Assertion and Reason Questions on Class 12 Physics
Directions:
These questions consist of two statements,
each printed as Assertion and Reason. While answering these questions,
you are required to choose any one of the following four responses. (a) If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion. (b) If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion. (c) If the Assertion is correct but Reason is incorrect. (d) If both the Assertion and Reason are incorrect.
chapter 1 electric charges and Fields
Q.1. Assertion: Electron move away from a region of lower potential to a region of higher potential. Reason: An electron has a negative charge.
Answer
a
Q.2. Assertion : A metallic shield in form of a hollow shell may be built to block an electric field. Reason : In a hollow spherical shield, the electric field inside it is zero at every point.
Answer
a
Q.3. Assertion : Electric lines of force never cross each other. Reason : Electric field at a point superimpose to give one resultant electric field.
Answer
b
Q.4.Assertion : The Coulomb force is the dominating force in the universe. Reason : The Coulomb force is weaker than the gravitational force.
Answer
d
Q.5. Assertion : In a cavity within a conductor, the electric field is zero. Reason : Charges in a conductor reside only at its surface.
Answer
a
Q.6. Assertion : When bodies are charged through
friction, there is a transfer of electric charge from one body to
another, but no creation or destruction of charge. Reason : This follows from conservation of electric charges.
Answer
(a) Conservation of electric charge states that the total
charge of an isolated system remains unchanged with
time
Q.7. Assertion : The tyres of aircraft are slightly conducting. Reason : If a conductor is connected to ground, the extra charge induced on conductor will flow to ground.
Answer
(b) Both the statements are independently correct.
Q.8. Assertion : Some charge is put at the centre of a conducting sphere. It will move to the surface of the sphere. Reason : Conducting sphere has no free electrons at the centre.
Answer
(a) Because of repulsion, the free electrons will mole to the
outer surface.
Q.9. Assertion : Coulomb force and gravitational force follow the same inverse-square law. Reason : Both laws are same in all aspects.
Answer
(c) Coulomb force and gravitational force follow the
same inverse-square law. But gravitational force has
only one sign which is always attractive, while
coulomb force can be of both signs which are
attractive and repulsive.
Q.10. Assertion : The coulomb force is the dominating force in the universe. Reason : The coulomb force is weaker than the gravitational force.
Answer
(d) Gravitational force is the dominating force in nature
and not coulomb’s force. Gravitational force is the
weakest force. Also, Coulomb’s force > > gravitational
force.
Q.11. Assertion : If there exists coulomb attraction between two bodies, both of them may not be charged. Reason : In coulomb attraction two bodies are oppositely charged.
Answer
(b) Coulomb attraction exists even when one body is
charged, and the other is uncharged.
Q.12. Assertion : A deuteron and an -particle are
placed in an electric field. If F1 and F2 be the forces acting on them
and a1 and a2 be their accelerations respectively then, a1 = a2. Reason : Forces will be same in electric field.
Answer
c
Q.13. Assertion : The property that the force with
which two charges attract or repel each other are not affected by the
presence of a third charge. Reason : Force on any
charge due to a number of other charge is the vector sum of all the
forces on that charge due to other charges, taken one at a time.
Answer
(b) Force on any charge due to a number of other
charges is the vector sum of all the forces on that
charge due to the other charges, taken one at a time.
The individual force are unaffected due to the
presence of other charges. This is the principle of
superposition of charges.
Q.14. Assertion : A metallic shield in form of a hollow shell may be built to block an electric field. Reason : In a hollow spherical shield, the electric field inside it is zero at every point.
Answer
(a) The electrostatic shielding is possible by metallic
conductor.
Q.15. Assertion : A point charge is brought in an
electric field, the field at a nearby point will increase or decrease,
depending on the nature of charge. Reason : The electric field is independent of the nature of charge.
Answer
(c) The electric field will increase if positive charge is
brought in an electric field.
Q.16. Assertion : Consider two identical charges placed distance 2d apart, along x-axis.
The equilibrium of a positive test charge placed at the point O
midway between them is stable for displacements along the x-axis. Reason: Force on test charge is zero.
Answer
(b) If +ve charge is displaced along x-axis, then net force
will always act in a direction opposite to that of
displacement and the test charge will always come back
to its original position.
Q.17. Assertion : When a conductor is placed in an
external electrostatic field, the net electric field inside the
conductor becomes zero after a small instant of time. Reason : It is not possible to set up an electric field inside a conductor.
Answer
(c) Statement-1 is correct. The induced field cancels the
external field. Statement-2 is false. When a current is
set up in a conductor, there exists an electric field
inside it.
Q.18. Assertion : A uniformly charged disc has a pin hole at its centre. The electric field at the centre of the disc is zero. Reason : Disc can be supposed to be made up of many rings. Also electric field at the centre of uniformly charged ring is zero.
Answer
(a) The electric field due to disc is superposition of electric
field due to its constituent ring as given in Reason.
Q.19. Assertion : Electric lines of field cross each other. Reason : Electric field at a point superimpose to give one resultant electric field.
Answer
(d) Two field lines never intersect.
Q.20. Assertion : On bringing a positively charged rod near the uncharged conductor, the conductor gets attracted towards the rod. Reason : The electric field lines of the charged rod are perpendicular to the surface of conductor.
Answer
(b) Though the net charge on the conductor is still zero
but due to induction negatively charged region is nearer
to the rod as compared to the positively charged region.
