flux and (b) alternating emf versus time generated by a loop of wire rotating in a magnetic field, (ii) Why is choke coil needed in the use of fluorescent tubes with AC mains?   [Delhi 2014]
Ans.  23.Define the term self-inductance of a solenoid. Obtain the expression for the magnetic energy stored in an inductor of self-inductance L to build up a current I through it.[All India 2014]
Ans.   24.The current flowing in the two coils of self-inductance =16 mH and L2 = 12 mH are increasing at the same rate. If the power supplied to the two coils are equal, find the ratio of
(i)induced voltages
(ii)the currents and
(iii)the energies stored in the coil at a given instant. [Foreign 2014]
Ans.  Ans.  26.(i) Define self-inductance. Write its SI  units.
(ii) Derive an expression for self-inductance of a long solenoid of length /, cross-sectional area A having N number of turns.[Delhi 2009]
Ans.(i) (ii) 27.Define the coefficient of mutual  induction. A long solenoid of length / and radius is enclosed coaxially within another long solenoid of length l and radius r2 (r2 > r1  and >> r2). Deduce the expression for the mutual inductance of this pair of Solenoids.   [Delhi 2009; Ail India 2008]
Ans.  28.Deduce an expression for the  mutual inductance of two long coaxial solenoids but having different radii and different number of turns.                 [All India 2009]
Ans.  5 Marks Questions

29.(i) Draw a labelled diagram of AC  generator and state its working principle
(ii)How is magnetic flux linked with the armature coil changed in a generator ?
(iii)Derive the expression for maximum value of the induced emf and state the rule that gives the direction of the induced emf.
(iv)Show the variation of the emf  generated versus time as the armature is rotated with respect to the direction of the magnetic fields.    [Delhi 2014 C]
Ans.    30.State the working of AC generator with the help of a labelled diagram. The coil of an AC generator having N turns, each of area A, is rotated with a constant angular velocity to. Deduce the expression for the alternating emf generated in the coil.What is the source of energy generation in this device?
[Delhi 2010, All India 2008]
Ans.     