08-02-2013, 04:30 PM
MAGNETIC FLUX, INDUCED E.M.F,
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1.* Two concentric circular coils are perpendicular to each other. Coil I carries a current i. If this current is changed, will this induce a current in the coil II?
II
[No- Field due to one coil is parallel to the plane of the second coil. So flux does not change.]
2. A closed loop of wire is being moved with constant velocity without changing its orientation inside a uniform magnetic field. Will this induce a current in the loop?
[Ans: No there is no change in ΦB] [1]
3.* A cylindrical bar magnet is kept along the axis of a circular coil and near it as shown in the fig. Will there be any induced current at the terminals of the coil when the magnet is rotated a) about its own axis b) about an axis perpendicular to the length of the magnet?
N S
Fig (i) Fig(ii)
Ans Fig. (i) No e.m.f will be induced, as these is no change in flux.
Fig (ii) Yes, Φ changes continuously. So e.m.f is induced in the coil.
4. A wire is kept along the N→S direction and is allowed to fall freely. Will an e.m.f be induced in the wire? [1]
5. A wire is kept along the E→W direction and is allowed to fall freely. Will an e.m.f be induced in the wire? [1]
(Yes)
6. A vertical magnetic pole falls down through the plane of magnetic meridian. Will any e.m.f be induced between its ends? (1)
Ans: No, because the pole intercepts neither Bv or BH 51 STUDY MATERIAL CLASS XII 2011-12
7. A wheel with a certain number of spokes is rotated in a plane normal to earth’s magnetic field so that an emf is induced between the axle and rim of the wheel, keeping all other things same, number of spokes is changed. How is the e.m.f affected? (1)
(Hint: Number of spokes does not affect the net emf)
8. Name the various methods of producing induced emf? (2)
8. Name the various methods of producing induced emf? (2)
9. Explain any two applications of eddy current. (2)
10. The magnetic flux linked with a coil passing perpendicular to the plane of the coil changes with time Φ = 4t2 + 2t + 3, where “t” is the time in seconds. What is magnitude of e.m.f induced at t = 1 second? Ans: (e = dΦ/dt = ,e = 8t +2 If t = 1s e= 10V) 2 4 2 3 d t t dt
11. A wheel fitted with spokes of radius ‘r’ is rotating at a frequency of n revolutions per second in a plane perpendicular to magnetic field B Tesla. What is the e.m.f induced between the axle and rim of the wheel? [2]
Φ = BA
e = d(BA)/dt= B dA/dt, dA/dt= Πr2x n
e = B. Πr2n
12. Two coils P and S are arranged as shown in the figure.
(i) What will be the direction of induced current in S when the switch is closed?
(ii) What will be the direction of induced current in S when the switch is opened? [2]
Ans: (i) anticlockwise (ii) clockwise
13.* A conducting circular loop is placed in a uniform magnetic field B = 0.020T with its plane perpendicular to the field. Somehow, the radius of the loop starts shrinking at a constant rate of 1mm/s. Find the induced current in the loop at an instant when the radius is 2cm. (2)
Ans. (Ф= Πr2B d Ф/dt = 2ΠrB dr/dt e= 25μV
14. A 12V battery is connected to a 6Ω; 10 H coil through a switch drives a constant current in the circuit. The switch is suddenly opened. Assuming that it took 1ms to open the switch calculate the average e.m.f induced across the coil.
Ans. (I initial=2A I final= 0 e=-Ldi/dt 20000V) (2)
15. A coil of mean area 500 cm2 having 1000 turns is held perpendicular to a uniform magnetic field of 0.4 G. The coil is turned through 180 o in 1/10 seconds. Calculate the average induced e.m.f.
Ans. (0.04 V) (2)
16. A conducting rod of length l with one end pivoted is rotated with a uniform angular speed ω in a Vertical plane normal to uniform magnetic field B. Deduce an expression for e.m.f induced in this rod.