That is why the conductor gets attracted towards the
rod
Q.21. Assertion : Four point charges q1, q2, q3 and
q4 are as shown in figure. The flux over the shown Gaussian surface
depends only on charges q1 and q2.
Reason : Electric field at all points on Gaussian surface depends only on charges q1 and q2.
Answer
(d) Electric field at any point depends on presence of all
charges.
Q.22. Assertion : On disturbing an electric dipole
in stable equilibrium in an electric field, it returns back to its
stable equilibrium orientation. Reason : A restoring torque acts on the dipole on being disturbed from its stable equilibrium.
Answer
(a) The restoring torque brings it back to its stable
equilibrium.
Q.23. Assertion : On going away from a point charge or a small electric dipole, electric field decreases at the same rate in both the cases. Reason : Electric field is inversely proportional to square of distance from the charge or an electric dipole.
Answer
(d)
Q.24. Assertion : The electric flux of the electric field ∮ E.dA is zero. The electric field is zero everywhere on the surface. Reason : The charge inside the surface is zero.
Answer
(d)
Q.25. Assertion : On moving a distance two times the
initial distance away from an infinitely long straight uniformly
charged wire the electric field reduces to one third of the initial
value. Reason : The electric field is inversely proportional to the distance from an infinitely long straight uniformly charged wire.
Answer
(a)
Q.26. Assertion (A): No two electric lines of force can intersect each other. Reason (R) : Tangent at any point of electric line of force gives the direction of electric field.
Answer
(a)
Q.27. Assertion (A): Electric force acting on a
proton and an electron, moving in a uniform electric field is same,
where as acceleration of electron is 1836 times that of a proton. Reason (R): Electron is lighter than proton.
Answer
(a)
Q.28. Assertion (A): As force is a vector quantity, hence electric field intensity is also a vector quantity Reason (R): The unit of electric field intensity is newton per coulomb.
Answer
(b)
Q.29. Assertion (A) : Sharper is the curvature of spot on a charged body lesser will be the surface charge density at that point Reason (R): Electric field is non-zero inside a charged conductor.
Answer
(d)
Q.30. Assertion (A): The surface densities of two spherical conductors of different radii are equal. Then the electric field intensities near their surface are also equal. Reason (R) : Surface density is equal to charge per unit area.
Answer
(b)
Q.31. Assertion (A): Three equal charges are situated on a circle of radius r such that they form on equilateral triangle, then the electric field intensity at the centre is zero. Reason (R): The force on unit positive charge at the centre, due to the three equal charges are represented by the three sides of a triangle taken in the same order. Therefore, electric field intensity at centre is zero.
Answer
(a)
Q.32. Assertion (A): The electric lines of forces diverges from a positive charge and converge at a negative charge. Reason (A): A charged particle free to move in an electric field always move along an electric line of force.
Answer
(c)
Q.33. Assertion (A): Charging is due to transfer of electrons. Reason (R): Mass of a body decreases slightly when it is negatively charged.
Answer
(c)
Q.34. Assertion (A): Range of Coulomb force is infinite. Reason (R): Coulomb force acts between two charged particles.
Answer
(b)
Q.35. Assertion (A): A small metal ball is suspended
in a uniform electric field with an insulated thread. If high energy
X-ray beam falls on the ball, the ball will be deflected in the electric
field. Reason (R): X-rays emits photoelectron and metal becomes negatively charged.
Answer
(c)
Q.36. Assertion (A): If a point charge be rotated in a circle around a charge, the work done will be zero.
Reason (R): Work done is equal to dot product of force and distance
Answer
(a)
Chapter 2 Electrostatic Potential and Capacitance
Q.1. Assertion : If the
distance between parallel plates of a capacitor is halved and dielectric
constant is three times, then the capacitance becomes 6 times. Reason : Capacity of the capacitor does not depend upon the nature of the material.
Answer
c
Q.2. Assertion : A
parallel plate capacitor is connected across battery through a key. A
dielectric slab of dielectric constant K is introduced between the
plates. The energy which is stored becomes K times. Reason : The surface density of charge on the plate remains constant or unchanged.
Answer
c
Q.3. Assertion : The total charge stored in a capacitor is zero. Reason : The field just outside the capacitor is σ/ε0 . (σ is the charge density).
Answer
c
Q.4. Assertion : The
electrostatic force between the plates of a charged isolated capacitor
decreases when dielectric fills whole space between plates. Reason : The
electric field between the plates of a charged isolated capacitance
increases when dielectric fills whole space between plates.
Answer
d
Q.5. Assertion : Two concentric charged shells are given. The potential difference between the shells depends on charge of inner shell. Reason : Potential due to charge of outer shell remains same at every point inside the sphere.
Answer
a
Q.6. Assertion : Two equipotential surfaces cannot cut each other. Reason : Two equipotential surfaces are parallel to each other.
Answer
c
Q.7. Assertion: The potential difference between any two points in an electric field depends only on initial and final position. Reason: Electric field is a conservative field so the work done per unit positive charge does not depend on path followed.
Answer
a
Q.8. Assertion : Electric field inside a conductor is zero. Reason: The potential at all the points inside a conductor is same.
Answer
b
Q.9. Assertion : Electric field is discontinuous across the surface of a spherical charged shell. Reason : Electric potential is continuous across the surface of a spherical charged shell.
