[140] Important Short Questions + Solution of Physics for class 12 NEB: The Complete Study Guide

[140] Important Short Questions + Solution of Physics for class 12 NEB: The Complete Study Guide

1.       The angular velocity of the earth around the sun increases when it comes closer to the sun. Why?

Solution: The angular velocity of the Earth around the Sun increases when it comes closer to the Sun because of the conservation of angular momentum. The moment of inertia of the Earth about an axis measured through the Sun keeps on changing due to changes in its distance from the Sun. Hence, the angular velocity of the Earth around the Sun increases because the gravitational pull from the Sun gets stronger.

2.       If the earth were to shrink suddenly what would happen to the length of the day.

Solution: If the Earth were to shrink suddenly while keeping its mass constant, the length of a day would decrease. This is because the length of a day is determined by the Earth’s rotation on its axis, and the rotational speed of the Earth is determined by its mass and size. If the Earth were to shrink from its original size, then the moment of inertia would decrease. By the conservation of angular momentum, the angular velocity would increase due to its small size and thus the length of the day would decrease.

3.       What do you mean by moment of inertia?

Solution: The moment of inertia is a measure of an object’s resistance to changes in its rotational motion. It depends on the mass of the object and how that mass is distributed relative to the axis of rotation. The larger the moment of inertia, the more difficult it is to change the rotational speed of the object.

4.       State the principle of conservation of angular momentum.

Solution: The principle of conservation of angular momentum states that the total angular momentum of a closed system remains constant unless acted upon by external torques. This means that if no external torques are acting on a system, the angular momentum of the system will not change.

5.       Can you distinguish between a raw egg and a hard-boiled egg by spinning each on the table explain.

Solution: Yes, it is possible to distinguish between a raw egg and a hard-boiled egg by spinning each on a table. The egg that spins at a slower rate will be raw. This is because in a raw egg, the liquid matter inside tries to get away from its axis of rotation due to centrifugal force. This increases the rotational inertia of the egg and it spins slowly. On the other hand, the hard-boiled egg will have lower rotational inertia and will spin faster relative to the raw egg.

6.       A ballet dancer stretches her hands when she wants to come to rest why?

Solution: A ballet dancer stretches her hands when she wants to come to rest because it is based on the principle of conservation of angular momentum. When a ballet dancer stretches her arms, the moment of inertia of the dancer about the axis of rotation increases. Since no external torque is applied, total angular momentum remains constant. Due to the increase in moment of inertia, angular velocity decreases and the dancer comes to rest gradually.

7.       Why it is easier to open the cap of the pen with two fingers than with one finger?

Solution: It is easier to open the cap of a pen with two fingers than with one finger because when we use two fingers, two forces in opposite directions are applied. Rotational motion produced by a single force is less than two equal and opposite forces. Therefore, it is easier to open the cap with the help of two fingers rather than one finger.

8.       Will two spheres of equal mass one solid and other hollow have an equal moment of inertia? Explain.

Solution:  No, two spheres of equal mass, one solid and the other hollow, will not have an equal moment of inertia. The moment of inertia depends on the mass of the object and how that mass is distributed relative to the axis of rotation. For a solid sphere, the mass is distributed closer to the axis of rotation, while for a hollow sphere, the mass is distributed farther from the axis of rotation. This means that for a given mass and radius, a hollow sphere will have a larger moment of inertia than a solid sphere. 

9.       Explain why spokes are Fitted in the cycle wheel.

Solution: Spokes are fitted in the cycle wheel to increase the moment of inertia due to the increase in the distribution of mass. This opposes the change in the rotary motion of the wheel. Thus, spokes fitted to the cycle wheel give a steady motion.

10.   A pendulum clock is taken to the moon; will it gain or loss time. Why?

Solution: A pendulum clock taken to the moon will lose time. This is because the time period of a pendulum clock is determined by the acceleration due to gravity. On the moon, the acceleration due to gravity is lower than on Earth. This means that the time period of the pendulum will be longer on the moon, and hence, the pendulum will lose time.

