Board Paper of Class 12 2021 Physics Delhi (Set 055/1/4) - Solutions

General Instructions:

1. This question paper contains 55 questions out of which 45 questions are to be attempted. All questions carry equal marks.
2. This question paper consists of three sections , Section A, Section B and Section C.
3. Section A contains 25 questions. Attempt any 20 questions from Q. No. 1 to 25.
4. Section B contains 24 questions. Attempt any 20 questions from Q. No. 26 to 49.
5. Section C contains 6 questions. Attempt any 5 questions from . No. 50 to 55.
6. The first 20 questions in Section A and Sections B and first 5 questions attempted in Section C by a candidate will be evaluated.
7. There is only one correct option for every multiple choice question (MCQ). Marks will not be awarded for answering more than one option.
8. There is no negative marking.
This section consists of 25 multiple choice questions with over all choice to attempt any 20 questions. In case more than desirable number of questions are attempted, only first 20 questions will be considered for evaluation.

An electric dipole placed in a non-uniform electric field will experience :

1. only a force
2. only a torque
3. both force and torque
4. neither force nor torque

Ans. (c)

1. 1/π
2. 2/3
3. 9/4
4. π

Ans. (b)

1. IQ = IR
2. IR = IG
3. IP = IG
4. IQ = IG

Ans. (b)

1. 1/3
2. 3/4
3. 4/5
4. 9/8

Ans. (d)

1. 5 : 7
2. 7 : 5
3. 1 : 4
4. 1 : 1

Ans. (a)

A current I is flowing through the loop as shown in the figure (MA = R, MB = 2R). The magnetic field at the centre of the loop is μ0I/R times :

1. 5/16, into the plane of the paper
2. 5/16, out of the plane of the paper
3. 7/16, out of the plane of the paper
4. 7/16, into the plane of the paper

Ans. (d)

A capacitor and an inductor are connected in two different AC circuits with a bulb glowing in each circuit. The bulb glows more brightly when :

1. the number of turns in the inductor is increased
2. the separation between the plates of the capacitor is increased
3. an iron rod is introduced into the inductor
4. a dielectric is introduced into the gap between the plates of the capacitor

Ans. (d)

1. 150 V
2. 175 V
3. 220 V
4. 250 V

Ans. (d)

Ans. (d)

1. 1/k
2. k
3. k2
4. 1/k2

Ans. (b)

Infinity resistance in a resistance box has :

1. a resistance of 105
2. a resistance of 107
3. a resistance of ∾ resistance
4. a gap only

Ans. (d)

A plug (named infinity plug) is taken out from resistance box so air gap is introduced there. Since air is bad conductor of current. Therefore, no current flows in the circuit which implies resistance of infinity resistance is introduced in the circuit.

1. 122 J
2. 270 J
3. 420 J
4. 720 J

Ans. (b)

Lenz’s law is the consequence of the law of conservation of :

1. energy
2. charge
3. mass
4. momentum

Ans. (a)

We can take an example, when North Pole of magnet is moved towards a coil, one end of coil [near to magnet] develops north polarity hence repelling the magnet. Now we have to apply external force to move the magnet towards coil. This external work done gets converted into induced emf in coil. So Lenz's law is consequence of conservation of energy.

1. 30°
2. 45°
3. 60°

Ans. (b)

A long straight wire in the horizontal plane carries a current of 15 A in north to south direction. The magnitude and direction of magnetic field at a point 2.5 m east of the wire respectively are :

1. 1.2 µT, vertically upward
2. 1.2 µT, vertically downward
3. 0.6 µT, vertically upward
4. 0.6 µT, vertically downward

Ans. (a)

1. 104 V
2. 103 V
3. 102 V
4. 10 V

Ans. (c)

Ans. (d)

Ans. (c)

1. 10 Ω
2. 7 Ω
3. 3 Ω
4. 2 Ω

Ans. (d)

An electron is projected with velocity $\stackrel{\to }{\mathrm{v}}$ along the axis of a current carrying long solenoid. Which one of the following statements is true ?

1. The path of the electron will be circular about the axis.
2. The electron will be accelerated along the axis.
3. The path of the electron will be helical.
4. The electron will continue to move with the same velocity $\stackrel{\to }{\mathrm{v}}$ along the axis of the solenoid.

Ans. (d)

If the speed $\stackrel{\to }{\mathrm{v}}$ of a charged particle moving in a magnetic field $\stackrel{\to }{\mathrm{B}}$ ( $\stackrel{\to }{\mathrm{E}}$ is perpendicular to $\stackrel{\to }{\mathrm{B}}$ ) is halved, then the radius of its path will :

1. not change
2. become two times
3. become one-fourth
4. become half

Ans. (d)

A metal plate is getting heated. Which one of following statements is incorrect ?

1. It is placed in a space varying magnetic field that does not vary with time.
2. A direct current is passing through the plate.
3. An alternating current is passing through the plate.
4. It is placed in a time varying magnetic field.

Ans. (a)

Case (a); Space varying magnetic field means field at different points are different and constant in magnitude so there will be no change in flux of metal plate hence no induced current will be there in plate so no heat loss/dissipation.

Case (b), (c); Alternating current or direct current will heat up plate due to heat loss.

Case (d); In time varying magnetic field, emf gets induced so induced current heats up the plate.