Answer
b
Q.10. Assertion : Work done in moving a charge
between any two points in an electric field is independent of the path
followed by the charge, between these points. Reason: Electrostatic force is a non conservative force.
Answer
Q.11. Assertion : Two adjacent conductors of unequal dimensions, carrying the same positive charge have a potential difference between them. Reason : The potential of a conductor depends upon the charge given to it.
Answer
Q.12. Assertion : Electric potential and electric potential energy are different quantities. Reason : For a system of positive test charge and point charge electric potential energy = electric potential.
Answer
(c) Potential and potential energy are different quantities
and cannot be equated.
Q.13. Assertion : For a non-uniformly charged thin
circular ring with net charge is zero, the electric field at any point
on axis of the ring is zero. Reason : For a
non-uniformly charged thin circular ring with net charge zero, the
electric potential at each point on axis of the ring is zero.
Answer
(d) For a non-uniformly charged thin circular ring with net
zero charge, electric potential at each point on its axis is
zero. Hence electric field at each point on its axis must be
perpendicular to the axis. Therefore Assertion is false
and Reason is true.
Q.14. Assertion : For a charged particle moving from
point P to point Q, the net work done by an electrostatic field on the
particle is independent of the path connecting point P to point Q. Reason : The net work done by a conservative force on an object moving along a closed loop is zero.
Answer
a
Q.15. Assertion : Polar molecules have permanent dipole moment. Reason : In polar molecules, the centres of positive and negative charges coincide even when there is no external field.
Answer
c
Q.16. Assertion : Dielectric polarisation means formation of positive and negative charges inside the dielectric. Reason: Free electrons are formed in this process.
Answer
c
Q.17. Assertion : In the absence of an external electric field, the dipole moment per unit volume of a polar dielectric is zero. Reason : The dipoles of a polar dielectric are randomly oriented.
Answer
a
Q.18. Assertion : For a point charge, concentric spheres centered at a location of the charge are equipotential surfaces. Reason : An equipotential surface is a surface over which potential has zero value.
Answer
c
Q.19. Assertion : Electric energy resides out of the spherical isolated conductor. Reason : The electric field at any point inside the conductor is zero.
Answer
(a) As these is no electric field inside the conductor, and
so no energy inside it.
Q.20. Assertion : Two equipotential surfaces cannot cut each other. Reason : Two equipotential surfaces are parallel to each other.
Answer
(c) Reason is false because the work done in bringing a
unit positive charge from infinity to a point in equatorial
plane is equal and opposite for the two charges of the
dipole.
Q.21. Assertion: Two equipotential surfaces can be orthogonal. Reason: Electric field lines are normal to the equipotential surface.
Answer
(d) Two equipotential surfaces never intersect each other
so they cannot be orthogonal.
Q.22. Assertion: The equatorial plane of a dipole is an equipotential surface. Reason: The electric potential at any point on equatorial plane is zero.
Answer
b
Q.23. Assertion: The electric potential at any point on the equatorial plane of a dipole is zero. Reason: The
work done in bringing a unit positive charge from infinity to a point
in equatorial plane is equal for the two charges of the dipole.
Answer
d
Q.24. Assertion : A parallel plate capacitor is
connected across battery through a key. A dielectric slab of dielectric
constant k is introduced between the plates. The energy stored becomes k
times. Reason : The surface density of charge on the plate remains constant.
Answer
c
Q.25. Assertion : Two metal plates having charges Q,
–Q face each other at some separation and are dipped into an oil tank.
If the oil is pumped out, the electric field between the plates
increases. Reason : Electric field between the plates, Emed = Eair/K
Answer
c
Q.26. Assertion : When a dielectric slab is
gradually inserted between the plates of an isolated parallel-plate
capacitor, the energy of the system decreases. Reason : The force between the plates decreases.
Answer
c
Q.27. Assertion : A dielectric is inserted between the plates of a battery connected capacitor. The energy of the capacitor increases. Reason : Energy of the capacitor, U=CV2/2
Answer
Chapter 3 Current Electricity
Q.1. Assertion : In a simple battery circuit, the point of the lowest potential is positive terminal of the battery. Reason : The
current flows towards the point of the higher potential, as it does in
such a circuit from the negative to the positive terminal.
Answer
d
Q.2. Assertion : A larger dry cell has higher emf. Reason : The emf of a dry cell is proportional to its size.
Answer
d
Q.3. Assertion : A current continues to flow in superconducting coil even after switch is off. Reason : Superconducting coils show Meissner effect.
Answer
b
Q.4. Assertion : Voltmeter is connected in parallel with the circuit. Reason : Resistance of a voltmeter is very large.
Answer
b
Q.5. Assertion : Ohm’s law is applicable for all conducting elements. Reason : Ohm’s law is a fundamental law.
Answer
c
Q.6. Assertion : An electric bulb becomes dim, when the electric heater in parallel circuit is switched on. Reason : Dimness decreases after sometime.
Answer
Chapter 4 Moving Charges and Magnetism
Q.1. Assertion : Cyclotron is a device which is used to accelerate the positive ion. Reason : Cyclotron frequency depends upon the velocity.
Answer
c
Q.2. Assertion : Cyclotron does not accelerate electron. Reason : Mass of the electrons is very small.
Answer
c
Q.3. Assertion : In electric circuits, wires carrying currents in opposite directions are often twisted together Reason :
If the wires are not twisted together, the combination of the wires
forms a current loop, the magnetic field generated by the loop might
affect adjacent circuits or components.
