# HBSE Class 12 Physics Pre-Board Question Paper 2024 Answer Key

Haryana Board (HBSE) Class 12 Physics Pre Board Question Paper 2024 Answer Key. Haryana Board Class 12th Pre Board Question Paper PDF Download 2024. Haryana Board Class 12th Pre Board Question Paper Physics 2024. HBSE Class 12th Physics Pre Board Question Paper Solution 2024. HBSE Physics Pre Board Question Paper 2024 Class 12.

HBSE Class 12 Physics Pre-Board Question Paper 2024 Answer Key

Section – A (1 Mark)

1. With the increase of area of cross section of a conductor, its resistivity :
(a) increases
(b) decreases
(c) may increase or decrease
(d) does not change
Answer – (d) does not change

2. Torque acting on a dipole in electric field is given by :
(a) pE
(b) pEsinθ
(c) pEcosθ
(d) 0

3. Magnetic Susceptibility of diamagnetic substance is :
(a) small & negative
(b) small & positive
(c) large & positive
(d) none of these
Answer – (a) small & negative

4. Which phenomenon illustrates particle nature of light waves?
(a) Interference
(b) Diffraction
(c) Polarization
(d) None of these
Answer – (d) None of these
(Photoelectric effect illustrates particle nature of light waves)

5. Which of the following has highest frequency?
(a) X-rays
(c) Ultraviolet rays
(d) Gamma rays

6. The ratio of the intensities of two light waves is 16 : 9. The ratio of maximum and minimum intensities in their interference pattern will be :
(a) 49 : 3
(b) 49 : 1
(c) 25 : 7
(d) 256 : 81
Answer – (b) 49 : 1
Imax = (√16+√9)2 = (4+3)2 = 72 = 49
Imin = (√16–√9)2 = (4–3)2 = 12 = 1
Imax : Imin = 49 : 1

7. The least distance of distinct vision for a normal human eye is :
(a) 25 cm
(b) 54 cm
(c) 1 Dioptre
(d) None of these

8. Prism works on the principle of :
(a) Reflection
(b) Refraction
(c) Dispersion
(d) Diffraction

9. In the following nuclear reaction, what is X?
92U²³⁸ xTh²³⁴ + 2He⁴
(a) 92
(b) 2
(c) 90
(d) 4

10. Two light waves of equal amplitude and wavelength are superimposed. The amplitude of the resultant wave will be maximum when the phase difference between them is :
(a) Zero
(b) π/2
(c) π
(d) π/4

11. The de-Broglie wavelength (λ) associated with a particle of momentum (p) is given as :
(a) p/h
(b) h/p
(c) hp
(d) None of these
Wavelength = Plank’s constant / Momentum

12. At absolute zero, Germanium act as :
(a) Insulator
(b) Metal
(c) Non-metal
(d) Semi-conductor

13. The energy gap is maximum in :
(a) Superconductor
(b) Metal
(c) Non-metal
(d) Semi-conductor

14. In glass, the velocity of light is minimum for :
(a) Red
(b) Violet
(c) Yellow
(d) Green

15. Assertion (A) : In a uniform Electric Field, electrons move in the opposite direction of Electric Field.
Reason (R) : This is because of the negative charge of an electron.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Ascertion is true but reason is false.
(d) Both assertion and reason are false.
Answer – (a) Both assertion and reason are true and reason is the correct explanation of assertion.

16. Assertion (A) : The whole charge of a conductor cannot be transferred to another isolated conductor.
Reason (R) : The total transfer of charge from one to another is not possible.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Ascertion is true but reason is false.
(d) Both assertion and reason are false.
Answer – (b) Both assertion and reason are true but reason is not the correct explanation of assertion.

17. Assertion (A) : Total flux though a closed surface is zero if net charge enclosed by the surface is zero.
Reason (R) : Gauss Law is true for any closed Surface, no matter what its shape or size.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Ascertion is true but reason is false.
(d) Both assertion and reason are false.
Answer – (a) Both assertion and reason are true and reason is the correct explanation of assertion.

18. Assertion (A) : In young’s double slit experiment, all fringes are of equal width.
Reason (R) : The fringe width depends upon wavelength of light (λ) used, distance of screen from plane of slits (D) and slits separation (d).
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Ascertion is true but reason is false.
(d) Both assertion and reason are false.
Answer – (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Section – B (2 Marks)

19. The force of attraction between a positively charged particle and a negatively charged particle is F, when distance between them is made one-fourth, what will be the value of this force?
Answer : Initial Force (F) = kq1q2/r2
New force (F’) = kq1q2/(¼)2 = 16kq1q2/r2 = 16F
F’ = 16F
So, when the distance between the two charges is one-fourth, the force is increased 16 times.

OR

What are the number of electrons in 1µC?
Answer : Total charge = number of electrons × charge on one electron
Q = n × e
n = Q/e = 10–6/(1.6×10–19) = 6.25 × 1012 e
So, number of electrons = 6.25 × 1012 electrons

20. Explain the significance of negative energy of an electron in an orbit?
Answer – The energy of an electron in the orbit of an atom is negative. It shows that the electron is bound to the nucleus. Greater the value of negative energy, more tightly the electron is bound to the nucleus.

21. A circular coil of radius r carrying current I. What is magnetic field at the centre of circular coil?

22. Draw the diagram for a.c. circuit containing capacitor only.

23. State two properties of Electromagnetic waves.
Answer – (i) Electromagnetic waves are transverse in nature.
(ii) They propagate by varying electric fields and magnetic fields, such that these two fields are at right angles to each other and at a right angle with the direction of propagation of the wave.
(iii) Same speed in vacuum for all waves.
(iv) No material medium required for propagation.

