Physics 55/3/1 2018-2019 Science (English Medium) Class 12 Question Paper Solution

Two metallic spheres A and B kept on insulating stands are in contact with each other. A positively charged  rod P is brought near the sphere A as shown in the figure. The two spheres are separated from each other,  and the rod P is removed. What will be the nature of charges on spheres A and B?

A metal sphere is kept on an insulting stands. A negatively charged rod is brought near it, then the sphere is  earthed as shown. On removing the earthing, and taking the negatively charged rod away, what will be the  nature of charge on the sphere? Give reason for your answer. 

A carbon resistor is shown in the figure. Using color code, write the value of the resistance.

State the condition under which a large magnification can be achieved in an astronomical telescope.

How does the angle of minimum deviation of a glass prism vary if the incident violet light is replaced by red light?

On the basis of the graphs shown in the figure, answer the following questions :

(a) Which physical parameter is kept constant for the three curves?

(b) Which is the highest frequency among v₁, v_2, and v₃?

Define amplitude modulation in a communication system.

Five point charges, each of charge +q are placed on five vertices of a regular hexagon of side 'l '. Find the magnitude of the resultant force on a charge -q placed at the centre of the hexagon.

A simple pendulum consists of a small sphere of mass m suspended by a thread of length l. The sphere carries a positive charge q. The pendulum is placed in a uniform electric field of strength E directed vertically downwards. Find the period of oscillation of the pendulum due to the electrostatic force acting on the sphere, neglecting the effect of the gravitational force.

The figure shows a network of five capacitors connected to a 100 V supply. Calculate the total energy stored in the network.

A 0⋅5 m long solenoid of 10 turns/cm has the area of cross-section 1 cm². Calculate the voltage induced across its ends if the current in the solenoid is changed from 1 A to A in 0⋅1 s.

A small flat search coil of area 5cm² with 140 closely wound turns is placed between the poles of a  powerful magnet producing magnetic field 0.09T and then quickly removed out of the field region. Calculate:

(a) Change of magnetic flux through the coil, and

(b) emf induced in the coil.

For paraxial rays, show that the focal length of a spherical mirror is one-half of its radius of curvature.

Obtain Bohr’s quantisation condition for angular momentum of electron orbiting in nth orbit in hydrogen  atom on the basis of the wave picture of an electron using de Broglie hypothesis. 

Plot a graph showing the variation of undecayed nuclei N versus time t. From the graph, find out how one  can determine the half-life and average life of the radioactive nuclei.

Write two distinguishing features of nuclear forces.

Complete the following nuclear reactions for α and β deca :

(i) `"_92^238U ->?  + _2^4He + Q`

(ii) `"_11^22Na ->?  + _10^22Ne + v`

Explain briefly, using a proper diagram, the difference in the behavior of a conductor and a dielectric in the presence of an external electric field.

Define the term polarization of a dielectric and write the expression for a linear isotropic dielectric in  terms of electric field.

Twelve wires each having a resistance of 3 Ω are connected to form a cubical network. A battery of 10 V and negligible internal resistance is connected across the diagonally opposite corners of this network. Determine its equivalent resistance and the current along each edge of the cube.

A student uses the circuit diagram of a potentiometer as shown in the figure
(a) for a steady current I passing through the potentiometer wire, he gets a null point for the cell ε₁. and not for ε₂. Give the reason for this observation and suggest how this difficulty can be resolved.

(b) What is the function of resistance R used in the circuit? How will the change in its value affect the null point?

(c) How can the sensitivity of the potentiometer be increased?

Show that the time period (T) of oscillations of a freely suspended magnetic dipole of magnetic moment (m) in a uniform magnetic field (B) is given by `"T" = 2pi sqrt("I"/"mB")`, where I is a moment of inertia of the magnetic dipole.

Identify the following magnetic materials :

How are eddy currents generated in a conductor which is subjected to a magnetic field?

Write two examples of their useful applications .

How can the disadvantages of eddy currents be minimized?

An ac circuit as shown in the figure has an inductor of inductance L and a resistor of resistance R connected in series. Using the phasor diagram, explain why the voltage in the circuit will lead the current in phase.

The potential difference across the resistor is 160 V and that across the inductor is 120 V. Find the effective value of the applied voltage. If the effective current in the circuit is 1⋅0 A, calculate the total impedance of the circuit.

What will be the potential difference in the circuit when direct current is passed through the circuit?

An ac circuit consists of a series combination of circuit elements X and Y. The current is ahead of the voltage in phase by `pi/4`. If element X is a pure resistor of 100 Ω,

(a) name the circuit element Y.

