Fifty electric bulbs, all identical, are connected in series across the mains of a ${\rm{220}}\,\,{\rm{V}}$ supply. After one bulb is fused, the remaining ${\rm{49}}$ bulbs connected in series across the same mains. The illumination will be

More with ${\rm{50}}$ bulbs than with 48 bulbs

More with ${\rm{49}}$ bulbs than with 50 bulbs

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NEET • PHYSICS • CURRENT ELECTRICITY

Current electricity practice · Hard

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1 Hard

1 of 16

Application +4 | -1

Resistance of a wire at

20\,\,{\,^0}C

20\,\,\,\Omega

and at

500\,\,{\,^0}C

C is

60\,\,\,\Omega

. At what temperature its resistance is

25\,\,\,\Omega

View answer and explanation

Correct option: D

80\,{\,^0}\mathrm{C}

Option D (

80\,{\,^0}\mathrm{C}

) is correct because it is the temperature calculated using the linear relationship between resistance and temperature.

2 Hard

2 of 16

Application +4 | -1

The current in a circuit containing a battery connected to

2 \Omega

resistance is 0.9 A. When a resistance of

7 \Omega

is connected to the same battery, the current observed in the circuit is 0.3 A. Then the internal resistance of the battery is:

View answer and explanation

Correct option: B

0.5 \Omega

Option B (

0.5 \Omega

) is correct because it is the internal resistance calculated by setting up and solving a system of equations based on Ohm's law for the two different circuit configurations.

3 Hard

3 of 16

Application +4 | -1

The area of cross-section, length and density of a piece of a metal of atomic weight

60\, \mathrm{gm}

are

10^{-6} \mathrm{~m}^2, 1.0

\mathrm{m}

and

5 \times 10^3 \mathrm{~kg} / \mathrm{m}^3

respectively and every atom contributes one free electron. (Given Avogadro number

=6 \times 10^{23} / \mathrm{mol}

). Find the drift velocity of electrons in the metal when the current of

16 \mathrm{~A}

passes through it.

View answer and explanation

Correct option: A

2 \mathrm{~mm} / \mathrm{s}

Option A (

2 \mathrm{~mm} / \mathrm{s}

) is correct because it is the calculated drift velocity of electrons in the metal using the given parameters and Avogadro's number.

4 Hard

4 of 16

Analyzing +4 | -1

Statement-1: When a wire is stretched with in the proportionality limit such that its length becomes

n

times that of its initial value, the resistance of wire may become

n^2

times of its initial resistance.
Statement-2: The poisson's ratio of the wire's material can be

1 / 2

View answer and explanation

Correct option: A

Statement-1 is true, Statement-2 is true, Statement-2 is the correct explanation for Statement-1

Statement-1 is true, and Statement-2 is true, but Statement-2 is not the correct explanation for Statement-1. When a wire is stretched, its resistance becomes n² times the initial resistance. Poisson's ratio can be 1/2 for some materials.

5 Hard

5 of 16

Application +4 | -1

In the series combination of

n

cells each cell having emf

\varepsilon

and internal resistance

r

. If three cells are wrongly connected, then total emf and internal resistance of this combination will be

View answer and explanation

Correct option: D

(n \varepsilon-6 \varepsilon), n r

Option D (

(n \varepsilon-6 \varepsilon), n r

) is correct because the total emf is reduced by

6 \varepsilon

due to the wrongly connected cells, while the internal resistance remains

nr

6 Hard

6 of 16

Analyzing +4 | -1

Statement-1: If the radius of copper wire carrying a current is doubled, then the drift velocity of the electrons will become one fourth.
Statement-2: Drift velocity will change according to the relation,

I=n e A v_{{d}}

View answer and explanation

Correct option: A

Statement-1 is true, Statement-2 is true, Statement-2 is the correct explanation for Statement-1

Statement-1 is true, Statement-2 is true, Statement-2 is the correct explanation for Statement-1 because drift velocity is inversely proportional to the cross-sectional area, which depends on the radius squared.

7 Hard

7 of 16

Application +4 | -1

A resistor

R

has power of dissipation

P

with cell voltage

E

. The resistor is cut in

n

equal parts and all parts are connected in parallel with same cell. The new power dissipation is

View answer and explanation

Correct option: C

n^2 P

Option C (

n^2 P

) is correct because cutting the resistor into n parts reduces the resistance of each part by a factor of n, and connecting them in parallel reduces the equivalent resistance by a factor of

n^2

8 Hard

8 of 16

Application +4 | -1

To measure the internal resistance of a battery, potentiometer is used. For

\mathrm{R}=10 \Omega

, the balance point is observed at

\ell=500 \mathrm{~cm}

and for

\mathrm{R}=1 \Omega

the balance point is observed at

\ell=400 \mathrm{~cm}

. The internal resistance of the battery is approximately:

View answer and explanation

Correct option: D

0.3 \Omega

Option D (If both Assertion & Reason are False) is correct because the terminal potential difference can be greater than the emf during charging, and the potential drop due to internal resistance decreases the terminal potential difference.

