﻿ Obtain an expression for magnetic flux density ‘B’ at the ce

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## ISC Class 12 Physics Solved Question Paper 2009

21.

(i) What is meant by:

(1) Drift velocity and

(2) Relaxation time?

(ii) A metallic plug AB is carrying a current I (see Figure 4 below). State how the drift velocity of free electrons varies, if at all, from end A to end B.

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22. Figure below shows a uniform manganin wire XY of length 100 cm and resistance 9 Ω, connected to an accumulator D of emf 4 V and internal resistance 1 Ω through a variable resistance R. E is sa cell of emf 1.8 V connected to the wire XY via a jockey J and a central zero galvanometer G. It is found that the galvanometer shows no deflection when XJ = 80 cm. Find the value of R.

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# 23.Obtain an expression for magnetic flux density ‘B’ at the center of a circular coil of radius R and having N turns, when a current I is flowing through it.

Magnetic flux density at the centre of circular coil:

Consider a circular coil of one turn of radius R with its centre at O, carrying a current I. To find the magnetic flux density at the centre of the current carrying coil, it is divided into a large number of elements of length dl. According to Biot-Savart law, the magnetic field due to one element is,

dB =

The magnetic flux density due to different current elements at the centre act in the same direction. Therefore the magnetic flux due to the whole circular coil at the centre of the coil is given by,

B =

If there are N turns in the coil, then the field at the centre, B =

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24.

(i) State any two differences between a moving coil galvanometer and a tangent galvanometer.

(ii) What is the use of a Cyclotron?

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25. What is meant by the time constant of an LR circuit? When the current flowing through a coil P decreases from 5A to 0 in 0.2 seconds, an emf of 60 V is induced across the terminals of an adjacent coil Q. Calculate the coefficient of mutual inductance of the two coils P and Q.
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26.

When an alternating emf e = 300 Sin (100 πt + π/6) Volt is applied to a circuit, the current I through it is I = 5.0 Sin (100 πt - π/6) ampere. Find the :

(i) Phase difference between the emf and the current.

(ii) Average power consumed by the circuit.

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27. Obtain an expression for resonant frequency ‘f0’of a series LCR circuit.
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28.

In which part of the electromagnetic spectrum, do the following radiations lie:

(i) Having wavelength of 20 nm

(ii) Having frequency of 10 MHz

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29.  A thin convex lens which is made of glass (refractive index 1.5) has a focal length of 20 cm. It is now completely immersed in a transparent liquid having refractive index 1.75. Find the new focal length of the lens.
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30. Draw a labelled graph showing the variation in intensity of light with distance in a single slit Fraunhofer diffraction experiment.
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