Answer
a
Q.4. Assertion : The magnetic field produced by a current carrying solenoid is independent of its length and cross-sectional area. Reason : The magnetic field inside the solenoid is uniform.
Answer
b
Q.5. Assertion : A charge, whether stationary or in motion produces a magnetic field around it. Reason : Moving charges produce only electric field in the surrounding space.
Answer
d
Q.6. Assertion : A proton and an alpha particle
having the same kinetic energy are moving in circular paths in a uniform
magnetic field. The radii of their circular paths will be equal. Reason : Any
two charged particles having equal kinetic energies and entering a
region of uniform magnetic field B in a direction perpendicular to B,
will describe circular trajectories of equal radii.
Answer
c
Q.7. Assertion : If the current in a solenoid is
reversed in direction while keeping the same magnitude, the magnetic
field energy stored in the solenoid remains unchanged. Reason : Magnetic field energy density is proportional to the magnetic field.
Answer
c
Q.8. Assertion : The magnetic field at the centre of the circular coil in the following figure due to the currents I1 and I2 is zero.
Reason : I1 = I2 implies that the fields due to the current I1 and I2 will be balanced.
Answer
d
Q.9. Assertion : If the current in a solenoid is
reversed in direction while keeping the same magnitude, the magnetic
field energy stored in the solenoid decreases. Reason : Magnetic field energy density is proportional to square of current.
Answer
d
Q.10. Assertion : Free electrons always keep on
moving in a conductor even then no magnetic force act on them in
magnetic field unless a current is passed through it. Reason : The average velocity of free electron is zero.
Answer
b
Q.11. Assertion: To convert a galvanometer into an ammeter a small resistance is connected in parallel with it. Reason: The small resistance increases the combined resistance of the combination.
Answer
Chapter 5 Magnetism and Matter
Q.1. Assertion : We cannot think of a magnetic field configuration with three poles Reason : A bar magnet does exert a torque on itself due to its own field.
Answer
d
Q.2. Assertion : In high latitudes one sees colourful curtains of light hanging down from high altitudes Reason : The high energy charged particles from the sun are deflected to polar regions by the magnetic field of the earth.
Answer
a
Q.3. Assertion : The true geographic north direction is found by using a compass needle. Reason : The magnetic meridian of the earth is along the axis of rotation of the earth.
Answer
d
Q.4. Assertion : A disc-shaped magnet is deviated above a superconducting material that has been cooled by liquid nitrogen. Reason : Superconductors repel a magnet.
Answer
a
Q.5. Assertion : Magnetic Resonance Imaging (MRI) is a useful diagnostic tool for producing images of various parts of human body. Reason : Protons of various tissues of the human body play a role in MRI.
Answer
a
Q.6. Assertion : Diamagnetic materials can exhibit magnetism. Reason : Diamagnetic materials have permanent magnetic dipole moment.
Answer
c
Q.7. Assertion : Ferro-magnetic substances become paramagnetic above Curie temp. Reason : Domains are destroyed at high temperature.
Answer
a
Q.8. Assertion : If a compass needle be kept at magnetic north pole of the earth the compass needle may stay in any direction. Reason : Dip needle will stay vertical at the north pole of earth
Answer
b
Q.9. Assertion : The ferromagnetic substance do not obey Curie’s law. Reason : At Curie point a ferromagnetic substance start behaving as a paramagnetic substance.
Answer
b
Q.10. Assertion : The ferromagnetic substance do not obey Curie’s law. Reason : At Curie point a ferromagnetic substance start behaving as a paramagnetic substance.
Answer
c
Q.11. Assertion : A paramagnetic sample display greater magnetisation (for the same magnetic field) when cooled. Reason : The magnetisation does not depend on temperature.
Answer
d
Q.12. Assertion : Electromagnetic are made of soft iron. Reason : Coercivity of soft iron is small.
Answer
b
Q.13. Assertion : The sensitivity of a moving coil galvanometer is increased by placing a suitable magnetic material as a core inside the coil. Reason : Soft iron has high magnetic permeability and cannot be easily magnetized or demagnetized.
Answer
c
Q.14. Assertion : The poles of magnet can not be separated by breaking into two pieces. Reason : The magnetic moment will be reduced to half when a magnet is broken into two equal pieces.
Answer
Chapter 6 Electromagnetic Induction
Q.1. Assertion : Induced emf will always occur whenever there is change in magnetic flux. Reason : Current always induces whenever there is change in magnetic flux.
Answer
(c) Emf will always induces whenever, there is change in
magnetic flux. The current will induced only in closed
loop.
Q.2. Assertion : Faraday’s laws are consequence of conservation of energy. Reason : In a purely resistive ac circuit, the current legs behind the emf in phase.
Answer
(c) In purely resistive circuit, the current and emf are in
the same phase.
Q.3. Assertion : Only a change in magnetic flux will maintain an induced current in the coil. Reason : The presence of large magnetic flux through a coil maintain a current in the coil of the circuit is continuous.
Answer
(c)
Q.4. Assertion : Lenz’s law violates the principle of conservation of energy. Reason : Induced emf always opposes the change in magnetic flux responsible for its production.
Answer
(a) Lenz’s law (that the direction of induced emf is always
such as to oppose the change that cause it) is direct
consequence of the law of conservation of energy.