11.   A simple harmonic motion is represented in usual motion by y=rsin(wt +ϕ),find its acceleration.

Solution: The acceleration of a simple harmonic motion represented by y = r sin(wt + ϕ) can be found by taking the second derivative of the displacement with respect to time. The first derivative gives us the velocity:

v = dy/dt = r w cos(wt + ϕ)

Taking the derivative of the velocity with respect to time gives us the acceleration:

a = dv/dt = -r w2 sin(wt + ϕ)

So, the acceleration of the simple harmonic motion is a = -r w2 sin(wt + ϕ).

12.   Define Second's Pendulum.

Solution: A second’s pendulum is a pendulum whose time period is precisely two seconds; one second for a swing in one direction and one second for the return swing, for a complete cycle in two seconds. The length of a simple pendulum that has a period of two seconds is approximately 0.994 meters.

13.   On what factors does the period of a simple pendulum depend.

Solution: The period of a simple pendulum depends on two factors: the length of the pendulum and the acceleration due to gravity. The period of a simple pendulum is given by the formula $T = 2\pi \sqrt {\frac{l}{g}} $, where T is the period, L is the length of the pendulum, and g is the acceleration due to gravity. This means that the period of a simple pendulum is directly proportional to the square root of its length and inversely proportional to the square root of the acceleration due to gravity.

14.   Explain why soldiers are ordered to break steps while crossing the bridge.

Solution: Soldiers break steps while crossing a bridge to prevent it from collapsing due to mechanical resonance caused by their rhythmic marching.

15.   Why do we say that the velocity and acceleration of a body executing SHM are out phases?

Solution: In simple harmonic motion (SHM), velocity and acceleration are said to be out of phase because when the velocity is at its maximum value, the acceleration is zero. This is because acceleration is the rate of change of velocity. When the magnitude of velocity is maximum, there will be a stationary point. At that point, the acceleration is zero.

16.   What do you mean by damping? What is its cause?

Solution: Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. For example, friction can cause damping by converting mechanical energy of motion into heat.

17.   At what temperature the surface tension of a liquid is zero?

Solution: The surface tension of a liquid becomes zero at its critical temperature. At this temperature, there is no separation of surfaces between the vapor phase and the liquid phase.

18.   Why do clouds seem to be floating in the sky.

Solution: Clouds seem to be floating in the sky because they are composed primarily of small water droplets and, if it is cold enough, ice crystals. The water and ice particles in the clouds are simply too small to feel the effects of gravity. As a result, clouds appear to float on air.

19.   Why is the bottom of a ship made heavy explain?

Solution: The bottom of a ship is made heavy for two reasons. One is to keep the center of gravity of the ship below the center of buoyancy, which helps to keep the ship in a stable equilibrium position. The other reason is to increase the moment of inertia of the ship, which opposes changes in its rotational motion and helps to keep it steady.

20.   The purity of the gold can be tested by weighing it in year in water, how?

Solution: You can test the purity of gold by measuring its density using water displacement. Measure the mass of the gold and the water level in a graduated cylinder before and after submerging the gold. Calculate the difference in water levels and divide the mass of the gold by this volume to get its density. Compare this to the density of pure gold (19.3 g/cc) to determine its purity.

21.   Why is it easier to lift a body in a liquid than in air?

Solution: It is easier to lift a body in a liquid than in air because the liquid provides a greater amount of upthrust than air does. This is because the density of a liquid is greater than that of air. The upthrust acting on the body reduces its apparent weight, making it easier to lift.

22.   Define Surface tension.

Solution: Surface tension is a property of liquids that arises from the cohesive forces between their molecules. It is defined as the force acting per unit length on the surface of a liquid, which tends to minimize its surface area.

23.   Why does hot soup taste better than cold soup.

Solution: Hot soup is often considered to taste better than cold soup because the surface tension of hot soup is lower than that of cold soup. This allows it to spread over a larger area of the tongue, allowing more taste receptors to be activated.

24.   Explain why liquid drops are spherical in shape? Explain.

Solution: Liquid drops are spherical in shape because of surface tension. The inward forces on the surface molecules of a liquid drop tend to cause the surface to volume ratio as small as possible. Since surface to volume ratio is minimum for the spherical shape, a liquid drop is spherical.

25.   What factors does the surface tension of a liquid depend on, explain.

Solution: Surface tension is caused by the effects of intermolecular forces at the interface. Surface tension depends on the nature of the liquid, the surrounding environment and temperature. Liquids where molecules have large attractive intermolecular force will have a large surface tension.

