Pre Board Jan 2025 Exam Class 12th : answer key/ question paper

Pre Board Jan 2025 Exam Class 12th

Subject: -Physics (Theory)

TIME 3 Hrs.

M.M. 70

Instructions

• The question paper consists of 18 questions. All questions are compulsory.
• Section A contains question no. 1 which carries 20 objective type questions of one mark each.
• Section B contains question numbers 2 to 8 which carry 2 marks each.
• Section C contains question numbers 9 to 15 which carry 3 marks each.
• Section D contains question numbers 16 to 18 which carry 5 marks each.

Answer key 

(i) (a) expands

(ii) (a) C2N-1 'm-²

(iii) (c) increase in temperature

(iv) (d) work per unit charge

(v) (c) 90°

(vi) (c) small resistance in parallel

(vii) (b) The direction of the induced current

(viii) (b) glass to diamond

(ix) (c) -5 D

(x) (b) have high resolution

(xi) (c) Remains constant

(xii) (b) phosphorus and boron respectively

(xiii) (b) insulators

(xiv) (a) fusion

(xv) d) 15P30 and 14Si30

(xvi) (T)

(xvii) (T)

(xviii) (T)

(xix) (F)

(xx) (F)

Section-A (1 Mark Questions)

Q1. Objective Type Questions

MCQS

1. (i) When a soap bubble is charged it
   1. (a) expands
   2. (b) contracts
   3. (c) remain same
   4. (d) becomes elastic
2. (ii) The unit of permittivity of free space is
   1. (a) C2N-1 'm-²
   2. (b) N m² C
   3. (c) Fm
   4. (d) NC
3. (iii) Specific resistance of a conductor increases with
   1. (a) increase in cross-sectional area
   2. (b) decrease in length.
   3. (c) increase in temperature
   4. (d) decrease in temperature
4. (iv) Electromotive force of a cell is basically
   1. (a) force
   2. (b) power
   3. (c) current
   4. (d) work per unit charge
5. (v) What is angle of dip at magnetic poles of earth
   1. (a) zero
   2. (b) 45°
   3. (c) 90"
   4. (d) 180°
6. (vi) Galvanometer is converted into ammeter by connecting
   1. (a) large resistance in parallel
   2. (b) small resistance in series
   3. (c) small resistance in parallel
   4. (d) large resistance in series
7. (vii) Lenz's law gives:
   1. (a) The magnitude of the induced e.m.f
   2. (b) The direction of the induced current
   3. (c) both the magnitude and direction of the induced current
   4. (d) The magnitude of the induced current
8. (viii) Total internal reflection takes place when light travels from
   1. (a) glass to water
   2. (b) glass to diamond
   3. (c) air to water
   4. (d) air to glass
9. (ix) The focal length of concave lens is 20 centimeter its power is
   1. (a) -0.2 D
   2. (b) +5D
   3. (c)-5 D
   4. (d) co
10. (x) An astronomical telescope has large aperture to
    1. (a) reduce spherical aberration
    2. (b) have high resolution
    3. (c) increase span of observation
    4. (d) have lower dispersion
11. (xl) Kinetic energy of the photoelectrons varies with intensity of photons as
    1. Directly proportional
    2. Inversely proportional
    3. Remains constant
    4. None of the above
12. (xii) n-type and p-type silicon can be obtained by doping pure silicon with
    1. sodium and magnesium respectively
    2. phosphorus and boron respectively
    3. boron and phosphorus respectively.
    4. indium and sodium respectively
13. (xiii) Energy gap is maximum in
    1. metals
    2. insulators
    3. semiconductors
    4. superconductors
14. (xiv) Hydrogen bomb is based upon
    1. fusion
    2. fission
    3. transmutation
    4. chemical reaction
15. (xv) Which of the following pairs is an isobar?
    1. H and H²
    2. H² and H³
    3. C12 and C13
    4. 15P30 and 14Si30
16. (xvi) in diffraction of light, the intensity of secondary maxima goes on decreasing with the order of the maxima. (T/F)
17. (xvii) Transformer does not work on dc. (T/F)
18. (xviii) Apparent depth in water is greater than real depth. (T/F)
19. (xix) Electromagnetic waves need some material medium for their propagation. (T/F)
20. (xx) Photoelectric effect is convincing proof of wave nature of particle. (T/F)

Section-B (2 Marks Questions)

