Annual Exam Class12

PRE BOARD EXAMINATION 2020

CLASS 12TH PHYSICS

TIME 3:00Hrs MAXIMUM MARKS 70

General Instructions:

(1) All questions are compulsory. There are 33 questions in all.

(2) This question paper has five sections: Section A, Section B, Section C, Section D and Section E.

(3) Section A contains ten very short answer questions and four assertion reasoning MCQs of 1 mark each; Section B has two case based questions of 4 marks each

(4)Section C contains nine short answer questions of 2 marks each, Section D contains five short answer questions of 3 marks each and Section E contains three long answer questions of 5 marks each.

SECTION A

Q1Define the term 'intensity of radiation' in photon picture.

Q2 How are X-rays produced?

Q3 A plane EM- Wave travels in vaccum along Z-axis.What can you say about the direction of electric and magnetic field vectors?

Q4 Define ionisation energy.What is its value for a Hydrogen atom?

Q5 What happens when a forward bias is applied to a p-n junction?

Q6 An electron and a proton have the same kinetic energy.Which one of the two has the larger de Broglie wavelength and why?

Q7 An electron does not suffer any deflection while passing through a region of uniform magnetic field.What is the direction of the magnetic field?

Q8 Why AC Voltage is preferred over DC Voltage ?Give two reasons.

Q9. What is the ratio of speed of infrared rays and ultraviolet rays in vaccum ?

Q10 What is the phase difference between voltage and current in a LCR series circuit at resonance?

For question numbers 11, 12, 13 and 14, two statements are given-one labeled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:

(a) Both A and R are true and R is the correct explanation of A.

(b) Both A and R are true but R is not the correct explanation of A.

(d) A is false and R is also false.

11. Assertion (A):The charge on any body can be increased or decreased in terms of 'e'.

Reason (R): Quantisation of charge means that the charge on a body is the integral multiple of 'e'.

12. Assertion (A):The electric field inside a cavity is always zero.

Reason (R) : Charge reside only on the outer surface of a conductor with cavity.

13. Assertion (A) : A total reflecting prism is used to erect the inverted image without deviation.

Reason (R) : Rays of light incident parallel to the base of prism emerge out as parallel rays.

14. Assertion (A) : If objective and eyepiece of a microscope are interchanged ,then it can work as telescope .

Reason (R): The objective lens of telescope has much smaller focal length than the eyepiece.

SECTION-B

Questions 15 and 16 are case study based questions and are compulsory.

Each question carriers 1 mark.

15.ELECTROSTATIC SHIELDING:

The phenomenon of making a region free from any electric field is called electrostatic shielding or electrostatic screening.It is base on the fact that electric field becomes zero inside the cavity of hollow conductor.In a hollow conductor charges are distributed on the surface of conductor.Such a field free region is also called a Faraday Cage.Such a cage can block the effects of external field on its internal contents,or the effects of an internal field on the outside environment.Inside the Faraday cage ,electric field is always zero.Even if the conductor is charged or charges are induced on a neutral conductor by an external field ,all charges reside only o the outer surface of the conductor.Hence , any cavity of any shape and size is always shielded from outer electric region.

(i)In a region of constant potential

(a)the electric field is uniform

(b)the electric field is zero

(c)there can be no charge inside the region

(d)both (b) and (c)

(ii)If a conductor has non-zero potential and there are no charges anywhere else outside,then

(a)there must be charges on the surface or inside itself.

(b)there cannot be any charge in the body of the conductor.

(c)there must be charges only on the surface.

(d)both (a) and (b).

(iii)The work done in carrying a charge Q once round a circle of radius r with charge q at the centre of the circle is

(a)kQ/r

(b)kQq/r

(c)Qq/2r

(d)zero

(iv)At a point A, there is a electric field of 500V/m and potential difference of 3000V.The distance between the point charge and A is

(a)6m

(b)36m

(d)144m

16. Fibre optics ,or optical fibre refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic stand or fibre.Fiber optics is used for long distance and high performance data networking.Fibre optics are commonly used in telecommunication services such as internet ,television and telephones.