24. Define power of a lens and give its S.l. unit.
Answer – Power of a lens is its ability to converge or diverge the rays of light falling on it. The power P of a lens of focal length f (in m) is given by P = 1/f.
The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.

25. What will be de-Broglie wavelength associated with an electron moving under 100V patential difference?
Answer : Wavelength (λ) = 1.227/√V = 1.227/√100 = 1.227/10 = 0.1227 ≈ 0.123 nm

OR

How will be photoelectric current change on decreasing the wavelength of incident radiation for a given photosensitive material?
Answer – Photoelectric current is not affected on decreasing the wavelength of incident radiation provided its intesity remains unchanged.

Section – C (3 Marks)

26. State and prove Ampere’s Circuital law.

OR

State and explain Biot Savart Law.
Answer – Biot savart law states that “magnetic field due to a current-carrying conductor at a distance point is inversely proportional to the square of the distance between the conductor & point, and the magnetic field is directly proportional to the length of the conductor, current flowing in the conductor”.
It can be mathematically expressed as :
dB = μo/4π × Idlsinθ/r2

27. What are electric field lines? Draw these for a point charges +Q and –Q.
Answer – Electric field lines are imaginary lines, can be straight or curved, tangent to it at any point gives the direction of electric field.

28. Define mass defect and binding energy for a nucleus.
Answer : Mass defect – Mass defect is defined as, the difference between the sum of masses of the nucleons of nucleus and the rest mass of nucleus.
• Binding energy – Binding energy of a nucleus is the energy with which nucleons are bound in the nucleus.

29. What are the drawbacks of Bohr’s atomic Model?
Answer – Drawbacks of Bohr’s atomic Model :
(i) He could not explain the details of the Hydrogen and Helium atomic spectrum.
(ii) He did not explain the splitting of spectral lines in presence of a magnetic field.
(iii) The intensity of spectral lines was not explained by him.
(iv) He could not explain the Zeeman effect i.e. splitting of spectral lines in Magnetic field as well as the Stark effect i.e. splitting of spectral lines in electric field.

30. Explain diode as a full wave rectifier.
Answer – A full wave rectifier is a circuit which converts the entire alternating input voltage into direct voltage. This is achieved by using two junction diodes. The p-sides of the diode are connected to the input while the n-sides are connected together and along with the centre, tap form the output.
During the positive cycle of the alternating voltage, one diode conducts while the other doesn’t. This is reversed during the negative cycle. Hence, both positive and negative cycles are converted into a direct voltage.

Section – D (5 Marks)

31. What is total internal reflection? Explain it with two exemples.
Answer – Total internal reflection is a phenomenon of reflection of ray back to the same medium when passing from denser medium to rarer medium in a such away that angle of incidence greater than its critical angle.

32. What is Wheatstone bridge? What do you mean by balanced condition of Wheatstone bridge? Using Kirchhoff’s law derive the balanced condition of Wheatstone bridge.
Answer – It works on the principle of null deflection, which means the ratio of their resistances are equal and hence no current flows through the circuit. Under normal conditions, the bridge will be in the unbalanced condition where current flows through the galvanometer. The bridge will be in a balanced condition when no current flows through the galvanometer. One may achieve this condition by adjusting the known resistance and variable resistance.
The Wheatstone bridge principle states that if four resistances P, Q, R, and S are arranged to form a bridge with a cell and key between A and C, and a galvanometer between B and D then the bridge is said to be balanced when the galvanometer shows a zero deflection.
In balanced condition, Ig = 0
so, VB = VD or  P/Q = R/S .This is called condition of balance.

33. State and prove Gauss’s law in Electrostatics.
Answer – Gauss’s Theorem : The surface integral of electric field over a closed path is equal to 1/∈o times the total charge enclosed by surface.

Section – E (Case Study : 4 Marks)

34. When a conductor does not have a current through it, its conduction electrons move randomly, with no net motion in any direction. When the current flows through the conductor, these electrons actually still move randomly, but now they tend to drift with the drift speed vd. The drift speed is very less as compared to speeds in random thermal motion.
Questions :
(a) A steady current I flows through a metallic conductor whose area of cross-section (A) increases continuously from one end to the other, the drift velocity of free electron (vd) as a function of A will be …………..

(b) For Ohm’s law is obeyed, then what is the relation between electric field (E) and drift velocity (vd)?

(c) When a current flows in a conductor, what is the order of magnitude of drift velocity of electrons through it?

(d) Two nichrome wires of equal lengths but having radii in the ratio 1 : 3 are connected in series across an electric cell. The drift velocities of free electrons through them will be in the ratio of ………….
Answer – R1 : R2 = 1 : 3
Vd ∝ 1/R
Vd1 : Vd2 = R2 : R1 = 3 : 1

OR

What is the path of electrons : (i) in the absence,  (ii) in the presence of electric field?
Answer – (i) path are straight,  (ii) path are twisted or curved

35. A galvanometer is a device which is used to detect presence of current in a conductor. But it cannot be used directly as an ammeter. It has a very large resistance. If connected in series like an ammeter, it affects the value of current in a circuit. But it can be converted into an ammeter. Similarly, a galvanometer can be used as a voltmeter also. In both cases, a shunt resistance is connected with the galvanometer.
Questions :
(a) To convert a galvanometer of resistance RG into voltmeter a shunt resistance R is connected ………….

(b) To convert a galvanometer of resistance RG into ammeter a shunt resistance R is connected …………..