Q.5. Assertion : An induced current has a direction
such that the magnetic field due to the current opposes the change in
the magnetic flux that induces the current. Reason : Above statement is in accordance with conservation of energy.
Answer
(b)
Q.6. Assertion : Acceleration of a magnet falling through a long solenoid decreases. Reason :
The induced current produced in a circuit always flow in such direction
that it opposes the change to the cause that produced it.
Answer
(a)
Q.7. Assertion : Figure shows a horizontal solenoid
connected to a battery and a switch. A copper ring is placed on a smooth
surface, the axis of the ring being horizontal. As the switch is
closed, the ring will move away from the solenoid.
Reason : Induced emf in the ring, e=-dΦ/dt
Answer
(a)When switch is closed , the magnetic flux through the
ring will increase and so ring will move away form the
solenoid so as to compensate this flux. This is according
to Lenz’s law.
Q.8. Assertion : An emf can be induced by moving a conductor in a magnetic field. Reason : An emf can be induced by changing the magnetic field.
Answer
(b) In both the cases, the magnetic flux will change, and
so there is an induced current.
Q.9. Assertion : Figure shows a metallic conductor moving in magnetic field. The induced emf across its ends is zero.
Reason : The induced emf across the ends of a conductor is given by e = Bvℓ sinθ.
Answer
(a)
Q.10. Assertion : Eddy currents are produced in any metallic conductor when magnetic flux is changed around it. Reason : Electric potential determines the flow of charge.
Answer
(b) Both the statements are independently correct.
Q.11. Assertion : An induced emf appears in any coil in which the current is changing. Reason : Self induction phenomenon obeys Faraday’s law of induction.
Answer
(b)
Q.12. Assertion : When number of turns in a coil is doubled, coefficient of self-inductance of the coil becomes 4 times. Reason : This is because L α N2.
Answer
Chapter 8 Electromagnetic Waves
Q.1. Assertion : Electromagnetic wave are transverse in nature. Reason : The electric and magnetic fields in electromagnetic waves are perpendicular to each other and the direction of propagation.
Answer
(a) Transverse waves are those waves in which the
particles of the medium oscillate perpendicular to the
direction of wave propagation.
Q.2. Assertion : Electromagnetic waves interact with matter and set up oscillations. Reason : Interaction is independent of the wavelength of the electromagnetic wave.
Answer
(c) Electromagnetic waves interact with matter via their
electric and magnetic field which in oscillation of
charges present in all matter. The detailed interaction
and so the mechanism of absorption, scattering, etc.
depend of the wavelength of the electromagnetic
wave, and the nature of the atoms and molecules in
the medium.
Q.3. Assertion : Electromagnetic waves carry energy and momentum. Reason : Electromagnetic waves can be polarised.
Answer
(b) Consider a plane perpendicular to the direction of
propagation of the electromagnetic wave. If electric
charges are present in this plane, they will be set
and sustained in motion by the electric and magnetic
fields of the electromagnetic wave. The charge thus
acquired energy and momentum from the wave. This
illustrate the fact that an electromagnetic wave like
other waves carries energy and momentum.
Answer
(a) Electromagnetic waves have linear momentum as
well as energy. This concludes that they can exert
radiation pressure by falling beam of electromagnetic
radiation on an object.
Q.5. Assertion : The electromagnetic wave is transverse in nature. Reason : Electromagnetic wave propagates parallel to the direction of electric and magnetic fields.
Answer
(c) This electromagnetic wave contains sinusoidally
time varying electric and magnetic field which act
perpendicular to each other as well as at right angle
to the direction of propagation of waves, so
electromagnetic waves are transverse in nature.
Electromagnetic wave propagate in the perpendicular
direction to both fields.
Q.6. Assertion : The velocity of electromagnetic waves depends on electric and magnetic properties of the medium. Reason : Velocity of electromagnetic waves in free space is constant.
Answer
(b)
Q.7. Assertion : The basic difference between various types of electromagnetic waves lies in their wavelength or frequencies. Reason : Electromagnetic waves travel through vacuum with the same speed.
Answer
(a) The basic difference between various types of
electromagnetic waves lies in their wavelengths or
frequencies since all of them travel through vacuum
with the same speed. Consequently, the waves differ
considerably in their mode of interaction with matter.
Q.8. Assertion : Microwaves are better carrier of signals than optical waves. Reason : Microwaves move faster than optical waves.
Answer
(d) The optical waves used in optical fibre
communication are better carrier of signals than
microwaves. The speed of microwave and optical
wave is the same in vacuum.
Q.9. Assertion : Infrared radiation plays an important role in maintaining the average temperature of earth. Reason : Infrared radiations are sometimes referred to as heat waves
Answer
Chapter 9 Ray Optics and Optical Instruments
Q.1. Assertion : Plane mirror may form real image. Reason : Plane mirror forms virtual image, if object is real.
Answer
b
Q.2. Assertion : The focal length of the convex mirror will increase, if the mirror is placed in water.
Reason : The focal length of a convex mirror of radius R is equal to , f = R/2.
Answer
(d) Focal length of the spherical mirror does not depend
on the medium in which it placed.
Q.3. Assertion : The image formed by a concave mirror is certainly real if the object is virtual. Reason : The image formed by a concave mirror is certainly virtual if the object is real.
Answer
(c) The image of real object may be real in case of concave
mirror.
