**Class 12 Physics Model Question Solution 2078 **

*According to New Course 2078*

Grade: XII

Subject: Physics

Full marks: 75 (11 marks Obj+ 64 marks Sub)

Time: 3 Hour

Group A

Multiple Choice Questions (11x1 = 11)Time: 25 Minute

Tick the correct answer.

Group B: Short Answer Questions (8x5=40)

1. (i) Define surface tension. [1]

(ii) Establish a relation between surface tension and surface energy of a liquid. [2]

(iii) Two spherical rain drops of equal size are falling vertically through air with a certain terminal velocity. If these two drops were to coalesce to form a single drop and fall with a new terminal velocity, explain how the terminal velocity of the new drop compares to the original terminal velocity. [2]

2. Angular speed of a rotating body is inversely proportional to its moment of inertia.

(i) Define 'moment of inertia.[1]

(ii) Explain why angular velocity of the Earth increases when it comes closer to the Sun in its orbit. [2]

(iii) If the Earth were to shrink suddenly, what would happened to the length of the day? Give reason. [2]

Or

(i) State Bernoulli principle. [1]

(ii) Derive Bernoulli's equation [2]

(iii) You can squirt water from a garden hose a considerably greater distance by partially covering the openingwith your thumb. Explain how this works, [2]

3.(i) Define 'harmonics' in music. [1]

(ii) Calculate the frequency of a monotonous sound produced by a 30 cm 2 flute open at both ends and being played in the first harmonie. (Velocity of sound in air 330 m/s) [2]

(iii) The flute mentioned in question (ii) was being played by a passenger on a stationary bus. The bus then moves uniformly. Explain what change in the pitch of the flute sound, if any, a person sitting on a bench at the bus park will feel when the bus starts moving. [2]

4. (i) State the second law of thermodynamics [1]

(ii) A refrigerator transfers heat from a cold body to hot body. Does this not violate the second law of thermodynamics? Give reason. [2]

(iii) in the given figure, a heat engine absorbs Q1, amount of heat from a source at temperature T1, and rejects Q2 amount of heat to a sink at temperature T2 doing some external work W.

(a) Obtain an expression for the efficiency of this heat engine. [1]

(b) Under what condition does the efficiency of such engine become-zero percentage, if at all? [1]

5. A student wants to measure the magnetic flux density between the poles of two weak bar magnets mounted on a steel yoke as shown in the figure. The magnitude of the flux density is between 0.02T and 0.04T.

(i) Define Magnetic flux density. [1]

(ii) One way of measuring the magnetic flux density could be the use of a Hall probe. Suggest one reason why Hall probe is not a suitable instrument to measure the magnetic flux density for the arrangement shown in the above figure. [1]

(iii) Another method of measuring the magnetic flux density for the arrangement shown in the above figure is to insert a current-carrying wire between the poles of the magnet. Explain how the magnetic flux density can be determined using this method. You are allowed to use any additional apparatus. [3]

6. (a) Law of electromagnetic induction can be expressed mathematically E =-N dphi/dt

(b) (i) State what the symbols E and dphi/dt and represent in the equation. [2]

(ii) Explain the significance of the negative sign. [1]

(i) Two identical copper balls are dropped from the same height as shown in the figure. Ball P passes through a region of uniform horizontal magnetic filed of flux density B. Explain why ball P takes longer than ball Q to reach the ground.[2]

7. Ultraviolet radiation of frequency 1.5 × 10^15Hz is incident on the surface of an aluminium plate whose work function is 6.6 × .10^-19J

(i) Show that the maximum speed of the electrons enmitted from the surface of the aluminium is 8.6 × 10^5 m/s.[3]

(ii) State and explain what change, if any, occurs to the maximum speed of the emitted electrons when the intensity of the ultraviolet radiation is increased. [2]

8 (i) State Bohr's postulates of atomic model [3]

(ii) The figure shows Lymen series of energy transmission in hydrogen atom. Calculate the frequency of a photon emitted by an electron jumping from second excite state to ground level. [2]

Or

(i) Sketch the symbol of a p-n junction diode and indicate the polarity of its ends. [1]

(ii) Copy the outline of a diode bridge rectifier and complete it by adding diodes in the gaps. [2]

(ii) Explain what will happen if one of the four diodes is damaged so that it stops conducting totally in any direction. Sketch a graph to show how the pd across the Load R1, would vary with time in this situation. [2]

Group C: Long Answer Questions. (3x8=24)

9.Earthquake sets rocks and buildings in motion. When a rock is subjected to compression, a restoring force develops inside it. This restoring force is given by an equation F=-Ax where x is displacement and A is a constant.

(i) Prove that this force will make the rock vibrate with simple harmonic motion. [2]

(ii) Show that the speed of an object undergoing simple harmonic motion is given by the expression v= ±on(A^2-x^2) where the symbols carry standard meanings. [2]

(iii) Calculate the maximum speed of a building shaken by S-waves of 21Hz and amplitude 0.05m. [2]

(iv) Explain why tall buildings are more susceptible to damage by S-waves which generally have low frequency. [2]

10. The figure below shows the variation of emf and current with typical LRC circuit.

i) Explain whether the phase constant is positive or negative [2]

(ii) Sketch a phasor diagram for the given case. [2]

(iii) is the circuit more inductive or capacitive? Explain, [2]

(iv) To increse the rate at which energy is transferred to the load, should the inductance be increased or decreased? Justify your answer. [2]

Or

A student sets up a circuit as shown in the figure given below to measure the emf of a test cell.

(a) Explain why he is unable to find a balance point and state the change he must make in order to achieve the balance. [2]

(b) State how he would recognize the balance point. [1]

(c) He obtained the balance point for distance 37.5cm using standard cell of emf 1.50V. And for the test cell,the balance distance AB was 25.0 cm. Calculate the emf of the test cell. [2] (d) He could have used an ordinary voltmeter to measure the emf of the test cell directly. The student,however, argues that the above instrument is more precise than an ordinary voltmeter. Justify his logic. [2]

11. (a) Explain what is meant by quantization of charge. [2]

(b) In a Millikan's oil drop experiment, an oil drop of weight 1.5 × 10^-14 N is held stationary between plates 10mm apart by applying a p.d. of 470V between the plates.

(i) State the condition necessary for the drop to remain stationary. Also, sketch the forces acting on the oil drop. [2]

(ii) Calculate the charge on the oil-drop.

[2]

(iii) Explain what would happen if the above oil drop is suddenly struck by a stray alpha particle. [2]

OR

(a) Derive an expression N=

# $N=N_{0}e_{−λt}$

for a radioactive process where the symbols carry their standard meanings.(3)

(b) A student measured the activity of a sample of radioactive rock. Her results are presented in the graph

i) Explain why the data are scattered. [1]

(ii) Determine the half-life of this sample. [2]

(iii) How will the shape of this curve will change if she repeats the experiment with a sample with a larger decay constant. Give reason to your answer. [2]

Second excited state; n2=3

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