 

26.   Antiseptic used in cuts and wounds of human flesh have low surface tension, why?

Solution: It should have low surface tension because liquids with low surface tension can spread easily. So, antiseptic with low surface tension can spread over cuts and wounds easily and help to recover faster.

27.   State stokes law.

Solution: Stokes Law states that the force that retards a sphere moving through a viscous fluid is directly proportional to the velocity and the radius of the sphere, and the viscosity of the fluid.

28.   State Bernoulli's theorem.

Solution: Bernoulli’s theorem, also known as Bernoulli’s principle, states that the total mechanical energy of a moving fluid, which includes gravitational potential energy of elevation, fluid pressure energy, and kinetic energy of fluid motion, remains constant.

29.   Small air bubbles rise slowly while big bubbles rise rapidly through the liquid. why?

Solution: The terminal velocity of bubbles is approximated by Stokes Law (assuming a spherical bubble). The terminal velocity is proportional to the square of the radius as well as the density difference between the bubble and the fluid. A bubble twice as big as another will rise 4 times as fast. So, small air bubbles rise slowly while big bubbles rise rapidly through the liquid because of their terminal velocity.

30.   Define viscosity. Does it depend on temperature?

Solution: Viscosity is the measure of a fluid’s resistance to flow. It denotes opposition to flow and can be thought of as internal friction between the molecules of a fluid. Yes, viscosity does depend on temperature. In liquids, it usually decreases with increasing temperature, whereas in most gases, viscosity increases with increasing temperature.

31.   During certain windstorms, light roofs are blown off. Why?

Solution: During windstorms, light roofs can be blown off due to differences in air pressure. The air pressure inside a house is lower than outside. When wind enters a house from an open doorway or broken window, the air pressure inside will increase and push upwards towards the roof. Combine this with a suction pulling from on top of the roof and you have a push-and-pull scenario that increases the likelihood of the roof getting blown off from the house.

32.   Define Reynold's number?

Solution: Reynold's number is defined as the ratio of inertial forces to viscous forces within a fluid that is subjected to relative internal movement due to different fluid velocities.

33.   When a smooth flowing stream of water comes out of a faucet, it narrows as it falls. Explain.

Solution: When a smooth flowing stream of water comes out of a faucet, it narrows as it falls due to gravity. Like all objects, a stream of water also accelerates as it falls. Since the density of water is constant and water tends to hold together (cohesion), the only thing that can happen is the narrowing of the stream – which is exactly what happens.

34.   Why does the temperature of a gas undergo adiabatic expansion decrease?

Solution: In an adiabatic expansion, a gas does work at the cost of its kinetic energy and so its random motion is reduced. According to the kinetic theory, the temperature of a gas is associated with its random motion. This is why the temperature of a gas drops in an adiabatic expansion.

35.   Why does a gas have two specific heat capacities?

Solution: A gas has two specific heat capacities because the measurement of specific heat for gases can be affected by many state variables of the system such as temperature, pressure, and volume. Therefore, we use two methods to measure the specific heat of gases, which are at constant volume (Cv) and constant pressure (Cp). 

36.   Is Cp always greater than Cv?

Solution: Yes, Cp is always greater than Cv for gases. When a gas is heated at constant pressure, expansion in the volume with an increase in the internal energy of the system takes place. Hence, Cp is greater than Cv.

37.   What are the limitations of the first law of thermodynamics.

Solution: The first law of thermodynamics has several limitations. 

·         It does not state anything about the heat flow direction. 

·         It does not indicate whether heat can flow from a cold body to a hot body. 

·         The first law also does not tell anything about the condition under which heat can be converted into mechanical work.

38.   Why does a refrigerator consume more power in summer than in winter to cool the same quantity of food by the same degree?

Solution: A refrigerator consumes more power in summer than in winter to cool the same quantity of food by the same degree because heat pumps are designed to move thermal energy opposite to the direction of spontaneous heat flow by absorbing heat from a cold space and releasing it to a warmer one. Due to surrounding temperature, a refrigerator consumes more power in summer than in winter to cool the same quantity of food by the same degree.