Q.4. Assertion :
The image of an extended object placed perpendicular to the principal
axis of a mirror, will be erect if the object is real but the image is
virtual. Reason : The image of an extended object,
placed perpendicular to the principal axis of a mirror, will be erect if
the object is virtual but the image is real.
Answer
b
Q.5. Assertion : An object is placed at a distance of f from a convex mirror of focal length f its image will form at infinity. Reason : The distance of image in convex mirror can never be infinity
Answer
d
Q.6. Assertion : The image of a point object situated at the centre of hemispherical lens is also at the centre. Reason : For hemisphere Snell’s law is not valid.
Answer
(c) The rays from centre of hemisphere cut at the centre after
refraction – Snell’s law is valid in each case of refraction.
Q.7. Assertion: The focal length of an equiconvex lens of radius of curvature R made of material of refractive index μ = 1.5, is R. Reason : The focal length of the lens will be R/2.
Answer
(c)
Q.8. Assertion : If the rays are diverging after emerging from a lens; the lens must be concave. Reason : The convex lens can give diverging rays.
Answer
(d) If the rays cross focal point of convex lens, they
become diverging.
Q.9. Assertion : The resolving power of a telescope is more if the diameter of the objective lens is more. Reason : Objective lens of large diameter collects more light.
Answer
(a) RP α diameter of objective.
Q.10. Assertion : The optical instruments are used to increase the size of the image of the object. Reason : The optical instruments are used to increase the visual angle.
Answer
Chapter 10 Wave Optics
Q.1. Assertion : According to Huygen’s principle, no backward wave-front is possible. Reason : Amplitude
of secondary wavelet is proportional to (1 + cos θ) where θ is the
angle between the ray at the point of consideration and the direction of
secondary wavelet.
Answer
b
Q.2. Assertion : Thin film such as soap bubble or a thin layer of oil on water show beautiful colours when illuminated by white light. Reason : It happens due to the interference of light reflected from upper and lower face of the thin film.
Answer
a
Q.3. Assertion : No interference pattern is detected when two coherent sources are infinitely close to each other. Reason : The fringe width is inversely proportional to the distance between the two sources.
Answer
a
Q.4. Assertion : It is necessary to have two waves of equal intensity to study interference pattern. Reason : There will be an effect on clarity if the waves are of unequal intensity.
Answer
(d) For interference, the waves may be of unequal
intensities.
Q.5. Assertion : White light falls on a double slit with one slit is covered by a green filter. The bright fringes observed are of green colour. Reason : The fringes observed are coloured.
Answer
(c) Interference will take place in green light only
Q.6. Assertion : In YDSE, if a thin film is introduced in front of the upper slit, then the fringe pattern shifts in the downward direction. Reason : In YDSE if the slit widths are unequal, the minima will be completely dark.
Answer
d
Q.7. Assertion : In Young’s double slit experiment
if wavelength of incident monochromatic light is just doubled, number of
bright fringe on the screen will increase. Reason : Maximum number of bright fringe on the screen is inversely proportional to the wavelength of light used
Answer
a
Q.8. Assertion : In YDSE number of bright fringe or dark fringe can not be unlimited Reason : In YDSE path difference between the superposing waves can not be more than the distance between the slits.
Answer
b
Q.9. Assertion : Interference pattern is made by using yellow light instead of red light, the fringes becomes narrower. Reason : In YDSE, fringe width is given by β=λD/d
Answer
a
Q.10. Assertion : Coloured spectrum is seen when we look through a muslin cloth. Reason : It is due the diffraction of white light on passing through fine slits.
Answer
a
Q.11. Assertion : Diffraction takes place for all types of waves mechanical or non-mechanical, transverse or longitudinal. Reason : Diffraction’s effect are perceptible only if wavelength of wave is comparable to dimensions of diffracting device.
Answer
Chapter 11 Dual Nature of Radiation and Matter
Q.1. Assertion : In process of photoelectric emission, all emitted electrons do not have same kinetic energy. Reason :
If radiation falling on photosensitive surface of a metal consists of
different wavelength then energy acquired by electrons absorbing photons
of different wavelengths shall be different.
Answer
(b) Both statement I and II are true; but even it radiation
of single wavelength is incident on photosensitive
surface, electrons of different KE will be emitted.
Q.2. Assertion : Though light of a single frequency (monochromatic) is incident on a metal, the energies of emitted photoelectrons are different. Reason : The energy of electrons emitted from inside the metal surface, is lost in collision with the other atoms in the metal.
Answer
(a) When a light of single frequency falls on the electrons
of inner layer of metal, then this electron comes out of
the metal surface after a large number of collisions
with atom of it’s upper layer.
Q.3. Assertion : The photoelectrons produced by a monochromatic light beam incident on a metal surface have a spread in their kinetic energies. Reason : The work function of the metal is its characteristics property.
Answer
(b) The kinetic energy of emitted photoelectrons varies
from zero to a maximum value. Work function depends
on metal used.
Q.4. Assertion : Photoelectric saturation current increases with the increase in frequency of incident light. Reason : Energy of incident photons increases with increase in frequency and as a result photoelectric current increases.
Answer
(d) Photoelectric saturation current is independent of
frequency. It only depends on intensity of light.
Q.5. Assertion : Photosensitivity of a metal is high if its work function is small. Reason : Work function = hf0 where f0 is the threshold frequency.
Answer
(b) Less work function means less energy is required for
ejecting out the electrons.
