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Physics Part-I

Electromagnetic Induction

Figure 6.20 shows a metal rod PQ resting on the smooth rails A

Physics Part-I

Class 12

NCERT

Chapter 1: Electric Charges and Fields

47 questions

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Chapter 3: Current Electricity

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Chapter 4: Moving Charges and Magnetism

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Chapter 5: Magnetism and Matter

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Chapter 6: Electromagnetic Induction

28 questions

It is desired to measure the magnitude of field between the poles of a powerful loud speaker magnet. A small flat search coil of area 2

c m^{2}

with 25 closely wound turns, is positioned normal to the field direction, and then quickly snatched out of the field region. Equivalently, one can give it a quick 90° turn to bring its plane parallel to the field direction). The total charge flown in the coil (measured by a ballistic galvanometer connected to coil) is 7.5 mC. The combined resistance of the coil and the galvanometer is 0.50 Ω. Estimate the field strength of magnet.

Figure 6.20 shows a metal rod PQ resting on the smooth rails AB and positioned between the poles of a permanent magnet. The rails, the rod, and the magnetic field are in three mutual perpendicular directions. A galvanometer G connects the rails through a switch K. Length of the rod = 15 cm, B = 0.50 T, resistance of the closed loop containing the rod = 9.0 mΩ. Assume the field to be uniform.
(a) Suppose K is open and the rod is moved with a speed of

12 c m s^{-1}

in the direction shown. Give the polarity and magnitude of the induced emf.
(b) Is there an excess charge built up at the ends of the rods when
K is open? What if K is closed?
(c) With K open and the rod moving uniformly, there is no net
force on the electrons in the rod PQ even though they do experience magnetic force due to the motion of the rod. Explain.
(d) What is the retarding force on the rod when K is closed?
(e) How much power is required (by an external agent) to keep
the rod moving at the same speed (

= 12 c m s^{-1}

) when K is closed?
How much power is required when K is open?
(f ) How much power is dissipated as heat in the closed circuit?
What is the source of this power?
(g) What is the induced emf in the moving rod if the magnetic field
is parallel to the rails instead of being perpendicular?

An air-cored solenoid with length 30 cm, area of cross-section 25

c m^{2}

and number of turns 500, carries a current of 2.5 A. The current is suddenly switched off in a brief time of

1 0^{-3} s

. How much is the average back emf induced across the ends of the open switch in the circuit? Ignore the variation in magnetic field near the ends of the solenoid.

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Chapter 7: Alternating Current

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Chapter 8: Electromagnetic Waves

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Question

Medium

Solving time: 6 mins

Figure 6.20 shows a metal rod PQ resting on the smooth rails AB and positioned between the poles of a permanent magnet. The rails, the rod, and the magnetic field are in three mutual perpendicular directions. A galvanometer G connects the rails through a switch K. Length of the rod = 15 cm, B = 0.50 T, resistance of the closed loop containing the rod = 9.0 mΩ. Assume the field to be uniform.
(a) Suppose K is open and the rod is moved with a speed of $12 c m s^{-1}$ in the direction shown. Give the polarity and magnitude of the induced emf.
(b) Is there an excess charge built up at the ends of the rods when
K is open? What if K is closed?
(c) With K open and the rod moving uniformly, there is no net
force on the electrons in the rod PQ even though they do experience magnetic force due to the motion of the rod. Explain.
(d) What is the retarding force on the rod when K is closed?
(e) How much power is required (by an external agent) to keep
the rod moving at the same speed ( $= 12 c m s^{-1}$ ) when K is closed?
How much power is required when K is open?
(f ) How much power is dissipated as heat in the closed circuit?
What is the source of this power?
(g) What is the induced emf in the moving rod if the magnetic field
is parallel to the rails instead of being perpendicular?

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Physics Part-I (NCERT)

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Eleanor Discussed

12 c m s^{-1}

= 12 c m s^{-1}

14 mins ago

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Text solutionVerified

(a)

Induce EMF is

e = Bl v = 9 \times 1 0^{- 3} V

The polarity of the induced emf is such that end P shows positive while end Q shows negative ends.

(b)

Yes; when key K is closed, excess charge is maintained by the continuous flow of current.

When key K is open, there is excess charge built up at both ends of the rods.

When key K is closed, excess charge is maintained by the continuous flow of current.

(c)

Magnetic force is cancelled by the electric force set-up due to the excess charge of opposite nature at both ends of the rod.

(d)

Retarding force exerted on the rod, $F = I Bl$

$I = e / R = 9 \times 1 0^{- 3} /9 \times 1 0^{- 3} = 1 A$

so, $F = 1 \times 0.5 \times 0.15 = 75 \times 1 0^{- 3} N$

(e)

Retarding force, $F = B I l$

where, $I = e / R = 1 A$

so, $F = 75 \times 1 0^{- 3} N$

Also, Power $P = F v = 75 \times 1 0^{- 3} \times 0.12 = 9 mW$

(f)

Power dissipated = $I^{2} R = 9 mW$

The source of this power external agent.

(g)

Zero. As, in this case, no emf is induced in the coil because the motion of the rod does not cut across the field lines.

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Practice more questions from Physics Part-I (NCERT)

It is desired to measure the magnitude of field between the poles of a powerful loud speaker magnet. A small flat search coil of area 2

c m^{2}

12 c m s^{-1}

= 12 c m s^{-1}

An air-cored solenoid with length 30 cm, area of cross-section 25

c m^{2}

and number of turns 500, carries a current of 2.5 A. The current is suddenly switched off in a brief time of

1 0^{-3} s

. How much is the average back emf induced across the ends of the open switch in the circuit? Ignore the variation in magnetic field near the ends of the solenoid.

View all

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Question Text	Figure 6.20 shows a metal rod PQ resting on the smooth rails AB and positioned between the poles of a permanent magnet. The rails, the rod, and the magnetic field are in three mutual perpendicular directions. A galvanometer G connects the rails through a switch K. Length of the rod = 15 cm, B = 0.50 T, resistance of the closed loop containing the rod = 9.0 mΩ. Assume the field to be uniform. (a) Suppose K is open and the rod is moved with a speed of $12 c m s^{-1}$ in the direction shown. Give the polarity and magnitude of the induced emf. (b) Is there an excess charge built up at the ends of the rods when K is open? What if K is closed? (c) With K open and the rod moving uniformly, there is no net force on the electrons in the rod PQ even though they do experience magnetic force due to the motion of the rod. Explain. (d) What is the retarding force on the rod when K is closed? (e) How much power is required (by an external agent) to keep the rod moving at the same speed ( $= 12 c m s^{-1}$ ) when K is closed? How much power is required when K is open? (f ) How much power is dissipated as heat in the closed circuit? What is the source of this power? (g) What is the induced emf in the moving rod if the magnetic field is parallel to the rails instead of being perpendicular?
Topic	Electromagnetic Induction
Subject	Physics
Class	Class 12
Answer Type	Text solution:1
Upvotes	88

Text solutionVerified

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