Class XII, CBSE exam questions, Physics

Slight typo/omission is present in 1 or 2 questions. It will be rectified. Subsequent question papers will have the correct version.

SUB: PHYSICS                                                          MARKS: 50

CLASS: II PUC                                                           TIME: 1.5 Hrs


Total 25 marks


(Compulsory, very short-answer questions)

Each carries 1 mark.  Answer all.                                             (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?

(Optional 10 out of 18, short-answer questions)

Answer any TEN questions, 2 marks each.                                              (2×10=20)


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. What happens to the charge on a conducting shell? What happens to lines of forces when a conducting shell is present in the path of the lines of forces?

9. 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 AU, where AU is the unit of distance and 1 AU is the distance of the Sun from the earth. {This is a brain teaser}

10. 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.

11. Define flux of the uniform electric field vectorF.  Write out Gauss law of electrostatics.

12. A charge is placed on a line which intersects the axis of a Dipole Moment Vector P by 900 angle. What will be the potential V and electric field E at this point if this point is distance r apart from the center of the dipole moment?

13. A charge is placed on a line which is parallel to the axis of a Dipole Moment Vector P. What will be the potential V and electric field E at this charge’s location if this point is distance r apart from the center of the dipole moment?

14. 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.

15. Write the expression for the electric potential of a dipole P when the point of reference where we determine the potential is on the plane of the dipole so that its distance r apart from the center of the dipole. Differentiate this equation to obtain the electric field vector at this point. Take the angle of the reference line wrt Dipole-Axis asJ.



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

17. 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.  ?

18. In what energy band will a spectrum from a star appears if a star is known to radiate collapse of Hydrogen atom of level 2 to ground state? What will be the frequency of such radiation?

19. How many atoms are there in our periodic table as of this year? Name two radioactive elements. Mention in a sentence how radioactivity of Carbon is useful.

Ray Optics and Instruments

20. 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?

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

22. Write in 2-3 sentences what’s Total Internal Reflection (TIR). Which of these is not an example of TIR? Mirage, Prism, Diamond, Optical Fiber, Lightening, Magnetic induction.

23. What’s the mathematical expression for the power of a lens? If the focal length of a lens is 150 mm, what’s the power of this lens?


Total 25 marks (Optional, 5 out of 8, Long-answer questions)

SECTION-A {Electrostatics}

Each carry 5 marks, answer only three                                    (5×3)

Question 1

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.





Question 2

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.

Question 3

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?

Question 4

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


Each carry 5 marks, answer only one.                                  (5×1)

Question 5

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.

Question 6        

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} energy of the Hydrogen-atom.

SECTION-C {Ray Optics and Optical Instruments}

Each carry 5 marks, answer only one.                                  (5×1)

Question 7

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.


Question 8        

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.

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