39.   Can a room be cooled leaving the doors of electric refrigerator open in a closed room? Explain.

Solution: No, a room cannot be cooled by leaving the doors of an electric refrigerator open in a closed room. In fact, the temperature of the room will rise because the refrigerator extracts heat from its freezing chambers and rejects it to the surrounding air in the room.

40.   Explain the significance of the second law of thermodynamics.

Solution: The second law of thermodynamics is significant because it explains why certain processes occur and others do not. It states that as energy is transferred or transformed, more and more of it is wasted. The second law also states that there is a natural tendency of any isolated system to degenerate into a more disordered state. 

41.   Petrol engines are less effective than diesel engines. Explain why?

Solution: Because diesel has a lower ignition temperature than petrol. Diesel is more combustible than petrol, therefore less fuel is required to generate the same amount of power. Secondly, petrol doesn't undergo complete combustion and hence there is a small wastage of fuel. This is why diesel engines are more efficient than petrol engines.

42.   Explain why the temperature of the gas drops in the adiabatic process?

Solution: During adiabatic expansion, the gas is allowed to expand quickly work is done by the gas during its expansion. So, its internal energy decreases. As heat can't be enter the same system for the surroundings, the temperature of gas falls.

43.   State second law of thermodynamics.

Solution: It states that, “It is impossible to get continuously supply work done from a body by cooling it to a temperature than that of its surrounding.”

44.   Can longitudinal waves be polarized? Explain.

Solution: No, because they are the wave in which the vibration of a party called takes place in the direction of the motion of the waves. Thus, the longitudinal wave is already polarized in the direction of the motion of the waves. Therefore, longitudinal waves are not polarized.

45.   Distinguish between progressive waves and stationery waves.

Solution:

Progressive Wave

Stationary waves

No particle is permanently at rest.

All the particles of nodes are permanently at rest.

At every point there is vibration in pressure.

Pressure vibration is maximum at nodes and zero at antinodes.

Energy is transmitted from particle to particle.

Energy is not transmitted from particle to particle.

 

46.   Frequency is the most fundamental property of a wave. Why?

Solution:  Frequency is the only physical quantity that is independent of the nature of media through which the wave is propagating. When a wave travels its wavelength as well as velocity may changes but its frequency remains unchanged. This is the reason for frequency to be a fundamental property.

47.   Which types of waves propagate in the liquid, Explain.

Solution:  For the propagation of the transverse wave, the modulus rigidity of the medium is responsible and for the propagation of the longitudinal wave, the bulk modulus of elasticity is responsible. A sold has both moduli of rigidity and bulk modulus but liquids only have bulk modulus. Hence only longitudinal waves can propagate in the liquids.

48.   The speed of sound in humid air is more than that in dry air, why?

Solution: The velocity off sound is inversely proportional to the square root of its density.  The density of water vapor is smaller than that of dry air, and the presence of moisture un air reduces the density of air in atmosphere. So, Velocity of Sound is more in moist (humid) air than dry air.

49.   Define an organ pipe.

Solution:  The hollow wood or metallic tube which is used to produce musical sound is called organ pipe.

50.   When we start filling an empty bucket with water, the pitch of sound produced goes on changing. Why?

Solution:  a bucket can be treated as pipe closed at one end. The frequency of the note produced =v/4l, where v is velocity of sound and l is length of air column, which is equal to depth of water level from the open end. As a bucket is filled with water length (l) decreases. Therefore, frequency of sound produces goes on increasing.

51.   Bells are made of metal but not wood. Why?

Solution:  Bells are made from metals because metals have the capability to produce sound when hit with a solid object, i.e, they are sonorous. Wood is not sonorous, and is a non-metal.

 

52.   Why is sounding board used in a string instrument?

Solution: the stringed instrument is provided with a hollow box called a soundboard. When the strings are shaped into vibrations forced vibration are produced in the sound board. Since the soundboard has a large area, it sets the large volume of air into vibration. This produces a loud sound of the same frequency as that of the string.

53.   When the tension in a given stretched string is increased by four times, by what factor does the velocity of transverse wave in the string change?

Solution:

54.   What do you mean by resonance?

Solution: A phenomenon in which an external force or a vibrating system forces another system around it to vibrate with greater amplitude at a specified frequency of operation.

