Physics MCQs for NEET — Practice Questions with Answers

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A galvanometer has 30 divisions and a sensitivity 16μA/div. It can be converted into a voltmeter to read 3 V by connecting (approximately):

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In a circuit, 5 percent of total current passes through a galvanometer. If the resistance of the galvanometer is G then the value of the shunt is :

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Explanation

igi=SG+S

5100=SG+S

S=G19

A voltmeter has a range 0-V with a series resistance R. With a series resistance 2R, the range is 0-V'. The correct relation between V and V' is :

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Explanation

For conversion of galvanometer (of resistances) into voltmeter, a resistance R is connected in series.

ig=VR+G and ig=V'2R+G

VR+G=V'2R+GV'V=2R+GR+G=2(R+G)G(R+G)

=2G(R+G)V'=2VVG(R+G)V'<2V 

If an ammeter is to be used in place of a voltmeter then we must connect with the ammeter a :

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Explanation

If ammeter is used in place of voltmeter (i.e. in parallel) it may damage due to large current in circuit. Hence to control this large amount of current a high resistance must be connected in series.

A galvanometer of resistance 36 Ω is changed into an ammeter by using a shunt of 4 Ω. The fraction f0 of total current passing through the galvanometer is :

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Explanation

igi=SG+S=436+4=440=110  

A galvanometer, having a resistance of 50 Ω gives a full scale deflection for a current of 0.05 A. The length in meter of a resistance wire of area of cross-section 2.97× 10–2 cm2 that can be used to convert the galvanometer into an ammeter which can read a maximum of 5 A current is (Specific resistance of the wire = 5 × 10–7 Ωm

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Explanation

iig=1+GS50.05=1+50S

S=5099=ρ×lAl=5099×2.97×102×1045×107=3m.

An ammeter reads up to 1 ampere. Its internal resistance is 0.81 ohm. To increase the range to 10 A the value of the required shunt is :

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Explanation

iig=1+GS101=1+0.81SS=0.09Ω

 The dimension of the magnetic field intensity B is:

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Explanation

(b)  F = Bil  B=FiL=MLT-2AL=MT-2A-1  

An electron moves with a constant speed v along a circle of radius r. Its magnetic moment will be (e is the electron's charge)

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Explanation

(b) M=iπr2=ev2πr×πr2M=12evr

The field normal to the plane of a wire of n turns and radius r which carries a current i is measured on the axis of the coil at a small distance h from the centre of the coil. This is smaller than the field at the centre by the fraction

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Explanation

(a)  Field at the centre  B1=μ04π×2πinr=μ02·nir

      Field at a distance from the centre

      B2=μ04π·2πnir2(r2+h2)3/2=μ02·nir2r31+h2r23/2     =B11+h2r2-3/2 =B11-32·h2r2

              (By binomial theorem)

     Hence B2 is less than B1 by a fraction =32h2r2 

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