(b) calculate the rms value of current, if rms of voltage is 141 V.

(c) what will happen if the ac source is replaced by a dc source

Name the radiation of the electromagnetic spectrum which is used for the following:

Radar and Give the frequency range.

Name the radiation of the electromagnetic spectrum which is used for the following:

To photograph internal parts of the human body and Give the frequency range

Name the radiation of the electromagnetic spectrum which is used for the following:

For taking photographs of the sky during the night and foggy conditions . Give the frequency range

A converging beam of light traveling in air converges at a point P as shown in the figure. When a glass sphere of refractive index 1⋅5 is introduced in between the path of the beam, calculate the new position of, the image. Also, draw the ray diagram for the image formed.

Explain how an unpolarised light gets polarised when incident on the interface separating the two transparent media.

Greenlight is an incident at the polarising angle on a certain transparent medium. The angle of refraction is 30°.
Find
(i) polarising angle, and
(ii) refractive index of the medium.

Plot a graph to show the variation of stopping potential with frequency of incident radiation in relation to photoelectric effect.

Use Einstein's photoelectric equation to show how from this graph,
(i) Threshold frequency, and
(ii) Planck's constant can be determined.

How does one explain the emission of electrons from a photosensitive surface with the help of Einstein’s photoelectric equation? 

The work function of aluminum is 4⋅2 eV. If two photons each of energy 2⋅5 eV are incident on its surface, will the emission of electrons take place? Justify your answer.

The stopping potential in an experiment on photoelectric effect is 1.5V. What is the maximum kinetic energy of the photoelectrons emitted? Calculate in Joules.

(i) Write the truth tables of the logic gates marked P and Q in the given circuit.
(ii) Write the truth table for the circuit. 

Why are NOR gates considered as universal gates?

Explain how a potential barrier is developed in a p-n junction diode.

Draw the circuit arrangement for studying the V-I characteristics of a p-n junction diode in reverse bias. Plot the V-I characteristics in this case.

What do you mean by bandwidth of a signal? Give its importance. 

Differentiate between Analog and Digital communication. 

Write the functions of the transducer and repeaters.

State and explain the law used to determine the magnetic field at a point due to a current element. Derive the expression for the magnetic field due to a circular current-carrying loop of radius r at its center.

A long wire with a small current element of length 1 cm is placed at the origin and carries a current of 10 A along the X-axis. Find out the magnitude and direction of the magnetic field due to the element on  the Y-axis at a distance 0.5 m from it.

Derive the expression for the magnetic field due to a current-carrying coil of radius r at a distance x from the center along the X-axis.

A straight wire carrying a current of 5 A is bent into a semicircular arc radius 2 cm as shown in the figure. Find the magnitude and direction of the magnetic field at the center of the arc

(a) Can the interference pattern be produced by two independent monochromatic sources of light? Explain.
(b) The intensity at the central maximum (O) in Young's double-slit experimental set-up shown in the figure is I_O. If the distance OP equals one-third of the fringe width of the pattern, show that the intensity at point P, would `"I"_°/4`

(c) In Young's double-slit experiment, the slits are separated by 0⋅5 mm and the screen is placed 1⋅0 m away from the slit. It is found that the 5th bright fringe is at a distance of 4⋅13 mm from the 2nd dark fringe. Find the wavelength of light used.

Derive the relation a sin θ = λ for the first minimum of the diffraction pattern produced due to a single slit of width 'a' using light of wavelength λ.

State with reason, how the linear width of the central maximum will be affected if
(i) monochromatic yellow  light is replaced with red light, and
(ii) distance between the slit and the screen is increased.

Using the monochromatic light of the wavelength in the experimental set-up of the diffraction pattern as well as in the interference pattern where the slit separation is 1 mm, 10 interference fringes are found to be within the central maximum of the diffraction pattern. Determine the width of the single slit, if the screen is kept at the same distance from the slit in the two cases.

Draw a circuit diagram of an n-p-n transistor with its emitter-base junction forward biased and base-collector junction reverse biased. Briefly describe its working.
Explain how a transistor in its active state exhibits a low resistance at its emitter-base junction and high resistance at its base-collector junction.

Derive the expression for the voltage gain of a transistor amplifier in CE configuration in terms of the load resistance R_L, current gain β_a and input resistance.
Explain why input and output voltages are in the opposite phase.

Write the important considerations which are to be taken into account while fabricating a p-n junction diode to be used as a Light Emitting Diode (LED). What should be the order of the band gap of an LED, if it is required to emit light in the visible range? Draw a circuit diagram and explain its action.

Draw the V-I characteristics of an LED. State two advantages of LED lamps over convertional  incandescent lamps.