9 Hard

9 of 16

Analyzing +4 | -1

Fifty electric bulbs, all identical, are connected in series across the mains of a

\mathrm{220}\,\,\mathrm{V}

supply. After one bulb is fused, the remaining

\mathrm{49}

bulbs connected in series across the same mains. The illumination will be

View answer and explanation

Correct option: D

In the ratio

{\left( {50} \right)^2}:\,\,{\left( {49} \right)^2}

in the first and second case respectively

The correct answer is In the ratio

{\left( {50} \right)^2}:\,\,{\left( {49} \right)^2}

in the first and second case respectively because the power is proportional to the square of the number of bulbs.

10 Hard

10 of 16

Understanding +4 | -1

n

cells each of emf

\varepsilon

and internal resistance

r

are connected in parallel, then the total emf and internal resistances will be

View answer and explanation

Correct option: A

\varepsilon, \frac{r}{n}

Option A is correct because when 'n' identical cells are connected in parallel, the emf remains the same, but the internal resistance is reduced by a factor of 'n'.

11 Hard

11 of 16

Application +4 | -1

A battery is charged to a voltage of

15 \mathrm{~V}

for 8 hours with

10 \mathrm{~A}

current. On discharge, the battery supplies a current of

5 \mathrm{~A}

for 15 hours at a mean terminal voltage of

14 \mathrm{~V}

. The efficiency of the battery is

View answer and explanation

Correct option: C

87.5 \%

Option B (Halved) is correct because conductance is inversely proportional to the length of the wire. Doubling the length halves the conductance.

12 Hard

12 of 16

Application +4 | -1

It takes 12 minutes to boil 1 litre of water in an electric kettle. Due to some defect it becomes necessary to remove

20 \%

turns of heating coil of the kettle. After repair, how much time will it take to boil 1 litre of water?

View answer and explanation

Correct option: A

9.6

minute

Option A (

9.6

minute) is correct because the time taken to boil water is inversely proportional to the square of the number of turns in the heating coil.

13 Hard

13 of 16

Application +4 | -1

In copper, each copper atom releases one electron. If a current of 1.1 A is flowing in the copper wire of uniform cross-sectional area of diameter 1 mm, then drift velocity of electrons will approximately be:

(Density of copper

=

9 \times 10^3 \mathrm{~kg} / \mathrm{m}^3,

Atomic weight of copper = 63)

View answer and explanation

Correct option: B

0.1 mm/s

Option B (0.1 mm/s) is correct because it is the approximate drift velocity calculated using the formula

v_d = \frac{I}{nAe}

, where

n

is the number density of electrons.

14 Hard

14 of 16

Application +4 | -1

A transformer operating at primary voltage of

4 \mathrm{~kV}

and secondary voltage of

90 \mathrm{~V}

serves a load of

90 \mathrm{~KW}

at secondary. Assuming the transformer to be operating at

90 \%

efficiency with purely resistive load, the loads in the primary and secondary are

View answer and explanation

Correct option: D

160 \Omega, 0.09 \Omega

Option D (

160 \Omega, 0.09 \Omega

) is correct because it calculates the primary and secondary resistances based on the given voltages, power, and efficiency of the transformer.

15 Hard

15 of 16

Application +4 | -1

Five cells each of emf

E

and internal resistance

r

send the same amount of current through an external resistance

R

whether the cells are connected in parallel or in series. Then the ratio

\left(\frac{R}{r}\right)

View answer and explanation

Correct option: D

Option D (1) is correct because the condition for the same current in series and parallel configurations leads to the external resistance being equal to the internal resistance.

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16 Hard

16 of 16

Application +4 | -1

In a simple metre-bridge circuit, the gaps are bridge by coils

P

and

Q

having the smaller resistance. A balance is obtained when the jockey key makes contact at a point of the bridge wire 40 cm from the

P

end. On shunting the coil

Q

with a resistance of

50 \Omega,

the balance point is moved through 10 cm. The resistance

P

and

Q

are

View answer and explanation

Correct option: B

\frac{50}{3} \Omega, \frac{50}{2} \Omega

respectively

Option B (

\frac{50}{3} \Omega, \frac{50}{2} \Omega

respectively) is correct because it satisfies the conditions of the meter bridge circuit before and after shunting the coil Q.

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