55.   Why is an end correction necessary for an organ pipe?

Solution: Rayleigh find out that when an air in a organ pipe vibrates, the reflection of the sound waves takes place a little above the rim of the organ pipe. Due to this reason, the vibrating length of the air column is greater than the actual length of the organ pipe. Therefore, in order to get accurate value for frequency of vibration, the end correction is taken into account.

56.   State Doppler's effect.

Solution: The apparent change in frequency of a wave when there is relative motion between source and observer is called Doppler effects.

57.   How is it that one can recognize a friend from his voice without seeing him?

Solution: Through the quality of the sound one can recognize his friends from his voice with out seeing him. The quality of sound released from the vocal of person is different for everyone because the overtone is different for different person. The quality of sound helps to distinguish between two sounds. The overtones present in different voices are different even at same frequency. In this way one can recognize his friend from voice.

58.   Justify the proverb "An empty vessel makes much noise".

Solution: The air molecules in empty vessel vibrates with greater amplitude and greater intensity than liquid molecules filled vessel.

59.   Why are longitudinal waves called pressure waves.

Solution: The propagation of the longitudinal waves in medium involves change in pressure and volume of the air during compression and rarefaction formation. So, longitudinal waves are called as pressure waves.

60.   Define beats and beat frequency.

Solution:  The beat frequency is equal to the complete value of the alteration in the frequency of the two waves. The count of beats per second is equivalent to the difference in frequencies of two waves is called beat frequency.

61.   Differentiate between a plane wave front and a spherical wave front.

Solution:

Plane wave front

Spherical wave front

As the spherical or cylindrical wave front advances, its curvature decreases progressively. so a small portion of such wave front at a large distance from a point source or linear source will appear a plane and hence termed as plane wave front.

It is produced by the point source of light. This is because all such point which are equidistance from the point source lie on sphere.

 

62.   What is meant by wave front and wavelets?

Solution: Wave front is the locus of all adjacent points which are equidistance from the source of light and are vibrating in same phase. Every point on the wave front acts as a source of disturbance, these disturbance from the points are called wavelets.

63.   What is Huygens's principle?

Solution: Huygens’s principle is based on following assumptions:

Each point on a wave front act as a new source of disturbance. The disturbance from these points is called secondary wavelets. These wavelets spread out in all direction in medium with the velocity of light.

The new wave front is tangent plane or tangent envelope to all secondary wavelets from the secondary source.

64.   The width of fringes obtained in a double slit experiment is 4mm. If one slit is closed, what will happen to the interference fringes?

Solution: When one slits is completely closed then diffraction pattern will be observed instead of interference pattern.

65.   What happens on the interference fringes in a young's double slit experiment when

i) the screen is moved away.

ii) the source is replaced by another source of shorter wavelength.

Solution:

i) When the screen is moved away from the double-slit, the fringes become wider and farther apart.

ii) If the source is replaced by another source of shorter wavelengths, then the fringe, width decreases, and interference bands are not seen directly.

66.   What are the conditions for destructive and constructive interference of light waves.

Solution:

For constructive interference, the path difference between two interfering waves should be an integral multiple of the wavelength.

For destructive interference, the path difference between interfering waves should be half of odd number integral multiple of wavelength.

67.   Distinguish between interference and diffraction.

Solution:

Interference

Diffraction

Interference is the phenomenon of two or more waves overlapping and producing a resultant wave with a higher or lower amplitude depending on their phase relationship.

Diffraction is the bending of waves around an obstacle or through an aperture, resulting in the spreading out of the wavefront.

It is caused by the superposition of waves from two or more coherent sources.

It is caused by the interaction of waves with an obstacle or aperture.

It results in the formation of interference fringes or bands of bright and dark regions.

It results in the formation of a diffraction pattern or series of alternating bright and dark regions.

68.   What is the diffraction of light?

Solution: the phenomenon of bending of the light around the corner and spreading around the region of a geometrical shadow is called diffraction of light.

