**CBSE Guess Paper**

**Physics**

**Class XII**

Q1. What is quantization of charge. What is its cause? Can a body have a charge of 18*10^{-20 }C?

Q2. How are n-P-n and P-n-P transistors represented symbolically in a circuit?

Q3. Write any one equation representing nuclear fusion reaction?

Q4. In a hydrogen atom, an electron revolves around a proton. Which of these two exerts a greater electrostatic force on the other?

Q5. Define gyromagnetic ratio. What is it’s value?

Q6. What is the order of voltages that can be built up using a Van De Graff generator?

Q7. Describe how a metallic rod can be made positively charged by the method of induction.

Q8. The instantaneous current flowing from an a.c. source is I=5 sin 314t. What is the rms. value of current?

Q9. A velocity selector is to be designed for particles of velocity 10m/s. What magnetic field should be employed if the electric field in it is 100 N/C.

Q10. The resistivity of a metal X is 3.2 x 10^{-8} while the free electron density is 5 x 10^{28} m^{-3}. Find the drift velocity of electrons if a potential gradient of 1 Vm^{-1} is applied across X.

Q11. What type of materials are used for making

**(a) Permanent magnets**

(b) Transformer cores.

Give two line reasons for each

Q12. Free electrons in a conductor are not at rest. How can you explain the absence of electric current without potential difference across its ends?

Q13. Define the term resistivity and write its S.I. unit. Derive the expression for the resistivity of a conductor in terms of number density of free electrons and relaxation time.

Q14. State Gauss theorem and using it derive the value of electric field at a point R mts away from an infinite line of charge having charge density λ C/m.

Q15. An alpha particle and a proton accelerated by the same potential difference enter into a magnetic field. Find the ratio of their radius and the ratio of their frequency.

Q16. A long cylinder of radius R_{o} is carrying a current I_{o} ,which is uniformly distributed over its cross section. Derive an expression for the magnitude of magnetic field inside as well as outside the wire. Plot a curve to show the variation of magnetic field with radial distance.

Q17. Explain the following terms

(a) Electrostatic shielding (b) Faraday cages (c) Dielectric constant

(d) Polarization (e) Electric potential (e) Electric flux

Q18. (a)A spherical Gaussian surface encloses a charge 8.85 x 10^{-8}C. Calculate the electric flux passing through the surface. If the radius of the Gaussian surface is doubled, how would the flux change

(b) A charge of 8.85 x 10^{-8}C is placed inside a cube of side 1mm. Find the total flux and the flux through one of the faces.

Q19. (a) Distinguish between metals, Insulators and semiconductors on the basis of their energy bands.

(b) Why are photodiodes used preferably in reverse bias condition? A photodiode is fabricated with band gap of 2.8 ev. Can it defect a wavelength of 6000 nm? Justify.

Q20. Define the term wavefront. Draw the wavefront and corresponding rays in the case of a (i) diverging sphericalwave

(ii) Plane wave

Using Huygen’s construction of a wavefront, explain the refraction of a plane wave front at a plane surface and hence verify snell’s law.