CBSE EXAMINATION PAPER-2023
PHYSICS
(Solved)
General Instructions :
Read the following instructions carefully and follow them :
- This question paper contains 42 questions. All questions are compulsory.
- This question paper is divided into 5 sections.
- Section A – questions number 1 to 2 are case based questions
- Section B – questions number 3 to 20 are multiple choice questions
- Section C – questions number 21 to 29 are very short answer
- Section D – questions number 30 to 36 are short answer
- Section E – questions number 37 to 42 are long answer
- There is no overall choice given in the question paper. However, an internal choice has been provided in few questions.
- Use of calculator is NOT allowed.
Section A
A capacitor is a system of two conductors separated by an insulator. The two conductors have equal and opposite charges with a potential difference between them. The capacitance of a capacitor depends on the geometrical configuration (shape, size and separation) of the system and also on the nature of the insulator separating the two conductors. They are used to store charges. Like resistors, capacitors can be arranged in series or parallel or a combination of both to obtain desired value of capacitance.
Section B
An electric dipole of length 2 cm is placed at an angle of 30° with an electric field 2 x 10^5 N/C. If the dipole experiences a torque of 8 x 10^⁻3 Nm, the magnitude of either charge of the dipole, is:
Two long parallel wires kept 2 m apart carry 3A current each, in the same direction. The force per unit length on one wire due to the other is:
When two parallel wires carry currents in the same direction, they experience an attractive force due to the magnetic fields generated by their currents. The formula for the force per unit length between two parallel currents is given by F/L = (μ₀/2π) * (I₁ * I₂) / d, where μ₀ is the permeability of free space, I₁ and I₂ are the currents in the wires, and d is the distance between them. For currents of 3A each and a distance of 2m apart, the force per unit length calculates to be 4.5 x 10⁻⁷ N/m, and since the currents are in the same direction, the force is attractive.
Which of the following has its permeability less than that of free space?
Aluminium has its permeability less than that of free space, making it the correct answer. The other materials listed, such as Copper chloride, Nickel, and Copper, do not have this property.
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:
Using Faraday's law of electromagnetic induction, the induced emf (E) in the coil can be calculated using the formula E = -N * (dΦ/dt), where N is the number of turns and dΦ/dt is the rate of change of magnetic flux. The area (A) of the square coil is (0.1 m * 0.1 m) = 0.01 m². The magnetic flux (Φ) is given by B * A, so dΦ/dt = A * (dB/dt) = 0.01 m² * 1 T/s = 0.01 V. Therefore, the induced emf E = 100 * 0.01 V = 1 V. The correct answer is 1 V.
In a Young's double-slit experiment, the screen is moved away from the plane of the slits. What will be its effect on the following?
(A) Angular separation of the fringes.
(B) Fringe-width.
The energy of a photon of wavelength λ is:
The ratio of the nuclear densities of two nuclei having mass numbers 64 and 125 is:
The nuclear density of different nuclei is generally constant, as it is largely independent of mass number. Therefore, when comparing two nuclei with different mass numbers, the ratio of their densities will essentially be 1. This is because both nuclei occupy a similar volume per nucleon despite their differing mass numbers.
During the formation of a p-n junction:
The diagram shows four energy levels of an electron in the Bohr model of hydrogen atom. Identify the transition in which the emitted photon will have the highest energy.
The highest energy photon is emitted during the transition from the highest energy level (n=4) to the lowest energy level (n=1), which corresponds to option IV. This is because the energy of a photon is directly related to the difference in energy levels; the greater the difference, the higher the energy of the emitted photon.
Which of the following graphs correctly represents the variation of a particle's momentum with its associated de-Broglie wavelength?
The correct option is 'Inversely proportional'. According to the de-Broglie hypothesis, the momentum (p) of a particle is inversely proportional to its wavelength (λ), which is expressed by the formula p = h/λ, where h is Planck's constant. This indicates that as the wavelength increases, the momentum decreases, hence their relationship is inversely proportional.
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
The correct option is 'Two of them connected in parallel and the combination in series to the third.' Connecting two capacitors in parallel adds their capacitances, yielding 4 μF + 4 μF = 8 μF. Then, connecting the resulting 8 μF in series with the third 4 μF capacitor gives an effective capacitance of 1/(1/8 + 1/4) = 6 μF.
Assertion (A): The resistance of an intrinsic semiconductor decreases with increase in its temperature.
Reason (R): The number of conduction electrons as well as holes increase in an intrinsic semiconductor with a rise in its temperature.
Assertion (A): The equivalent resistance between points A and B in the given network is 2R. Reason (R): All the resistors are connected in parallel.
The assertion A states that the equivalent resistance is 2R, but this is likely incorrect if all resistors are connected in parallel. In a parallel connection, the overall resistance is generally less than the smallest individual resistor. Therefore, the reason R is false, which makes the assertion A also false regarding the provided context.
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 magnetic moment m in a magnetic field B is given by the dot product of m and B.