69.   Does polarizing angle depend on wavelength of light.

Solution: the angle of incident is known as polarizing angle for which the reflected ray is completely polarized. The refractive index of a transparent medium depends upon the wavelength by the Cauchy relations:

\[\mu  = A + \frac{B}{{{\lambda ^2}}} + \frac{C}{{{\lambda ^4}}}\]

Again, we know the polarizing angle θp = tan-1µ from these two relations it can be said polarizing angle depend upon the wavelength of the light.

70.   Can sound waves be polarized? Justify your answer.

Solution: No, sound wave cannot be polarized. It is because sound waves are longitudinal waves. (q.no. 44)

71.   Differentiate unpolarized and polarized light.

Solution:

Unpolarized

Polarized

The light having vibration of the electric field vector in all direction in a plane perpendicular to the direction of propagation of the light is known as unpolarized light.

The light having vibration of electric field vector along single straight line perpendicular to the direction of propagation of the light is known as polarized light.

Changing intensity of emergent light is observed when polarized light passed through a single rotating crystal.

There is no change in intensity of the emergent light, when unpolarized light passed through a single rotating crystal.

72.   What is the principle of potentiometer?

Solution:

The principal of the potentiometer is that for a wire having uniform area of cross section and uniform composition, the potential drop is directly proportional to the length of wire.

73.   Is polarization possible in longitudinal waves? why?

Solution: No, because they are the wave in which the vibration of a party called takes place in the direction of the motion of the waves. Thus, the longitudinal wave is already polarized in the direction of the motion of the waves. Therefore, longitudinal waves are not polarized.

74.   State the principle of potentiometer and write down its one application.

Solution: The principal of the potentiometer is that for a wire having uniform area of cross section and uniform composition, the potential drop is directly proportional to the length of wire.

Application: Potentiometer is used to measure the potential difference between two points.

75.   Why do we prefer a potentiometer with longer wire.

Solution: With the increase in length potentiometer becomes more sensitive. The fall of potential is proportional to the length of the wire only if the wire is of a uniform thickness.

76.   Why do we prefer a potentiometer to measure the emf of a cell rather than a voltmeter?

Solution: The potentiometer measures the potential difference with less error as compared to voltmeter. If the potentiometer is used, it doesn’t draw any current from the cell and hence gives the actual value of the EMF of the cell whereas when a voltmeter is used to measure the EMF of the shell, it draws some current from the cell and hence it gives a reading less than the actual value of the EMF of the cell.

77.   A voltmeter has high resistance, explain, why?

Solution: A voltmeter measures the voltage difference between the two different points, but it should not change the amount of current going through the element between these two points. So, it should have very high resistance so mat it doesn't draw current through it

78.   State and explain Kirchhoff's law of electric circuits.

Solution: Two Kirchhoff’s laws are:

(i) First Law: It states that the algebraic sum of the current at a junction of an electric circuit is zero.

(ii) Second Law: It states that in a closed loop the electric circuit, algebraic sum of the emfs is equal to the algebraic sum of the potential difference in the various part of the loop.

79.   The resistance of an ammeter must essentially be very small, why?

Solution: An ammeter is a low resistance galvanometer. When it is connected in series in a circuit, the resistance of the circuit does not increase appreciably and consequently but if the resistance of the ammeter is high, the net resistance of the circuit becomes very high and the current in the circuit reduces and hence ammeter is not able to measure accurate current. Due to this reason, the resistance of an ammeter must be essentially be small.

80.   What is thermoelectric effect?

Solution: the phenomenon in which electrical energy is produced from the thermal energy is known as thermoelectric effect.

81.   Does the thermoelectric effect obey the law of conservation of energy? Justify.

Solution: Yes, thermoelectric effect obeys the law of conservation of energy. In the Seebeck effect we see heat energy is converted into electrical energy whereas in the Thomson effect electrical energy is converted into heat energy and in the Peltier effect one junction absorbed the heat while another junction evolves or releases the heat. Hence, we can say that thermoelectric effect obeys the law of conservation of energy

82.   What is the temperature of inversion? On what factors does it depend.

Solution: the temperature of the hot junction at which thermal EMF is zero and changes its polarity beyond it is known as temperature of inversion. Temperature of inversion depends on the temperature of the cold junction and also the type of the thermocouple used.

83.   What is neutral temperature? On what factors does it depend?

Solution: the temperature of the hot junction at which Thermo-EMF becomes maximum and neutral temperature is constant for a given thermocouple.