(i)When a ray of light enters from denser medium to rarer medium it bends

(a)towards normal

(b)away from normal

(c)perpendicular to normal

(d)parallel to normal

(ii)The outer rarer coating of fiber is called

(a)cladding

(b)core

(c)coat

(d)mantle

(iii)The critical angle for glass is

(a)50⁰

(b)42⁰

(d)30⁰

(iv)critical angle of light passing from glass to air is minimum for

(a)red

(b)green

(c)Violet

(d)yellow

SECTION-C

All questions are compulsory. In case of internal choices, attempt any one.

17. A charge Q is distributed over the surfaces of two concentric hollow spheres of radii 'r' and 'R' (R>>r),such that their surface charge densities are equal .Derive the expression for the potential at the common point.

18. What is the emf induced in the coil of self inductance of 5 mH when current changes from 5A to 1A in 20 s?

19.Derive an expression for the Ohm's law and also write Ohms law in vector form.

20. Define the drift velocity and also find the expression for it.

Define Mobility.Derive the expression between current and drift velocity.

21. A slab of material of dielectric constant K has some area A as the area of the parallel plate capacitor but of thickness d/2 ,where d is the separation between the plates.Find the expression for the capacitance when the slab is inserted between the pates of the capacitor?

22.Calculate the (i) momentum and (ii) de-Broglie wavelength of the electrons accelerated through a potential difference of 3600 V.

23.State the Bohr's three postulates.

Given the value of the ground state energy of hydrogen atom is -13.6eV.Find out its K.E and P.E in the ground state.

24. Find the ratio of the potential difference that must be applied across the parallel and series combination of two capacitors C1 and C2 with their capacitaces in the ratio 1:2 So that the energy stored in the two cases becomes the same.

225.Distinguish between n-type and p-type

sesemiconductors on the basis of energy level diagram.

SECTION-D

All questions are compulsory. In case of internal choices, attempt any one.

26.(a)Define angle of declination,angle of dip and horizontal component.Define magnetic dipole moment and write it's SI unit.

27. Draw the phasor diagram of series LCR circuit and find the expression for the impedance and power factor.

28.Prove the lens maker's formula.Write all the factors on which the focal-length of a double convex lens depends.

29.(a)Derive expression for nuclear energy density .

(b) Why nuclear fusion is also as thermo- nuclear fusion?

30. With the help of circuit diagram describe the construction and working of half wave rectifier.

With the help of circuit diagram describe the construction and working of a full wave rectifier.

SECTION-E

All questions are compulsory. In case of internal choices, attempt any one.

31.

(i) State Gauss'Law. Derive an expression for electric field due to infinitely long straight wire.

Show that the electric field between two thin infinite plane oppsitely charged sheet is zero?

If N drops of same size each having the same charge ,coalesce to form a bigger drop .How will the following vary with respect to single small drop?

1)Total charge on bigger drop.

2)Potential on bigger drop.

3)Capacitance of bigger drop.

ii)What are the parameters on which capacitance of a parallel plate capacitor depends.

32. (i) Using Biot-Savart's law, deduce an expression for the magnetic field at the axis of a circular current carrying loop.

ii)Write Biot-savarts law in vector form.

iii)Define velocity selector and describe how it can be used select the particles on the basis of their speed.

With the help of the diagram describe the construction and working of a moving coil galvanometer.

How a Moving coil galvanometer can be converted into an ammeter and a voltmeter.

Q33 Derive the expression for the total energy stored in a hydrogen atom and also find the Bohr's radius. Define mass defect and write the expression for it for a nucleus( X) OR Differentiate between interference and diffraction with at least four points of differences.Describe the single slit experient and find the conditions of secondary maxima and minima.Also find the width of Central maxima.