# Class XII, CBSE exam questions, Physics, Updated Version 2 Reply

SUB: PHYSICS                                                                       MARKS: 50

CLASS: II PUC                                                                        TIME: 1.5 Hrs

Instructions:

* Numerical problems solved without writing the relevant formulae carry no marks

** Answers without relevant diagrams/circuits wherever necessary will not carry any marks

PART-A

## I. Answer all the following questions                                       (1×5)

1. If two charges are placed a distance r apart in a dielectric medium and in free-space where will be the Coulomb’s interaction be stronger?

2. What’s the Electric Potential of a Dipole Vector in the vertical direction to it?

3. What is the work done when a charge makes a trajectory on an equi-potential surface?

4. Write the de-Broglie formula for wavelength of a wave-particle if its momentum is p?

5. What’s the relation between angle of incidence and angle of reflection?

PART-B

## II. Answer any Five of the following questions, 2 marks each.   (2×5=10)

6. State how density of electric lines of force is related to the Electric Field Vector intensity E. Also mention another three properties of the lines of forces.

7. Write the vector expression for Coulomb’s law and state 3 forces that are conservative forces by nature.

8. How one defines the direction of a Dipole Vector? Define a unit dipole vector p. Write the expression for dipole vector by using p and the magnitude of charge = 3 Q and separation between the charge as 1 Angstrom.

9. What’s the energy of an alpha particle if we place it in a potential difference of 100 Volt assuming its at rest at this location? What will be the energy of a proton if its placed at the same location at rest.

10. Define atomic number, atomic mass and neutron number. What’s the atomic number and atomic mass of Oxygen?

11. Write out the complete expression of n-th level atomic spectra {energy level} of a Hydrogen atom. What energy will be released if the electron jumps from level=3 to level = 2.  ?

12. Write out the mirror equation. If an image is at 10 cm and object at 100 cm what’s the focal length of the mirror?

13. If angle of incidence is 300 and angle of refraction is 150 what will be the refractive index of the medium?

PART-C

## III. Answer any one of the following questions                                   (5×1)

14. Derive a) the Electric Field Vector E|| for the Dipole Vector P in a direction of alignment with the Dipole Vector, also called End-On configuration b) the scalar potential V^ due to the dipole in a vertical direction from the center of the dipole.

15. Derive a) the Electric Field vector Potential E^for the Dipole Vector P in a vertical direction from the center of the dipole, also called End-On configuration b) the scalar potential V(r) in a general direction at angle J to the Dipole.

## IV. Answer any two of the following questions                                   (5×2)

16. Define Electric flux F (Phi) in 1 or 2 lines and write out expression for it for a uniform electric field E. State Gauss Law in terms of charge and flux. State the mathematical vector form of Coulomb’s law of electrostatics force. What’s the flux of a vector if it is normal to the area vector?

17. Define Lines of force and flux of a vector. State Gauss law and derive its mathematical form by considering spherical symmetry.

18. Obtain an expression for the Nuclear Size of an atom by solving for distance of closest approach (r) which is also known as Impact Parameter (sayl). Use an incoming alpha-particle of mass mα and charge q that’s deflected by the presence of a nucleus of atomic number Z to obtain l. Now replace the variables by some numerical: mα = 6.8×10-27kg, e = 1.6 x 10-19 coul, Z=10, and vα= 2.50×107 m/s. Express the answer in Femto-meters.

## V. Answer any three of the following questions                                   (5×3)

19. A proton has a radius R and the charge Q is uniformly distributed. Use Bohr’s atomic energy level formula for the 1st excited state {n=2} to obtain energy of the Hydrogen-atom.

20. State the mirror equation and thin lens formula. For each case determine the focal length if the object distance is 100 cm and image distance is 25 cm.

21. Draw a simple and neat diagram showing a convex lens, optical center and focal point {Focus}. Now define power of a lens and obtain an expression for it.

22. Write the expression for the Electrostatics Potential of a point charge. Calculate the potential for a Nitrogen nucleus at a distance of 1 femto-meter (1 fm = 10-15 meters) {Z nitrogen=14}. Express this in terms of energy, in unit of Joule, by multiplying to the potential the charge of an alpha-particle = 2e.

## V. Answer any one of the following questions                                   (5×1)

23.  You obtained the following readings in one experiment you have performed to determine the focal length of a convex lens by u-v method. Following the data in the table, u, v, x, y and plot the graph between x and y and obtain the slope S. Then determine the focal length from the slope.

 SN Object O cm Lens L cm Image I cm u (cm) v (cm) x=20/u y=20/v 1 1.0 29.0 45.5 2 6.0 29.0 48.5 3 11.0 29.0 59.0 4 16.0 35.0 59.0 5 21.0 40.0 64.0

Determine (x1, y1), (x2, y2), (x, y), s = slope, y0 , f = focal length.