Physics Delhi Set 3 2022-2023 Science (English Medium) Class 12 Question Paper Solution

An electric dipole of length 2 cm is placed at an angle of 30° with an electric field 2 × 10⁵ N/C. If the dipole experiences a torque of 8 × 10^-3 Nm, the magnitude of either charge of the dipole is ______.

4 µC

7 µC

8 mC

2 mC

Two horizontal thin long parallel wires, separated by a distance r carry current I each in the opposite directions. The net magnetic field at a point midway between them will be ______.

zero

`((mu_0I)/(2pir))` vertically downward

`((2mu_0I)/r)` vertically upward

`((mu_0I)/(pir))` vertically downward

Which of the following cannot modify an external magnetic field as shown in the figure?

Nickel

Silicon

Sodium chloride

Copper

A square-shaped coil of side 10 cm, having 100 turns is placed perpendicular to a magnetic field which is increasing at 1 T/s. The induced emf in the coil is ______.

0.1 V

0.5 V

0.75 V

1.0 V

Which one of the following electromagnetic radiation has the least wavelength?

Gamma rays

Microwaves

Visible light

X-rays

In Young's double-slit experiment, the screen is moved away from the plane of the slits. What will be its effect on the following?

1. The angular separation of the fringes.
2. Fringe-width.

Both (i) and (ii) remain constant.

(i) remains constant, but (ii) decreases.

(i) remains constant, but (ii) increases.

Both (i) and (ii) increase.

E, c and `v` represent the energy, velocity and frequency of a photon. Which of the following represents its wavelength?

`(hν)/c^2`

hν

`(hc)/E`

`(hν)/c`

The ratio of the nuclear densities of two nuclei having mass numbers 64 and 125 is ______.

`64/125`

`4/5`

`5/4`

1

The energy required by an electron to jump the forbidden band in silicon at room temperature is about ______.

0.01 eV

0.05 eV

0.7 eV

1.1 eV

The diagram shows the four energy levels of an electron in the Bohr model of the hydrogen atom. Identify the transition in which the emitted photon will have the highest energy.

I

II

III

IV

Which of the following graphs correctly represents the variation of a particle momentum with its associated de-Broglie wavelength?

The capacitors, each of 4 µF are to be connected in such a way that the effective capacitance of the combination is 6 µF. This can be achieved by connecting ______.

All three are in parallel.

All three are in series.

Two of them connected in series and the combination in parallel to the third.

Two of them connected in parallel and the combination in series to the third.

What is the ratio of inductive and capacitive reactance in an ac circuit?

ω²LC

LC²

`(LC)/omega^2`

`omega^2L`

In an interference experiment, a third bright fringe is obtained at a point on the screen with a light of 700 nm. What should be the wavelength of the light source in order to obtain the fifth bright fringe at the same point?

420 nm

750 nm

630 nm

500 nm

The radius of the n^th orbit in the Bohr model of hydrogen is proportional to ______.

`n^2`

`1/n^2`

n

`1/n`

• Assertion (A): The given figure does not show a balanced Wheatstone bridge.
• Reason (R): For a balanced bridge small current should flow through the galvanometer.

Both Assertion (A) and Reason (R) are true and (R) is the correct explanation of (A).

Both Assertion (A) and Reason (R) are true and (R) is NOT the correct explanation of (A).

Assertion (A) is true and Reason (R) is false.

Assertion (A) is false and Reason (R) is also false.

• Assertion (A): The deflecting torque acting on a current-carrying loop is zero when its plane is perpendicular to the direction of the magnetic field.
• Reason (R): The deflecting torque acting on a loop of the magnetic moment `vecm` in a magnetic field `vecB` is given by the dot product of `vecm` and `vecB`.

Both Assertion (A) and Reason (R) are true and (R) is the correct explanation of (A).

Both Assertion (A) and Reason (R) are true and (R) is not the correct explanation of (A).

Assertion (A) is true and Reason (R) is false.

Assertion (A) is false and Reason (R) is also false.

Plot a graph showing the variation of photoelectric current, as a function of anode potential for two light beams having the same frequency but different intensities I₁ and I₂ (I₁ > I₂). Mention its important features.

How will the de-Broglie wavelength associated with an electron be affected when the velocity of the electron decreases? Justify your answer.

How will the de-Broglie wavelength associated with an electron be affected when the accelerating potential is increased? Justify your answer.

How would the stopping potential for a given photosensitive surface change if the frequency of the incident radiation were increased? Justify your answer.

How would the stopping potential for a given photosensitive surface change if the intensity of incident radiation was decreased? Justify your answer.

How are electromagnetic waves produced? 

Write the two characteristics of electromagnetic waves.

Three-point charges Q, q and -q are kept at the vertices of an equilateral triangle of side L as shown in the figure. What is 

1. the electrostatic potential energy of the arrangement? and
2. the potential at point D?

Two identical circular loops P and Q, each of radius R carrying current I are kept in perpendicular planes such that they have a common centre O as shown in the figure.

Find the magnitude and direction of the net magnetic field at point O.

A long straight conductor kept along X' X axis, carries a steady current I along the +x direction. At an instant t, a particle of mass m and charge q at point (x, y) moves with a velocity `vecv` along +y direction. Find the magnitude and direction of the force on the particle due to the conductor. 

