CBSE Class 12 Physics Electromagnetic Induction And Alternating Current Worksheet Set A

Conceptual and application type questions

1 What is the power dissipated in an AC circuit in which voltage and current are given by V = 230sin(ωt+π/2) and I = 10 sinωt? Give reason

2 Two identical loops ,one of copper and other of constantan ,are removed from a magnetic field within the same time interval. In which loop will the induced current be greater? Justify.

3 A conducting coil is moved relative to a magnetic field .Will there be induced emf and induced current always? Justify

4.What is the significance of power factor in i) electrical appliances ii) transmission of electric power?

5 Two identical bar magnets are dropped from the same height simultaneously , one falls through a copper tube and the other falls through air, will they reach ground at the same time? Justify.

Important Questions for NCERT Class 12 Physics Electromagnetic Induction

Question. A circular disc of radius 0.2 meter is placed in a uniform magnetic field of induction  1/∏ (Wb /m²) in such a way that its axis makes an angle of 60° with ^-B. The magnetic flux linked with the disc is
(a) 0.08 Wb
(b) 0.01 Wb
(c) 0.02 Wb
(d) 0.06 Wb

Answer :  C

Question. A 800 turn coil of effective area 0.05 m2 is kept perpendicular to a magnetic field 5 × 10^–5 T. When the plane of the coil is rotated by 90° around any of its coplanar axis in 0.1 s, the emf induced in the coil will be
(a) 0.02 V
(b) 2 V
(c) 0.2 V
(d) 2 × 10^–3 V

Answer :  A

Question. A coil of resistance 400 W is placed in a magnetic field. If the magnetic flux f (Wb) linked with the coil varies with time t (sec) as f = 50t2 + 4. The current in the coil at t = 2 sec is
(a) 0.5 A
(b) 0.1 A
(c) 2 A
(d) 1 A 

Answer :  A

Question. A conducting circular loop is placed in a uniform magnetic field, B = 0.025 T with its plane perpendicular to the loop. The radius of the loop is made to shrink at a constant rate of 1 mm s–1. The induced emf when the radius is 2 cm, is
(a) 2Π μV
(b) Π μV
(c) Π/2 μV
(d) 2 μV 

Answer :  B

Question. A rectangular coil of 20 turns and area of cross-section 25 sq. cm has a resistance of 100 W. If a magnetic field which is perpendicular to the plane of coil changes at a rate of 1000 tesla per second, the current in the coil is
(a) 1 A
(b) 50 A
(c) 0.5 A
(d) 5 A 

Answer :  C

Question. A magnetic field of 2 × 10–2 T acts at right angles to a coil of area 100 cm2, with 50 turns. The average e.m.f. induced in the coil is 0.1 V, when it is removed from the field in t sec. The value of t is
(a) 10 s
(b) 0.1 s
(c) 0.01 s
(d) 1 s

Answer :  B

Question. A metal ring is held horizontally and bar magnet is dropped through the ring with its length along the axis of the ring. The acceleration of the falling magnet is
(a) more than g
(b) equal to g
(c) less than g
(d) either(a) or (c) 

Answer :  C

Electromagnetic Induction and Alternating Currents 

1. The current passing through the wire A, B is increasing. In which direction does the induced current flow in the closed loop.

2. State Faraday’s Law of electromagnetic induction. Express it mathematically.

3. State Lenz Law. On which law of conservation is it based?

4. How are eddy currents produced?

5. Power factor of an a.c. circuit is 0.5. What will be the phase difference between voltage and current in the circuit?

6. What is the significance of a Q-factor in a series LCR resonant circuit?

7. What is the principle of a.c. generator?

8. What is the power consumed in (i) purely inductive and (ii) purely capacitive a.c. circuits?

9. Draw the graph to show the variation of Xc with the frequency of the a.c. source used.

10. Which is the best method of reducing current in an a.c. circuit and why?

11. If the no of turns of a solenoid is doubled, keeping the other factors constant how does the selfinductance of the solenoid change?

12. Sketch the variation of inductive reactance and capacitive reactance with the frequency of a.c. source.

13. What is the frequency of direct current?

14. What is the relation between peak value and root mean square value of alternating e.m.f.?

15. What is the mean value of an alternating current?

16. When are the voltage and current in LCR-circuit in same phase?

17. a capacitor blocks d.c. Why?

