The order of coloured rings in a carbon resistor is red, yellow, blue and silver. The resistance of the

carbon resistor is:

a) 24 x 10^{6} Ω ± 5%

b) 24 x 10^{6} Ω ± 10%

c) 34 x 10^{4} Ω ± 10%

d) 26 x 10^{4} Ω ± 5%

Concept: Temperature Dependence of Resistance

A circular coil carrying a current I has radius R and number of turns N. If all the three, i.e. the current

I, radius R and number of turns N are doubled, then, the magnetic field at its centre becomes:

(a) Double

(b) Half

(c) Four times

(d) One fourth

Concept: Magnetic Force

A circular coil carrying a current I has radius R and number of turns N. If all the three, i.e. the current

I, radius R and number of turns N are doubled, then, the magnetic field at its centre becomes:

(a) Double

(b) Half

(c) Four times

(d) One fourth

Concept: Magnetic Force

An object is kept on the principal axis of a concave mirror of focal length 10 cm. at a distance of 15

cm from its pole. The image formed by the mirror is:

(a) Virtual and magnified

(b) Virtual and diminished

(c) Real and magnified

(d) Real and diminished

Concept: Ray Optics - Mirror Formula

Einstein's photoelectric equation is:

a) `E_"max" = hlambda - varphi_0`

b) `E_"max"= (hc)/lambda varphi_0`

c) `E_"max" = hv + varphi_0`

d) `E_"max" = (hv)/lambda + varphi_0`

Concept: Einstein’s Equation - Particle Nature of Light

In Bohr’s model of the hydrogen atom, the radius of the first orbit of an electron is r_{0} . Then, the radius of the third orbit is:

a) `r_0/9`

b) `r_0`

c) `3r_0`

d) `9r_0`

Concept: Bohr'S Model for Hydrogen Atom

In a potentiometer experiment, balancing length is found to be 120 cm for a cell E_{1} of emf 2V. What will be the balancing length for another cell E_{2} of emf 1.5V? (No other changes are made in the experiment.)

Concept: Potentiometer

How will you convert a moving coil galvanometer into a voltmeter?

Concept: Moving Coil Galvanometer

A moving charged particle q travelling along the positive x-axis enters a uniform magnetic field B.

When will the force acting on q be maximum?

Concept: Force on a Moving Charge in Uniform Magnetic and Electric Fields

Why is the core of a transformer laminated?

Concept: Eddy Currents

Ordinary (i.e. unpolarized) light is incident on the surface of a transparent material at the polarising angle. If it is partly reflected and partly refracted, what is the angle between the reflected and the refracted rays?

Concept: Brewster's Law

Define coherent sources of light

Concept: Coherent and Incoherent Sources and Sustained Interference of Light

Name a material which is used in making control rods in a nuclear reactor.

Concept: Atomic Masses and Composition of Nucleus

Define current density. Write an expression which connects current density with drift speed

Concept: Electrical Resistivity and Conductivity

A long horizontal wire P carries a current of 50A. It is rigidly fixed. Another wire Q is placed directly above and parallel to P, as shown in Figure 1 below. The weight per unit length of the wire Q is 0.025 Nm-1 and it carries a current of 25A. Find the distance 'r' of the wire Q from the wire P so that the wire Q remains at rest

Concept: Force on a Moving Charge in Uniform Magnetic and Electric Fields

Calculate force per unit length acting on the wire B due to the current flowing in the wire A.(See

Figure 2 below)

Concept: Cells in Series and in Parallel

Explain Curie’s law for a paramagnetic substance.

Concept: Magnetic Properties of Materials

A rectangular coil having 60 turns and area of 0.4m^{2} is held at right angles to a uniform magnetic field of flux density 5 × 10^{-5}T. Calculate the magnetic flux passing through it.

Concept: Magnetic Flux

What is motional emf? State any two factors on which it depends

Concept: Induced Emf and Current

What is the ratio of the speed of gamma rays to that of radio waves in a vacuum?

Concept: Electromagnetic Waves

Identify the part of the electromagnetic spectrum which is suitable for radar system used in aircraft navigation.

Concept: Electromagnetic Spectrum

A biconvex lens made of glass (refractive index 1.5) has two spherical surfaces having radii 20 cm and 30 cm. Calculate its focal length.

Concept: Lenses

State any two difference between the primary rainbow and secondary rainbow

Concept: Dispersion by a Prism

State de Broglie hypothesis

Concept: de-Broglie Relation

With reference to the photoelectric effect, define threshold wavelength

Concept: Einstein’s Equation - Particle Nature of Light

Calculate the minimum wavelength of the spectral line present in Balmer series of hydrogen

Concept: Hydrogen Spectrum

What are means by pair annihilation? Write a balanced equation for the same.