Assertion (A) is true because when the plane of the loop is perpendicular to the magnetic field, the angle between the magnetic moment and the magnetic field is 90 degrees, leading to zero torque. Reason (R) is also true as it correctly states that the torque is determined by the dot product of the magnetic moment and the magnetic field. However, Reason (R) does not directly explain Assertion (A), as it merely describes the relationship without addressing the specific case of zero torque. Therefore, the correct option is that both are true, but R is not the correct explanation of A.
Which of the following statements about a series LCR circuit connected to an ac source is correct ?
At resonance, the reactance of the inductor (XL) equals the reactance of the capacitor (XC), resulting in the impedance being purely resistive. Consequently, the voltage drops across the inductor and capacitor become equal, but their phase is opposite. Thus, the statement 'At resonance, the voltage drop across the capacitor is more than that across the inductor' is not true. Instead, the correct understanding of resonance aligns with the behavior of the circuit's impedance and the relationships between voltage and reactance. Correct answer is If the frequency of the source is increased, the impedance of the circuit first decreases and then increases.
Section C
(a) How will the de Broglie wavelength associated with an electron be affected when (i) the velocity of the electron decreases? and (ii) accelerating potential is increased ? Justify your answer.
Identify the electromagnetic wave whose wavelength range is from about
(a) 10^-12 m to about 10^-8 m.
(b) 10^-3 m to about 10^-1 m.
Write one use of each.
Depict the orientation of an electric dipole in (a) stable and (b) unstable equilibrium in an external uniform electric field. Write the potential energy of the dipole in each case.
(b) A long straight wire AB carries a current I. A particle (mass m and charge q) moves with a velocity v, parallel to the wire, at a distance d from it as shown in the figure. Obtain the expression for the force experienced by the particle and mention its directions.
Draw a graph showing the variation of potential energy of a pair of nucleons as a function of their separation. Indicate the region in which the nuclear force is (a) attractive and (b) repulsive.
The potential difference applied across a given conductor is doubled. How
will this affect (i) the mobility of electrons and (ii) the current density in
the conductor ? Justify your answers.
Two coils C1, and C2, are placed close to each other. The magnetic flux φ2 linked with the coil C2, varies with the current I1, flowing in coil C1, as
shown in the figure. Find
(i) the mutual inductance of the arrangement, and
(ii) the rate of change of current (dI1/dt) that will induce an emf of 100 V
in coil C2.
(b) How would the stopping potential for a given photosensitive surface change if (i) the frequency of the incident radiation were increased ? and (i) the intensity of incident radiation were decreased ? Justify your answer.
(a) Write the expression for the Lorentz force on a particle of charge q moving with a velocity V in a magnetic field B. When is the magnitude of this force maximum ? Show that no work is done by
this force on the particle during its motion from a point r1, to point r2.
Section D
(a) A plane wave-front propagating in a medium of refractive index ‘μ1, is incident on a plane surface making an angle of incidence (i). It enters into a medium of refractive index μ2(μ2>μ1).
Use Huygen’s construction of secondary wavelets to trace the retracted wave-front. Hence verify Snell’s law of refraction.
An alternating voltage of 220 V is applied across a device X. A current of 0.22 A flows in the circuit and it lags behind the applied voltage in phase
by π/2 radian. When the same voltage is applied across another device Y,
the current in the circuit remains the same and it is in phase with the
applied voltage.
(i) Name the devices X and Y and,
(ii) Calculate the current flowing in the circuit when the same voltage is
applied across the series combination of X and Y.
State the basic principle behind the working of an ac generator. Briefly describe its working and obtain the expression for the instantaneous value of emf induced.
(a) Briefly describe how the current sensitivity of a moving coil galvanometer can be increased.
(b) A galvanometer shows full scale deflection for current Ig,. 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.
(a) (i) Differentiate between ‘distance of closest approach’ and ‘impact
parameter’.
(ii) Determine the distance of closest approach when an alpha particle of kinetic energy 3.95 MeV approaches a nucleus of Z = 79, stops and reverses its directions.
(b) Using Huygen’s construction, show how a plane wave is reflected from a surface. Hence verify the law of reflection.
(b) (i) State three postulates of Bohr’s theory of hydrogen atom.
(ii) Find the angular momentum of an electron revolving in the second orbit in Bohr’s hydrogen atom.
Section E
(a) (i) 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.
(ii) 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 drift velocity of electrons in wire A to that in wire B.
(a) 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 terms : (i) minority carrier injection in forward biasing and (ii) breakdown voltage in reverse biasing.
(a) (i) Draw a ray diagram to show the working of a compound microscope. Obtain the expression for the total magnification for the final image to be formed at the near point.
(ii) 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.
(b) (i) A cell emf of (E) and internal resistance (r) is connected across a variable load resistance (R). Draw plots showing the variation of terminal voltage V with (i) R and (ii) the current (I) in the load.
(ii) 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 difference across the equivalent cell.
(b) Name two important processes involved in the formation of a p-n
junction diode. With the help of a circuit diagram, explain the working of junction diode as a full wave rectifier. Draw its input and output waveforms. State the characteristic property of a junction diode that makes it suitable for rectification.
(b) (i) 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.
(ii) 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.
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