84.   What is thermopile?

Solution: Thermopile is the device which is used to detect or measure the amount of heat radiation.

85.   What is the better representation of the Lorentz force?

Solution: The Lorentz magnetic force is given by the following relation:
$\overrightarrow F $=q ($\overrightarrow V $ × $\overrightarrow B $)
Here q is the magnitude of the moving charge.
The direction of the magnetic force is perpendicular to the plane containing the velocity vector $\overrightarrow V $ and the magnetic field vector $\overrightarrow B $.

86.   State Fleming Left hand rule.

Solution: Fleming's left-hand rule can be stated as stretching the forefinger, middle finger, and thumb of the right hand such that they are mutually perpendicular to each other. 

87.   Why are the pole pieces of magnets cut into cylindrical form in a galvanometer.

Solution: The major purpose of making pole pieces of magnets into cylindrical form is to ensure that the magnetic field produced in the moving coil galvanometer is strong and also radial

88.   What do you mean by hall effect? Why is it more effective in semiconductors.

Solution:  When a magnetic field is applied to a current carrying conductor, a voltage is developed across the specimen in the direction perpendicular to both the current and the magnetic field. This effect is known as halls effect.

The density of charge carriers is less in semiconductors. Since Hall voltage is inversely proportional to the number of charge carriers, it is more effective in the case of semiconductors.

89.   Define one ampere in terms of the force between current carrying conductors.

Solution: One ampere is that current following in each of two infinitely long parallel conductors 1 meter apart such that the force permitter length on each conductor is 2×10-7N.

90.   State ampere's theorem.

Solution: Ampere's Circuital law states that, “the line integral of the magnetic field around any closed path in free space is equal to µ0 times the total current enclosed by the path.

\[\oint {\mathop B\limits^ \to  } .d\mathop l\limits^ \to   = {\mu _0}I\]

91.   What is Curie temperature?

Solution: The temperature at which a ferromagnetic substance loses its ferromagnetism and attains Para magnetism is known as Curie Temperature.

92.   Distinguish between diamagnetic and paramagnetic substances in terms of their relative permeability and susceptibility.

Solution:

Diamagnetic

Paramagnetic substances

Its relative permeability is less than 1, usually 0.9999.

Its relative permeability is more than 1.

Magnetic Susceptibility is small and negative.

Magnetic Susceptibility is small and positive.

 

93.   What is the significance of the area of a hysteresis loop.

Solution: The area of the hysteresis loop measures the amount of energy that is basically lost per cycle of magnetization. The area of the hysteresis curve for steel is more than that of soft iron. The magnetic materials whose B-H curve is more are useful for making permanent magnets.

94.   Permanent magnets are made of steel, why?

Solution: Steel is used for making permanent magnet because its magnetism does not destroy easily by external magnetic field. 

95.   What is retentivity and coercivity of a ferromagnetic material?

Solution: The tendency of the magnetic material to maintain magnetism, even in the absence of a magnetizing field, is known as retentivity.

Coercivity in a ferromagnetic material is the intensity of the applied magnetic field (H field) required to demagnetize that material, after the magnetization of the sample has been driven to saturation by a strong field. 

Note: Ferromagnetic materials have high permeability and low retentivity.

96.   Above Curie temperature a ferromagnetic material became para magnetic, why?

Solution: A ferromagnetic material follows Curie law and the alignment of the atomic magnetic moments in it is disturbed with increase of the temperature. Above the Curie temperature this alignment is completely random due to thermal motion of the atom and the material becomes paramagnetic.

97.   Define Eddy Current.

Solution: the induced circulating current produced in a metal itself due to change in magnetic flux linked with the metal is known as Eddy current.

98.   Define magnetic flux.

Solution: The number of magnetic lines of force crossing through the surface is known as magnetic flux.

99.   State Faraday's law of electromagnetic induction.

Solution: Friday’s law of electromagnetic induction states that

When the magnetic flux linking a conductor are equal changes an EMF is induced in it.

The induced EMF lasts as long as the change in magnetic flux continues.

The magnitude of induced EMF in a conductor or a coil is directly proportional to the rate of change of flux linkage.