Concept: Energy Levels

What is meant by the terms half-life of a radioactive substance and binding energy of a nucleus?

Concept: Mass-energy and Nuclear Binding Energy - Nuclear Binding Energy

In a communication system, what is meant by modulation?

Concept: Modulation and Its Necessity

State any two types of modulation

Concept: Modulation and Its Necessity

Obtain an expression for an intensity of electric field at a point at the end of position, i.e., the axial position of an electric dipole.

x

Concept: Electrical Potential Energy of a System of Two Point Charges and of Electric Dipole in an Electrostatic Field

Deduce an expression for equivalent capacitance C when three capacitors C_{1}, C_{2} and C_{3} connected in parallel.

Concept: Combination of Capacitors

ε_{1} and **ε _{2}** are two batteries having emf of 34V and 10V respectively and internal resistance of 1Ω and 2Ω respectively. They are connected as shown in the figure below. Using Kirchhoff’s Laws of electrical networks, calculate the currents I

_{1}and I

_{2}.

Concept: Kirchhoff’s Rules

An electrical bulb is marked 200V, 100W. Calculate the electrical resistance of its filament. If five such

bulbs are connected in series to a 200V supply, how much current will flow through them?

Concept: Ohm’s Law

For any prism, prove that :

'n' or `mu = sin((A + delta_m)/2)/sin(A/2)`

where the terms have their usual meaning

Concept: Dispersion by a Prism

When two thin lenses are kept in contact, prove that their combined or effective focal length F is

given by :

`1/F = 1/f_1 + 1/f_2`

where the terms have their usual meaning.

Concept: Refraction at Spherical Surfaces and by Lenses - Combination of Thin Lenses in Contact

In Young’s double slit experiment, show graphically how the intensity of light varies with distance

Concept: Interference of Light Waves and Young’S Experiment

In Fraunhofer diffraction, how is the angular width of the central bright fringe affected when slit separation is increased?

Concept: Fraunhofer Diffraction Due to a Single Slit

Write one balanced equation to show Nuclear fission

Concept: Nuclear Energy - Nuclear Fusion – Energy Generation in Stars

Write one balanced equation to show Nuclear fusion

Concept: Nuclear Energy - Nuclear Fusion – Energy Generation in Stars

Write one balanced equation to show Emission of `beta^-` (i.e. a negative beta particle)

Concept: Atomic Masses and Composition of Nucleus

With reference to semiconductor devices, define a p-type semiconductor and a Zener diode.

Concept: Semiconductor Diode

What is the use of Zener diode?

Concept: Semiconductor Diode

An alternating emf of 220 V is applied to a circuit containing a resistor R having the resistance of 160Ω and a capacitor ‘C’ is series. The current is found to lead the supply voltage by an angle

`θ = tan^(-1) ("3/4")`

a) Calculate

1) The capacitive reactance

2) The impedance of the circuit

3) Current flowing in the circuit

b) If the frequency of the applied emf is 50 Hz, what is the value of the capacitance of the capacitor ‘C’?

Concept: Power in Ac Circuit: the Power Factor

An A.C. generator generating an emf of ε = 300 sin (100 πt) V is connected to a series combination of 16μ F capacitor, 1 H inductor and 100 Ω resistor.

Calculate :

1) An impedance of the circuit at the given frequency.

2) Resonant frequency `f_0`

3)Power factor at the resonant frequency `f_0`.

Concept: Power in Ac Circuit: the Power Factor

Draw a labelled ray diagram of an image formed by a refracting telescope with the final image formed at infinity. Derive an expression for its magnifying power with the final image at infinity

Concept: Optical Instruments - Telescope

Derive Snell’s law on the basis of Huygen’s wave theory when the light is travelling from a denser to a rarer medium.

Concept: Snell’s Law

With the help of an experiment, state how will you identify whether a given beam of light is polarised or unpolarized?

Concept: Polarisation

The forward characteristic curve of a junction diode is shown in Figure 4 below :

Calculate the resistance of the diode at

(1) V = 0.5 V

(2) I = 60 mA

Concept: Diode as a Rectifier

Draw separate energy band diagram for conductors, semiconductors and insulators and

label each of them.

Concept: Energy Bands in Conductors, Semiconductors and Insulators

The arrangement given below represents a logic gate :

Copy the following truth table in your answer booklet and complete it showing outputs at C

and D.

A | B | C | D |

0 | 0 | ||

1 | 0 | ||

0 | 1 | ||

1 | 0 |

Concept: Digital Electronics and Logic Gates

Draw a labelled diagram of a common emitter amplifier, showing waveforms of the signal voltage and output voltage.

Concept: Junction Transistor - Transistor as an Amplifier (Ce-configuration)

## CISCE previous year question papers Class 12 Physics (Theory) with solutions 2017 - 2018

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