Q.6. Assertion : The photon behaves like a particle. Reason : If E and P are the energy and momentum of the photon, then p = E / c .
Answer
a
Q.7. Assertion : In an experiment on photoelectric
effect, a photon is incident on an electron from one direction and the
photoelectron is emitted almost in the opposite direction. It violate
the principle of conservation of linear momentum. Reason : It does not violate the principle of conservation of linear momentum.
Answer
d
Q.9. Assertion : Two sources of equal intensity always emit equal number of photons in any time interval. Reason : Two sources of equal intensity may emit equal number of photons in any time interval.
Answer
(d) Total number of emitted photons depends on energy
of each photon. The energy of photons of two sources
may be different.
Q.10. Assertion : Two photons of equal wavelength must have equal linear momentum. Reason : Two photons of equal linear momentum will have equal wavelength.
Answer
(d) To photons of equal wavelength will have equal
momentum (magnitude), but direction of momentum
may be different.
Q.11. Assertion : The kinetic energy of photoelectrons emitted from metal surface does not depend on the intensity of incident photon. Reason : The
ejection of electrons from metallic surface is not possible with
frequency of incident photons below the threshold frequency.
Answer
Chapter 12 Atoms
Q.1. Assertion : The force of repulsion between atomic nucleus and α-particle varies with distance according to inverse square law. Reason : Rutherford did α-particle scattering experiment.
Answer
(b) Rutherford confirmed that the repulsive force of α-
particle due to nucleus varies with distance according
to inverse square law and that the positive charges
are concentrated at the centre and not distributed
throughout the atom.
Q.2. Assertion : According to classical theory the proposed path of an electron in Rutherford atom model will be parabolic. Reason : According to electromagnetic theory an accelerated particle continuously emits radiation.
Answer
(d) According to classical electromagnetic theory, an accelerated
charged particle continuously emits radiation.
As electrons revolving in circular paths are constantly
experiencing centripetal acceleration, hence
they will be losing their energy continuously and the
orbital radius will go on decreasing, form spiral and
finally the electron will fall in the nucleus.
Q.3. Assertion : Bohr had to postulate that the electrons in stationary orbits around the nucleus do not radiate. Reason: According to classical physics all moving electrons radiate.
Answer
(b) Bohr postulated that electrons in stationary orbits
around the nucleus do not radiate.
This is the one of Bohr’s postulate, According to this
the moving electrons radiates only when they go from
one orbit to the next lower orbit.
Q.4. Assertion : Electrons in the atom are held due to coulomb forces. Reason : The atom is stable only because the centripetal force due to Coulomb’s law is balanced by the centrifugal force.
Answer
(c) According to postulates of Bohr’s atom model the electron
revolves around the nucleus in fixed orbit of definite
radii. As long as the electron is in a certain orbit it
does not radiate any energy.
Q.5. Assertion : Hydrogen atom consists of only one electron but its emission spectrum has many lines. Reason : Only Lyman series is found in the absorption spectrum of hydrogen atom whereas in the emission spectrum, all the series are found.
Answer
(b) When the atom gets appropriate energy from outside,
then this electron rises to some higher energy level.
Now it can return either directly to the lower energy
level or come to the lowest energy level after passing
through other lower energy levels hence all possible
transitions take place in the source and many lines are
seen in the spectrum.
Q.6. Assertion : Between any two given energy
levels, the number of absorption transitions is always less than the
number of emission transitions. Reason : Absorption
transitions start from the lowest energy level only and may end at any
higher energy level. But emission transitions may start from any higher
energy level and end at any energy level below it.
Answer
(a)
Q.7. Assertion : In Lyman series, the ratio of minimum and maximum wavelength is 3/4 Reason : Lyman series constitute spectral lines corresponding to transition from higher energy to ground state of hydrogen atom.
Answer
Chapter 13 Nuclei
Q.1. Assertion : Density of all the nuclei is same. Reason : Radius of nucleus is directly proportional to the cube root of mass number.
Answer
a
Q.2. Assertion : Neutrons penetrate matter more readily as compared to protons. Reason : Neutrons are slightly more massive than protons.
Answer
(b) Both statements are separately correct.
Q.3. Assertion : The mass number of a nucleus is always less than its atomic number. Reason : Mass number of a nucleus may be equal to its atomic number.
Answer
(d) In case of hydrogen atom mass number and atomic
number are equal.
Q.4. Assertion : The binding energy per nucleon, for nuclei with atomic mass number A > 100, decrease with A. Reason : The forces are weak for heavier nuclei.
Answer
(c) Nuclear force is nearly same for all nucleus.
Q.5. Assertion : Radioactivity of 108 undecayed
radioactive nuclei of half life of 50 days is equal to that of 1.2 × 108
number of undecayed nuclei of some other material with half life of 60
days. Reason : Radioactivity is proportional to half-life.
Answer
c
Q.6. Assertion : The ionising power of β-particle is less compared to β-particles but their penetrating power is more. Reason : The mass of β-particle is less than the mass of α- particle.
Answer
(b) β-particles, being emitted with very high speed
compared to α-particles, pass for very little time near
the atoms of the medium. So the probability of the
atoms being ionised is comparatively less. But due to
this reason, their loss of energy is very slow and they
can penetrate the medium through a sufficient depth.
Q.8. Assertion : ZXA undergoes 2α, 2β- particles and 2γ-rays, the daughter product is Z-2YA – 8. Reason :
In α- decay the mass number decreases by 4 and atomic number decreases
by 2. In β-decay the mass number remains unchanged, but atomic number
increases by 1.
