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Question : 29
Total: 30
(a) Describe a simple experiment (or activity) to show that the polarity of emf induced in a coil is always such that it tends to produce a current which opposes the change of magnetic flux that produces it.
(b) The current flowing through an inductor of self inductanceL
(i) Magnetic flux versus the current
(ii) Induced emf versus
(iii) Magnetic potential energy stored versus the current.
OR
(a) Draw a schematic sketch of an ac generator describing its basic elements. State briefly its working principle. Show a plot of variation of
(i) Magnetic flux and
(ii) Alternating emf versus time generated by a loop of wire rotating in a magnetic field.
(b) The current flowing through an inductor of self inductance
(i) Magnetic flux versus the current
(ii) Induced emf versus
(iii) Magnetic potential energy stored versus the current.
OR
(a) Draw a schematic sketch of an ac generator describing its basic elements. State briefly its working principle. Show a plot of variation of
(i) Magnetic flux and
(ii) Alternating emf versus time generated by a loop of wire rotating in a magnetic field.
Solution:
(a) Lenz's law: According to Lenz's law, the polarity of the induced emf is such that it opposes a change in magnetic flux responsible for its production.
When the north pole of a bar magnet is pushed towards the coil, the amount of magnetic flux linked with the coil increase. Current is reduced in the coil from a direction such that it opposes the increase in magnetic flux. This is possible only when the current induced in the coil is in anti-clockwise direction, with respect to an observer. The magnetic momentM
Similarly, when the north pole of the bar magnet is moved away from the coil, the magnetic flux linked with the coil decreases. To counter this decrease in magnetic flux, current is induced in the coil in clockwise direction so that its south pole faces the receding north pole of the bar magnet. This would result in an attractive force which opposes the motion of the magnet and the corresponding decrease in magnetic flux
(b) (i) Since,ϕ = L I
where,I =
ϕ =
with all turns of the coil at that time and,
L =
(ii) Induced emf,e =
= −
i.e.,e = − L (
)
(iii) Since, magnetic potential energy is given by
U =
L I 2
OR
(a)
It works on the process of electromagnetic induction, i.e., when a coil rotates continuously in a magnetic field, the effective area of the coil, linked (normally) with the magnetic field lines, changes continuously with time. This variation of magnetic flux with time results in the production of a (alternating) emf in the coil.
(i) Magnetic flux versus time
(ii) Alternating emf versus time
e = NAB ω s i n ω t = e 0 s i n ω t
The graph between alternating emf versus time is shown below:
(b) A choke coil is an electrical appliance used for controlling current in an a.c. circuit. Therefore, if we use a resistanceR
When the north pole of a bar magnet is pushed towards the coil, the amount of magnetic flux linked with the coil increase. Current is reduced in the coil from a direction such that it opposes the increase in magnetic flux. This is possible only when the current induced in the coil is in anti-clockwise direction, with respect to an observer. The magnetic moment
Similarly, when the north pole of the bar magnet is moved away from the coil, the magnetic flux linked with the coil decreases. To counter this decrease in magnetic flux, current is induced in the coil in clockwise direction so that its south pole faces the receding north pole of the bar magnet. This would result in an attractive force which opposes the motion of the magnet and the corresponding decrease in magnetic flux
(b) (i) Since,
where,
with all turns of the coil at that time and,
(ii) Induced emf,
i.e.,
(iii) Since, magnetic potential energy is given by
OR
(a)
It works on the process of electromagnetic induction, i.e., when a coil rotates continuously in a magnetic field, the effective area of the coil, linked (normally) with the magnetic field lines, changes continuously with time. This variation of magnetic flux with time results in the production of a (alternating) emf in the coil.
(i) Magnetic flux versus time
(ii) Alternating emf versus time
The graph between alternating emf versus time is shown below:
(b) A choke coil is an electrical appliance used for controlling current in an a.c. circuit. Therefore, if we use a resistance
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