Physics (Theory) 2014-2015 ISC (Commerce) Class 12 Question Paper Solution

A short electric dipole (which consists of two point charges, +q and -q) is placed at the centre 0 and inside a large cube (ABCDEFGH) of length L, as shown in Figure 1. The electric flux, emanating through the cube is:

a) `q"/"4piin_9L`

b) zero

c) `q"/"2piin_0L`

d) `q"/"3piin_0L`

The equivalent resistance between points. a and f of the network shown in Figure 2 is :

a) 24 Ω

b) 110 Ω

c) 140 Ω

d) 200 Ω

A moving electron enters ·a uniform and perpendicular magnetic field. Inside thema~netic field, the electron travels along:

a) a straight line.

b) a parabola

c) a circle

d) a hyperbola

A fish which is at a depth of l2 em .in water `(mu = 4/3)` is viewed by an observer on the bank of a lake. Its apparent depth as observed: by the observer is:

a) 3 cm

b) 9 cm

c) 12 cm

d) 16 cm

if `E_p` and `E_k` represent potential energy and kinetic energy respectively, of an orbital electron, then, according to B9hr's theory:

a)`E_k = -E_p"/"2`

b) `E_k = -E_p`

c) `E_k = -2E_p`

d) `E_k = 2E_p`

What is meant by the term Quantization of charge?

A resistor R is connected to a cell of-emf e and internal resistance r. The potential difference across the resistor R is found to be V. State the relation between e, V, Rand r.

Three identical cells each of emf 2V and internal resistance 10 Ω are connected in series to form a battery. The battery is then connected to a parallel combination of two identical resistors, each of resistance 6 Ω. Find the current delivered by the battery.

State how magnetic susceptibility is different for the three types of magnetic materials, i.e. diamagnetic, paramagnetic and ferromagnetic materials

An emf of 2V is induced in a coil when the current in it is changed from 0A to 10A in 0·40 sec. Find the coefficient of self-inductance of the coil.

How are electric vector `(vec "E")`, magnetic vector `(vec "B")` and velocity vector `(vec "C")` oriented in an electromagnetic wave?

State any two methods by which ordinary light can be polarised

A monochromatic ray of light falls on a regular prism. What is the relation between the angle of incidence and angle of emergence in the case of minimum deviation?

What type of lens is used to correct long-sightedness?

Write two important advantages of reflecting telescope over a refracting telescope.

State Moseley's law

Wavelengths of the first lines of the Lyman series, Paschen series and Balmer series, in hydrogen spectrum are denoted by `lambda_L, lambda_P and lambda_B` respectively. Arrange these wavelengths in increasing order.

What is the significance of binding energy per nucleon of a nucleus of a radioactive element?

Write one balanced equation to show Nuclear fission

Whatis the difference between analogue signal and. digital signal?

Derive an expression for the intensity of electric field at a point in broadside position or on [4)
an equatorial line of an electric dipole.

Two point charges of 10C each are kept at a distance of 3m in the vacuum. Calculate their electrostatic potential energy.

Four capacitors, C₁, C₂, C₃ andC₄ are connected as shown in Figure 3 below. Calculate equivalent capacitance of the circuit between points X . and Y.

Draw labelled graphs to show how electrical resistance varies with temperature for: 

1) a metallic wire.

2) a piece of carbon

Two resistors R1 = 400Ω and R₂ = 20 n are connected in parallel to a battery. If heating the power developed in R₁ is 25 W. find the heating power developed in R₂

With the help of a labelled diagram, show that the balancing condition of a Wheatstone bridge is

`"R"_1/"R"_2 = "R"_3/"R"_4` where the terms have their usual meaning.

An I0m long uniform metallic wire having a resistance of 20Ω IS used as a  potentiometer wire. This wire is connected in series with another resistance of 480Ω
and a battery of emf 5V having negligible internal resistance. If an unknown emf e is balanced across 6m of the potentiometer wire, calculate

1) the potential gradient across the potentiometer wire

2) the value of the unknown emf e.

Explain the term hysteresis

Name three elements of the earth's magnetic field which help in defining earth's magnetic field completely.

Obtain an expression for magnetic flux density B at the centre of a circular coil of radius R, having N turns and carrying a current I

A coil of self-inductance 2.5H and resistance 20Ω is connected to a battery of emf 120V having the internal resistance of 5 n. Find:

1) The time constant of the circuit.

2) The current in the circuit in steady state

Figure 4 below shows a capacitor C, an inductor L and a resistor R, connected in series
to an a.c. supply of 220 V

Calculate:

1) The resonant frequency of the given CLR circuit.

2) Current flowing through·the circuit.

3) Average power consumed by the circuit.

In a series LCR circuit, what is the phase difference between V_L and V_C where V_L is the potential difference across the inductor and V c is the potential difference across the capacitor?

On the basis of Huygens Wave theory of light, show that angle of reflection is equal to the angle of incidence. You must draw a labelled diagram for this derivation

State any one difference between interference of light and diffraction of light

Laser light of wavelength 630 nm is incident on a pair of slits which are separated by 1.8 mm. If the screen is kept 80 cm away from the two slits, calculate:

1) fringe separation i.e. fringe width.

2) distance of 10^th bright fringe from the centre of the interference pattern

Show graphically the intensity distribution in Fraunhofer's single slit diffraction experiment. Label the axes.

A point object O is placed at a distance of 15cm from a convex lens L of focal length 1 Ocm as shown in Figure 5 below. On the other side of the lens, a convex mirror M is placed such that its distance from the lens is equal to the focal length of the lens. The final image formed by this combination is observed to coincide with the object O. Find the focal length of the convex mirror

What is the chromatic aberration? How can it be minimised or eliminated?

Draw a labelled ray diagram showing the formation of a final image by a compound microscope at least distance of distinct vision

With regard to an astronomical telescope of refracting type~ state how you will increase its:

1) magnifying power

2) resolving power

In an experiment of the photoelectric effect, the graph of maximum kinetic energy E_K of the emitted photoelectrons versus the frequency v of the incident light is a straight line AB shown in Figure 6 below:

Find:

1) Threshold frequency of the metal

2) The work function of the metal.

3) Stopping potential for the photoelectrons emitted by the light of frequency `v = 30 xx 10^14 Hz`

State how de-Broglie wavelength (`lambda`) of moving particles varies with their linear momentum (p).

State any one phenomenon in which moving particles exhibit wave nature.

On the basis of Bohr's theory, derive an expression for the radius of the n^th orbit of an electron of the hydrogen atom.

Using the constants given on page 8 ofthis·Paper, find the minimum wavelength of the emitted X rays, when an X-ray tube is operated at 50 kV.

Define half-life of a radioactive substance

Using the equation `N = N_0e^(-lambdat)` obtain the relation between half-life (T) and decay constant (`lambda`) of a radioactive substance.

With the help of a suitable example and an equation, explain the term pair production.

Draw a labelled diagram of a full wave rectifier. Show how output voltage varies with time if the input voltage is a sinusoidal voltage.

Give the truth table and circuit symbol for NAND gate