Physics MCQs for NEET — Practice Questions with Answers

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A galvanometer has a resistance of 25 ohm and a maximum of 0.01 A current can be passed through it. In order to change it into an ammeter of range 10 A, the shunt resistance required is 

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Explanation

ig=iSG+S0.01=10S25+S

   1000S=25+SS=25999Ω

Two resistances of 400 Ω and 800 Ω are connected in series with a 6-volt battery of negligible internal resistance. A voltmeter of resistance 10,000 Ω is used to measure the potential difference across 400 Ω. The error in the measurement of potential difference in volts approximately is :

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The length of a wire of a potentiometer is 100 cm, and the emf of its standard cell is E volt. It is employed to measure the e.m.f of a battery whose internal resistance is 0.5 Ω. If the balance point is obtained at l = 30 cm from the positive end, the e.m.f. of the battery is :

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Explanation

From the principle of potentiometer Vl

VE=lL; where V = emf of battery, E = emf of standard cell, L = Length of potentiometer wire

V=ElL=30E100

The current flowing in a coil of resistance 90 Ω is to be reduced by 90%. What value of resistance should be connected in parallel with it 

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Explanation

ig=i10 ⇒ Required shunt S=G(n1)=90(101)=10Ω 

A galvanometer of 50 ohm resistance has 25 divisions. A current of 4 × 10–4 ampere gives a deflection of one division. To convert this galvanometer into a voltmeter having a range of 25 volts, it should be connected with a resistance of :

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Explanation

Full deflection current ig=25×4×104=100×104A

Using R=VIgG=25100×10450=2450Ω in series.

In a metre bridge experiment, the null point is obtained at 20 cm from one end of the wire when resistance X is balanced against another resistance Y. If X < Y, then where will be the new position of the null point from the same end, if one decides to balance a resistance of 4X against Y 

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Explanation

In balancing condition, R1R2=l1l2=l1100l1

XY=2080=14 .....(i)

and 4XY=l100l .....(ii)

From equation (i) and (ii) :

44=l100ll=50cm 

In a potentiometer experiment, the balancing with a cell is at length 240 cm. On shunting the cell with a resistance of 2 Ω, the balancing length becomes 120 cm. The internal resistance of the cell is :

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Explanation

r=Rl1l21=22401201=2Ω 

Potentiometer wire of length 1 m is connected in series with 490 Ω resistance and 2V battery. If 0.2 mV/cm is the potential gradient, then the resistance of the potentiometer wire is :

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Explanation

Potential gradient x=e(R+Rh+r).RL

⇒ ⇒ R = 4.9 Ω.

Two uniform wires A and B are of the same metal and have equal masses. The radius of wire A is twice that of wire B. The total resistance of A and B when connected in parallel is :

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Explanation

RARB=rBrA4RARB=124=116RB=16RA

When RA and RB are connected in parallel then equivalent resistance Req=RARB(RA+RB)=1617RA

If RA=4.25Ω then Req=4Ω i.e. option is correct.

You are given several identical resistances each of value R = 10 Ω and each capable of carrying maximum current of 1 ampere. It is required to make a suitable combination of these resistances to produce a resistance of 5 Ω which can carry a current of 4 amperes. The minimum number of resistances of the type R that will be required for this job 

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Explanation

Suppose n resistors are used for the required job. Suppose equivalent resistance of the combination is R' and according to energy conservation, it's current rating is i'.

Energy consumed by the combination = n × (Energy consumed by each resistance)

i'2R'=n×i2R

n=i'i2×R'R=412×510=8 

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