18. The frequency of a.c. source is doubled. How do R, XL and Xc get affected?

19. Which is more dangerous than d.c. for the same value?

20. A lamp is connected in series with a capacitor. Predict your observation for d.c. and a.c. connections. What happens in each case if the capacitance is reduced?

21. What is the unit of L/R?

22. As soon as the current is switched on in a high voltage wire, the bird sitting on it flies away, Why?

23. Fig shows a bar magnet M falling under gravity through an air cored coil C. Plot a graph showing variation of induced e.m.f. E with time (t). What does the area enclosed by the E-t curve depict? M

24. A rectangular loop of wire is being withdrawn out of the magnetic field with velocity v. The magnetic field is perpendicular to the plane of paper. What will be the direction of induced current, if any, in the loop?

5 mark questions:-

25. Derive expression for self-inductance of a long air-cored solenoid of length l, radius r and having number of turns N.

26. What do you mean by mutual inductance of two nearby coils? Find an expression for mutual inductance of a solenoid-coil system.

27. Define the term capacitive reactance; show graphically the variation of capacitive reactance with frequency of applied voltage by angle π/2.
Describe briefly the principle, construction and working of a transformer. Why is its core laminated?
 

Important Questions for NCERT Class 12 Physics Electromagnetic Induction

Question. Faraday’s laws are consequence of conservation of
(a) energy
(b) energy and magnetic field
(c) charge
(d) magnetic field

Answer :  A.

Question. Two coils of self inductance 2 mH and 8 mH are placed so close together that the effective flux in one coil is completely linked with the other.
The mutual inductance between these coils is
(a) 16 mH
(b) 10 mH
(c) 6 mH
(d) 4 mH 

Answer :  D

Question. A cycle wheel of radius 0.5 m is rotated with constant angular velocity of 10 rad/s in a region of magnetic field of 0.1 T which is perpendicular to the plane of the wheel. The EMF generated between its centre and the rim is
(a) 0.25 V
(b) 0.125 V
(c) 0.5 V
(d) zero

Answer :  B

Question. A straight line conductor of length 0.4 m is moved with a speed of 7 m/s perpendicular to a magnetic field of intensity 0.9 Wb/m2. The induced e.m.f. across the conductor is
(a) 5.04 V
(b) 25.2 V
(c) 1.26 V
(d) 2.52 V

Answer :  D

Question. A long solenoid of diameter 0.1 m has 2 × 104 turns per meter. At the centre of the solenoid, a coil of 100 turns and radius 0.01 m is placed with its axis coinciding with the solenoid axis. The current in the solenoid reduces at a constant rate to 0 A from 4 A in 0.05 s. If the resistance of the coil is 10 p2 W, the total charge flowing through the coil during this time is
(a) 16 mC
(b) 32 mC
(c) 16p mC
(d) 32p mC

Answer :  B

Question. The magnetic flux through a circuit of resistance R changes by an amount Df in a time Dt. Then the total quantity of electric charge Q that passes any point in the circuit during the time Dt is represented by

(a) Q = 1/R . ΔΦ /Δt
(b) Q = ΔΦ/R
(c) Q = ΔΦ /Δt
(d) Q= R.ΔΦ /Δt

Answer :  B

Question. The total charge, induced in a conducting loop when
it is moved in magnetic field depends on
(a) the rate of change of magnetic flux
(b) initial magnetic flux only
(c) the total change in magnetic flux
(d) final magnetic flux only.

Answer :  C

Question. The magnetic flux through a circuit of resistance R changes by an amount Df in a time Dt. Then the total quantity of electric charge Q that passes any point in the circuit during the time Dt is represented by      
(a) Q = 1/R ^. ΔΦ//Δt 
(b) Q = ΔΦ//R
(c) Q = ΔΦ//Δt 
(d) Q = R .ΔΦ//Δt

Answer :  B

Question. In which of the following devices, the eddy current effect is not used?   
(a) electric heater
(b) induction furnace
(c) magnetic braking in train
(d) electromagnet .

Answer :  A