1. Q2. What is emf of cell? On what factors its value depends? Or In a meter bridge two unknown resistances R and X when connected in two gaps give null point at 40 cm from one end. What is the ratio of R and X?
2. Q3. Define SI units of capacitance. Or A 0.5 µF capacitor is connected parallel to 0.75µF capacitor and arrangement is joined to 100 dc source. Calculate charge taken from the source and charge on each capacitor. 
3. Q4. Write 2 properties of paramagnetic substances.
4. Q5. What is self induction? What are its SI units?
5. Q6. Give 2 properties and four uses of infrared rays.
6. Q7. Write Debroglie hypothesis for matter wave and derive expression for Debroglie wavelength. OR The work function of sodium is 2.3 eV. Calculate the maximum wavelength for the light that will cause photoelectrons to be emitted from sodium. 
7. Q8. On the basis of Bohr model, find the kinetic, potential and total energy of an electron in terms of r and plot these three energies in graph of energy versus distance r.
8.  Q9. Derive an expression for the potential energy of a dipole in a uniform electric field.

Section-C (3 Marks Questions)

Q10. Explain principle, construction and working of meter bridge.

Q11. Using Biot Savart's law, derive an expression for magnetic field at a point on the axis of current carrying circular coil.

Or

A solenoid of infinite length consists of single layer of 1000 turns per unit length of a wire carrying a current of 2mA. Calculate the magnetic field on the axis at the middle of the solenoid.

Q12. Explain principle and working of transformer with the help of diagram.

Or

At 10 µF capacitor, 50 O resistance and 10 H inductor are connected in series to source of alternating voltage of 220 Volt. What is maximum current in the circuit when resonance occurs?

Q13. (a) What are optical fibers? Give its one use.

(b) The sun appears before the sunrise and after sunset for few minutes, why?

Or

In young double slit experiment, the slits are separated by 0.03 cm and screen is placed 15 m away. The distance between central fringe and 4th fringe is 1cm. Determine the wavelength of light used in experiment.

Q14. Define binding energy per nucleon. Draw a graph showing variation of binding energy per nucleon with mass number of different nuclei.

Q15. Differentiate between intrinsic and extrinsic semiconductors.

Q16. Read the passage carefully and give answers of questions:

Section-D (Comprehension Type Questions)

Ferromagnetic substances are those which gets strongly magnetised when placed in an external magnetic field. They have strong tendency to move from a region of weak magnetic field to strong magnetic field. The individual atoms in a ferromagnetic material possess a dipole moment as in a paramagnetic material. However they interact with one another in such a way that they spontaneously align themselves in a common direction overall macroscopic volume called domain. Each domain has a net magnetisation. Typical domain size is 1mm and the domain contains about 101" atoms. When we apply an external magnetic field Be the domains orient themselves in a direction of Bo and simultaneously the domain oriented in the direction of Bo grow in size. Thus, in a ferromagnetic material the field lines are highly concentrated. In non-uniform magnetic field, the sample tends to move towards the region of high field. We may wonder as to what happens when the external field is removed. In some ferromagnetic materials, the magnetisation persists. Such materials are called hard magnetic materials. Alnico an alloy of iron, aluminium, nickel, cobalt and copper is one such material. The naturally occurring lodestone is another such materials. They are used to form permanent magnets and as a compass needle. On the other hand, there is a class of ferromagnetic materials in which the magnetisation disappears on removal of external field. Soft iron is one such material. Such materials are called soft ferromagnetic materials. There are a number of elements, which are ferromagnetic like iron, cobalt, nickel, gadolinium etc. The relative magnetic permeability is greater than 1000. The ferromagnetic property depends on the temperature. At high enough temperature a ferromagnet becomes a paramagnet. The domain structure disintegrates with the temperature.

Questions:

(1) What is relative magnetic permeability of ferromagnetic materials?

(2) What is effect of temperature on ferromagnetic substance?

(3) What are examples of ferromagnetic materials?

(iv) What happens to ferromagnetic materials when placed in non uniform magnetic field?

(v) How many atoms does a domain contains?

Section-E (5 Marks Questions)

Q17. (a) State and prove Gauss theorem. (b) Deduce expressions for torque acting on an electric dipole placed in uniform electric field.

Or

Derive expressions for the capacity of parallel plate capacitor with a dielectric slab placed in between the plates of capacitor.

Q18. What is compound microscope? With the help of ray diagram, explain its working. Find an expression for its magnifying power.

Or

State Huygens principle. With the help of suitable diagram prove the laws of refraction on the basis of wave theory.