1. E0 I0
2. Zero

Ans. (d)

The speed acquired by a free electron when accelerated from rest through a potential difference of 100 V is :

1. 6 × 106 m s-1
2. 3 × 106 m s-1
3. 4 × 105 m s-1
4. 2 × 103 m s-1

Ans. (a)

Which one of the following is not affected by the presence of a magnetic field ?

1. A current carrying conductor
2. A moving charge
3. A stationary charge
4. A rectangular current loop with its plane parallel to the field

Ans. (c)

This section consists of 24 multiple choice questions with over all choice to attempt any 20 questions. In case more than desirable number of questions are attempted, only first 20 questions will be considered for evaluation.

1. 8 L
2. 6 L
3. 4 L
4. 2 L

Ans. (d)

1. 30°
2. 45°
3. 60°
4. 90°

Ans. (a)

Ans. (c)

Two students A and B calculate the charge flowing through a circuit. A concludes that 300 C of charge flows in 1 minute. B concludes that 3.125 × 1019 electrons flow in 1 second. If the current measured in the circuit is 5 A, then the correct calculation is done by :

1. A
2. B
3. both A and B
4. neither A nor B

Ans. (c)

Ans. (a)

Ans. (c)

Ans. (d)

Ans. (a)

1. 50 Hz

Ans. (d)

In the given network all capacitors used are identical and each one is of capacitance C. Which of the following is the equivalent capacitance between the points A and B ?

1. 6 C

Ans.(c)

Given, connecting wire is attached at given points, that makes same potential at the end of connection so, circuit can be redrawn as. All capacitors are identical

The given figure shows I – V graph of a copper wire whose length and area of cross-section are L and A respectively. The slope of this curve becomes :

1. less if the length of the wire is increased
2. more if the length of the wire is increased
3. more if a wire of steel of same dimension is used
4. more if the temperature of wire is increased

Ans. (a)

1. T
2. V

Ans. (c)

Two identical thick wires and two identical thin wires, all of the same material and the same length form a square in three different ways P, Q and R as shown. Due to the current in these loops the magnetic field at the centre of the loop will be zero in case of :

1. P and R only
2. Q and R only
3. P and Q only
4. P, Q and R

Ans. (a)

Ans. (d)

Which one of the following statements is true ?

1. An inductor has infinite resistance in a dc circuit.
2. A inductor and a capacitor both cannot conduct in a dc circuit.
3. A capacitor can conduct in a dc circuit but not an inductor.
4. An inductor can conduct in a dc circuit but not a capacitor.

Ans. (d)

1. 53 V
2. 43 V
3. 10 V
4. 6 V

Ans. (a)

If a charge is moved against a coulomb force of an electric field, then the :

1. intensity of the electric field increases
2. intensity of the electric field decreases
3. work is done by the electric field
4. work is done by the external source

Ans. (d)

Ans. (d)

A long solenoid carrying current produces a magnetic field B along its axis. If the number of turns in the solenoid is halved and current in it is doubled, the new magnetic field will be :

1. B/2
2. B
3. 2B
4. 4B

Ans. (b)

Question Nos. 45 to 49 are Assertion (A) and Reason (R) type questions. Given below are two statements labelled as Assertion (A) and Reason (R). Select the most appropriate answer from the options given below.
1. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
2. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
3. Assertion (A) is true, but Reason (R) is false.
4. Assertion (A) is false, but Reason (R) is true.

Assertion (A) : A bar magnet experiences a torque when placed in a magnetic field. Reason (R): A bar magnet exerts a torque on itself due to its own magnetic field.

Ans. (c)

(A): A bar magnet experiences a torque in magnetic field

= MB sin $\mathrm{\theta }$ So, assertion is true

(R): A bar magnet exerts torque due to other's magnetic field (not its own magnetic field)

So, reason is not true.

Assertion (A): In a series LCR circuit connected to an ac source, resonance can take place. Reason (R): At resonance XL = XC.

Ans. (b)

(A): When LCR connected to an AC source, LCR circuit shows the resonance at XL  XC because not its reason

(R): It is true.

Reason (R): The magnetic force is perpendicular to the velocity of the particle.

Ans. (a)

(A): A positive charged particle enters in magnetic field normally. It makes the circular path.

Ans. (a)

Assertion (A): A transformer is used to increase or decrease ac voltage only. Reason (R) : A transformer works on the basis of mutual induction.

Ans. (a)

(A): A transformer is used to increase or decrease AC voltage only (not dc) because AC voltage provides changing in current which produce magnetic flux.

(R): Transformer works on the principle of mutual induction between primary coil and secondary coil.

Both A and R are true but R is not the correct explanation of A.

This section consists of 6 multiple choice questions with an overall choice to attempt any 5 questions. In case more than the desirable number of questions are attempted, only the first 5 questions will be considered for valuation.

Two charged spheres A and B having their radii in the ratio 1 : 2 are connected together with a conducting wire. The ratio of their surface charge densities will be :

1. 1/2
2. 2
3. 1/4
4. 4

Ans. (b)

When two charged spheres are connected together, charge flows from one sphere to other in such a way that potentials of both spheres become equal.

Ans. (d)

Ans. (c)

For terminal B to be negative :

1. ε1 r2 > ε2 -r1
2. ε1 r2 < ε2 r1
3. ε1 r1 > ε2 r2
4. ε2 r2 = ε1 r1

Ans. (b)

Ans. (a)

Ans. (c)