Answer
a
Q.9. Assertion : The heavier nuclei tend to have larger N/Z ratio because neutron does not exert electric force. Reason : Coulomb forces have longer range compared to the nuclear force.
Answer
a
Q.10. Assertion : A free neutron decays to a proton
but a free proton does not decay to a neutron. This is because neutron
is an uncharged particle and proton is a charged particle. Reason : Neutron has larger rest mass than the proton.
Answer
d
Q.11. Assertion : Cobalt-60 is useful in cancer therapy. Reason : Cobalt -60 is source of γ- radiations capable of killing cancerous cells.
Answer
d
Q.12. Assertion : It is not possible to use 35Cl as the fuel for fusion energy. Reason : The binding energy of 35Cl is to small.
Answer
c
Q.13. Assertion : Energy is released when heavy nuclei undergo fission or light nuclei undergo fusion. Reason : For heavy nuclei, binding energy per nucleon increases with increasing Z while for light nuclei it decreases with increasing Z.
Answer
Chapter 14 Semiconductor Electronics
Q.1. Assertion : A pure semiconductor has negative temperature coefficient of resistance. Reason :
In a semiconductor on raising the temperature, more charge carriers are
released, conductance increases and resistance decreases.
Answer
(a) In semiconductors, by increasing temperature,
covalent bond breaks and conduction hole and
electrons increase.
Q.2. Assertion : If the temperature of a semiconductor is increased then its resistance decreases. Reason : The energy gap between conduction band and valence band is very small.
Answer
(a) In semiconductors the energy gap between conduction
band and valence band is small (1 eV). Due to
temperature rise, electron in the valence band gain
thermal energy and may jumpy across the small energy
gap, (to the conduction band). Thus conductivity
increases and hence resistance decreases.
Q.3. Assertion : In semiconductors, thermal collisions are responsible for taking a valence electron to the conduction band. Reason : The number of conduction electrons go on increasing with time as thermal collisions continuously take place.
Answer
c
Q.4. Assertion : A p-type semiconductors is a positive type crystal. Reason : A p- type semiconductor is an uncharged crystal.
Answer
(d) There is no charge on P-type semiconductor, because
each atom of semiconductor is itself neutral.
Q.5. Assertion : Silicon is preferred over germanium for making semiconductor devices. Reason : The energy gap in germanium is more than the energy gap in silicon.
Answer
(c) Silicon is cheaper than germanium, so it is preferred
over germanium. But energy gap in germanium is
smaller than silicon.
Q.6. Assertion : Electron has higher mobility than hole in a semiconductor. Reason : The mass of electron is less than the mass of the hole.
Answer
a
Q.7. Assertion : The number of electrons in a p-type
silicon semiconductor is less than the number of electrons in a pure
silicon semiconductor at room temperature. Reason : It is due to law of mass action.
Answer
a
Q.8. Assertion : When two semi conductor of p and n type are brought in contact, they form p-n junction which act like a rectifier. Reason : A rectifier is used to convent alternating current into direct current.
Answer
(b) Study of junction diode characteristics shows that the
junction diode offers a low resistance path, when
forward biased and high resistance path when reverse
biased. This feature of the junction diode enables it to
be used as a rectifier.
Q.9. Assertion : Diode lasers are used as optical sources in optical communication. Reason : Diode lasers consume less energy.
Answer
(c) Statement – 1 is True, Statement- 2 is False
Q.10. Assertion : The diffusion current in a p-n junction is from the p-side to the n-side. Reason : The diffusion current in a p-n junction is greater than the drift current when the junction is in forward biased.
Answer
(b) Diffusion current is due to the migration of holes and
electrons into opposite regions, so it will be from p-side
to n-side. Also in forward bias it will increases.
Q.11. Assertion : The drift current in a p-n junction is from the n-side to the p-side. Reason : It is due to free electrons only.
Answer
a
Q.12. Assertion : A p-n junction with reverse bias can be used as a photo-diode to measure light intensity. Reason : In a reverse bias condition the current is small but it is more sensitive to changes in incident light intensity.
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XII - PHYSICS VIDEO LECTURES HEY! CRUSH OUT ALL ONCEPT OF XII PHYSICS UNITS WEBLINKS Unit 1. Electrostatics Part 1 https:// www.youtube.com/watch?v=5JZjEmJqtus&list=PLNz32RYOjBepdhG CSA5OoVPha83vKJePl&index=19 Part 2 https:// www.youtube.com/watch?v=jeLUsBz0XlM&list=PLNz32RYOjBepdh GCSA5OoVPha83vKJePl&index=18 Part 3 https:// www.youtube.com/watch?v=5id9XWSaS4g&list=PLNz32RYOjBepdh GCSA5OoVPha83vKJePl&index=17 Part 4 https:// www.youtube.com/watch?v=bAb4Yt0NR7w&list=PLNz32RYOjBepd hGCSA5OoVPha83vKJePl&index=16 Unit 2. Current Electricity https:// www.youtube.com/watch?v=_7V9rjkE5YQ&list=PLNz32RYOjBepdh GCSA5OoVPha83vKJePl&index=15 Unit 3. Magnetic Effect of Current and Magnetism Part 1 https:// www.youtube.com/watch?v=8zKTFwIzXhM&list=PLNz32RYOjBepd
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