Two conductors, made of the same material have equal lengths but different cross-sectional areas A₁ and A₂ (A₁ > A₂). They are connected in parallel across a cell. Show that the drift velocities of electrons in two conductors are equal.

Two coils C₁ and C₂ are placed close to each other. The magnetic flux Φ₂ linked with coil C₂ varies with the current I₁ flowing in coil C₁ as shown in the figure. Find

1. The mutual inductance of the arrangement, and
2. The rate of change of current `((dI_1)/(dt))` will induce an emf of 100V in coil C₂.

A plane wavefront propagating in a medium of refractive index 'μ₁' is incident on a plane surface making the angle of incidence 'i' as shown in the figure. It enters into a medium of refractive index 'μ₂' (μ₂ > μ₁). Use Huygens' construction of secondary wavelets to trace the propagation of the refracted wavefront. Hence verify Snell's law of refraction.

Using Huygens's construction, show how a plane wave is reflected from a surface. Hence verify the law of reflection.

An alternating current I = 14 sin (100 πt) A passes through a series combination of a resistor of 30 Ω and an inductor of `(2/(5pi))` H. Taking `sqrt2` = 1.4 calculate the rms value of the voltage drops across the resistor and the inductor.

An alternating current I = 14 sin (100 πt) A passes through a series combination of a resistor of 30 Ω and an inductor of `(2/(5pi))` H. Taking `sqrt2` = 1.4 calculate the power factor of the circuit.

State the basic principle behind the working of an ac generator.

Briefly describe the working of ac generator.

Obtain the expression for the instantaneous value of emf induced.

How is current sensitivity increased?

A galvanometer shows full-scale deflection for current I_g. A resistance R₁ is required to convert it into a voltmeter of range (0 - V) and a resistance R₂ to convert it into a voltmeter of range (0 - 2V). Find the resistance of the galvanometer.

Calculate the binding energy of an alpha particle in MeV. Given

mass of a proton = 1.007825 u

mass of a neutron = 1.008665 u

mass of He nucleus = 4.002800 u

1u = 931 MeV/c²

A heavy nucleus P of mass number 240 and binding energy of 7.6 MeV per nucleon splits into two nuclei Q and R of mass number 110 and 130 and binding energy per nucleon of 8.5 MeV and 8.4 MeV respectively. Calculate the energy released in fission.

Explain how free electrons in a metal at constant temperature attain an average velocity under the action of an electric field. Hence, obtain an expression for it.

Consider two conducting wires A and B of the same diameter but made of different materials joined in series across a battery. The number density of electrons in A is 1.5 times that in B. Find the ratio of the drift velocity of electrons in wire A to that in wire B.

Answer the following question.

A cell of emf E and internal resistance r is connected across a variable resistor R. Plot the shape of graphs showing a variation of terminal voltage V with (i) R and (ii) circuit current I.

Three cells, each of emf E but internal resistances 2r, 3r and 6r are connected in parallel across a resistor R.

Obtain expressions for (i) current flowing in the circuit, and (ii) the terminal potential differences across the equivalent cell.

Draw the circuit arrangement for studying V-I characteristics of a p-n junction diode in (i) forward biasing and (ii) reverse biasing. Draw the typical V-I characteristics of a silicon diode.

Describe briefly the following term:

minority carrier injection in forward biasing.

Describe briefly the following term:

breakdown voltage in reverse biasing

Write the two processes that take place in the formation of a p-n junction.

Draw the circuit diagram of a full wave rectifier. Explain its working showing its input and output waveforms.

Write the property of a junction diode which makes it suitable for rectification of ac voltages.

Explain about the compound microscope and obtain the equation for magnification.

In a compound microscope an object is placed at a distance of 1.5 cm from the objective of focal length 1.25 cm. If the eye-piece has a focal length of 5 cm and the final image is formed at the near point, find the magnifying power of the microscope.

Draw a ray diagram for the formation of image of an object by an astronomical telescope, in normal adjustment. Obtain the expression for its magnifying power.

The magnifying power of an astronomical telescope in normal adjustment is 2.9 and the objective and the eyepiece are separated by a distance of 150 cm. Find the focal lengths of the two lenses.

Read the following paragraph and answer the questions.

1. An object AB is kept in front of a composite convex lens, as shown in the figure. Will the lens produce one image? If not, explain.
   
2. A real image of an object formed by a convex lens is observed on a screen. If the screen is removed, will the image still be formed? Explain.
3. A double convex lens is made of glass with a refractive index of 1.55 with both faces of the same radius of curvature. Find the radius of curvature required if the focal length is 20 cm.
   OR
   Two convex lenses A and B of focal lengths 15 cm and 10 cm respectively are placed coaxially 'd' distance apart. A point object is kept at a distance of 30 cm in front of lens A. Find the value of 'd' so that the rays emerging from lens B are parallel to its principal axis.

Read the following paragraph and answer the questions.

1. Find the equivalent capacitance between points A and B in the given diagram.
   
2. A dielectric slab is inserted between the plates of the parallel plate capacitor. The electric field between the plates decreases. Explain.
3. A capacitor A of capacitance C, having charge Q is connected across another uncharged capacitor B of capacitance 2C. Find an expression for (a) the potential difference across the combination and (b) the charge lost by capacitor A.
   OR
   Two slabs of dielectric constants 2K and K fill the space between the plates of a parallel plate capacitor of plate area A and plate separation d as shown in the figure. Find an expression for the capacitance of the system.