Download pre-mock examination (physics)

GURU HARKRISHAN PUBLIC SCHOOL , Karol Bagh
Class XII
Physics (Theory)
Pre-Mock (2014-15)
Time allowed: 3 h
General Instructions:
(i) All questions are compulsory.
Maximum Marks: 70
(ii) There is no overall choice. However an internal choice has been provided in one question of two marks
and all three questions of five marks each. You have to attempt only one of the choices in such questions.
(iii) Questions 1 to 5 are very short answer type and carry one mark each.
(iv) Questions 6 to10 are short answer type and carry two marks each.
(v) Questions 11 to 22 are short answer type and carry three marks each.
(vi) Question 23 is a value based question and carry four marks
(vii) Questions 24 to 26 are long answer type and carry five marks each.
(viii) Use of calculators is not permitted.
Q1. Arrange the following in descending order of wavelength:
X-ray, Radiowave, Blue light, Infrared
Q2. Which property of zener diode is used in regulating voltage?
Q3. Electric charges q, q, and -2q are placed at the corners of an equilateral triangle of sides l. What is the dipole
moment of the system?
Q4. When energy of charged capacitor is reduced to half, what amount of charge is left on the capacitor in terms
of the initial amount of charge?
Q5. The power factor of an a.c. circuit is 0.5. What will be the phase difference between voltage and current in
this
circuit?
Q6. Two point charges 10 μC and 40 μC are placed 6 cm apart in vaccum. At what distance from 10 μC charge, will
the net electric field be zero?
OR
Two capacitors of capacitances 6 μF and 12 μF are connected in series with a battery. The voltage across the 6
μF
capacitor is 2 V. Compute the total battery voltage.
Q7. When an ideal capacitor is charged by a dc battery, no current flows. However, when an a.c. source is used,
the
current flows continuously. How does one explain this, based on the concept of displacement current?
Q8. Draw a circuit diagram for a p-n junction in reverse bias. Sketch the voltage-current graph for the same.
Q9. Calculate the quality factor of a series of LCR circuit with L=2.0 H, C=2 μF and R= 10Ω. Mention the
significance of quality factor in LCR circuit.
Q10. The graph below shows variation of photoelectric current with collector plate potential for different
frequencies of incident radiations.
(i)
(ii)
Which physical parameter is kept constant for the three curves?
Which frequency is the highest?
Q11. State Gauss’s law in electrostatics. Use this law to derive an expression for the electric field due to an
infinitely long straight wire of linear charge density λ C m-1.
Q12. Deduce the expression for the electrostatic energy stored in a capacitor ‘C’ and having charge ‘Q’.
How will the (i) energy stored and (ii)the electric field inside the capacitor be effected when it is completely
filled
with a dielectric material of dielectric constant ‘K’?
Q13. (i) What happens when a diamagnetic substance is placed in a varying magnetic field?
(ii) Name the properties of magnetic material that make it suitable for making (a) a permanent magnet and
(b) a core of an electromagnet
Q14. With a certain unknown resistance X in the right gap, null point is obtained on the metre bridge wire for a
resistance of 12Ω in the left gap. On putting another 6Ω resistor in parallel with the 12Ω resistor in the left gap,
the null point is found to be shift by 25 cm. Find the value of X from these calculations.
Q15. Define magnetic meridian. A freely suspended bar magnet of magnetic moment 0.8 JT -1 is to be held normal
to the magnetic meridian at a place where the horizontal component of earth’s magnetic field is 0.4 gauss. How
much external torque must be applied on the bar magnet in this orientation?
Q16.The following figure shows the input waveform ‘A’ and ‘B’ and output wave form Y of a gate. Write its truth
table and identify the gate.
Q17. Using Bohr’s postulates for hydrogen atom, show that the total energy (E) of the electron in the stationary
states can be expressed as the sum of kinetic energy (K) and potential energy(U), where U=-2K. Hence, deduce the
expression for the total energy in the nth energy level of hydrogen atom.
Q18. Draw a plot showing the variation of binding energy per nucleon versus the mass number A. Explain with the
help of this plot the release of enrgy in the processes of nuclear fission and fusion.
Q19. A circular coil of radius 8.0 cm and 20 turns rotates about its vertical diameter with an angular speed of 50
rad s-1 in a uniform horizontal magnetic field of magnitude 3 x 10-2 T. Calculate:
(i)
Maximum emf induced in the coil.
(ii)
Average emf.
If the coil forms a closed loop of resistance 10 Ω, calculate the power dissipated as heat.
Q20. Explain the following giving reason for each:
(a) How does a Polaroid work to produce a linearly polarised light from an unpolarised beam of light?
(b) Why is it that light waves can be ploarised, but sound waves cannot be?
(c) Why are sun goggles made of polaroids preferred over those using coloured glasses?
Q21. Photoelectrons are emitted from a metal surface when UV light of wavelength λ=400 nm is incident on
it. The minimum negative potential required to stop the emission of electrons is 0.40 V. Calculate:
(i)
the energy of the incident photons.
(ii)
the maximum kinetic energy of the photo-electrons emitted
(iii)
the work function of the metal.
Express all answers in eV.
Q22. A wire frame of size 5mm x 2mm is placed at a distance of 8cm from a convex lens of focal length 10cm held
close to the eye.
(a)How big will the frame appear to be?
(b) What is the angular magnification produced by the lens?
Q23. Vijay was preparing an electronic project for science exhibition. He required a capacitance of 2μF having a
capacity
to operate under 1KV potential. He went to a shop to purchase it. Shopkeeper was having only 1μF
capacitors of 400
V rating. Vijay calculated minimum number of capacitors of 1μF so that he can arrange them to form a
capacitor of
2μF value.
(a)
What values do you judge in vijay?
(b)
Show the calculations done by vijay?
Q24. You are given two convex lenses of short aperture having focal lengths 4 cm and 8 cm respectively. Which
one of these will you use as an objective and which one as an eyepiece for constructing a compound microscope?
Draw a ray diagram to show the formation of the image of a small object due to a compound microscope. Derive
an expression for its magnifying power.
OR
Derive lens maker's formula by considering concave lens.
For a converging lens r1=r2=24 cm and refractive index 1.6 (a) calculate its focal length in air and (b) if the lens is
split vertically into two identical parts, what is the focal length of each part?
Q25.
(a) Define the term 'electrostatic potential'. Give the dependence of electrostatic potential due to a small electric
dipole at a far off point lying on (i) the axial line, and (ii) equatorial line.
(b) Briefly explain the principle of capacitor. Obtain the expression for the capacitance of a parallel plate capacitor
with a conducting slab of thickness 't' between the plates
OR
State Gauss’ theorem and find out expression for electric field at a point near a surface distribution of charge of
infinite extent. Draw the graph between electric field and separation of point of observation from plane sheet
having continuous charge distribution.
Q26. (a) With the help of a diagram, explain the principle and working of a moving coil galvanometer.
(b) What is the importance of a radial magnetic field and how is it produced?
(c) Why is it that while using a moving coil galvanometer as a voltmeter a high resistance in series is required
whereas in an ammeter a shunt is used?
OR
(a) State the principle on which AC generator works. Draw a labelled diagram and explain its working.
(b) A conducting rod held horizontally along East-West direction is dropped from rest from a certain height
near the Earth's surface. Why should there be an induced emf across the ends of the rod? Draw a plot showing
the instantaneous variation of emf as a function of time from the instant it begins to fall.