100.           A bar magnet falls through copper ring. Will its acceleration be equal to g? Justify.

Solution: When the bar magnet approaches the ring, the magnetic flux linked with the ring increases so an EMF is induced in the ring. If the ring forms a complete conducting loop, the current is also induced. According to the Lenz law, the current opposes the approach of a magnet and hence the acceleration of a magnet becomes less than the value of acceleration due to gravity.

101.           Bird sitting on a high-tension line wire flying up when current is switched on, why?

Solution: As soon as the current is switched on in a high voltage wire the bird sitting on it flies away. The reason being due to induced current flowing throughout the body of the bird, which flows in the opposite direction through the wings of the birds. Hence, the wings experience a force of mutual repulsion.

102.           Lenz law closes the principle of conservation of energy. Explain.

Solution: Yes, Lenz's law follows the principle of conservation of energy. The law of conservation of energy states that energy can neither be created nor be destroyed, but it can be changed from one form to another. Lenz's law states that the direction of current is such that it opposes the change in the magnetic flux. So, an extra effort is required to do work against the opposing force. This extra effort is converted into electrical energy, which can be viewed as the law of conservation of energy.

103.           Does Lenz Law violate principle of conservation of energy, explain.

Solution: No, Lenz law do not violate the law of conservation of energy. In fact, Lenz's law follows the principle of conservation of energy. The law of conservation of energy states that energy can neither be created nor be destroyed, but it can be changed from one form to another. Lenz's law states that the direction of current is such that it opposes the change in the magnetic flux. So, an extra effort is required to do work against the opposing force. This extra effort is converted into electrical energy, which can be viewed as the law of conservation of energy.

104.           220-volt AC is more danger than 220-volt DC, why?

Solution: 220-volt a.c. means the effective or virtual value of a.c. is 220 volts, i.e., Ev=220 volt. But 220-volt d.c. has the same peak value (i.e., 220 volts only). Moreover, the shock of a.c. is attractive and that of d.c. is repulsive. Hence 220-volt a.c. is more dangerous than 220 volts d.c.

105.           Why is choke coil preferred over resistance in AC?

Solution: Choke coil is preferred over resistor in ac circuit because the phase difference between current and voltage is Φ=90˚ due to which power loss becomes zero.

i.e., P = EvIvCosΦ = EvIvCos90=0

Since choke coil acts as inductor and has large value of self-inductance making power dissipation zero(nearly).

106.           Define rms values of alternating current.

Solution: RMS value of AC current is that steady current which when passed through a resistance for a given time produces the same amount of heat as the alternating current does in the same resistance in the same time.

107.           Define quality factor of resonance in a series LCR circuit.

Solution: the quality factor of the resonance in the series LCR circuit is defined as the sharpness of a tuning of a resonance. The quality factor of series resonant circuit is defined as the ratio of the voltage developed across the inductance or capacitance at resonance to the impressed voltage, which is the voltage across the resistance.

108.           Explain why two parallel wires carrying current in the opposite direction repel each other?

Solution: When two free parallel wires carry currents in opposite direction, the two magnetic fields created by it will also be opposite to each other, so they will repel.

109.           Can a uniform magnetic field be used to speed up a charged particle? Explain.

Solution: The magnetic field accelerates the charged particle by changing the direction of velocity. The magnetic field doesn't change the speed of the charged particle.

The reason is that the magnetic field doesn’t affect the speed is because the magnetic field applies a force perpendicular to the velocity. Hence, the force can’t do work on the particle. As a result, the particle can’t change its kinetic energy. So it cannot change the speed.

110.           Can a holes be created in a metal? Justify your answer.

Solution: In a conductor, the forbidden band is missing and the conduction band and valance band are overlapped. Therefore, no holes can be created in a metal.

111.           At high frequencies, a capacitor becomes a short circuit, and an inductor becomes an open circuit explain.

Solution: At high frequencies the capacitive reactance of a capacitor becomes very small, nearly approaches to zero and capacitor behaves like a wire. So, it acts as a short circuit.

While in the inductor there is large opposition to the flow of a.c. at very high frequency and the current cannot flow through the inductor.

112.           What is meant by watt-less current.

Solution: The current through pure inductor or pure capacitor which consumes no power for its maintenance in the circuit is known as ideal current or what less current.

113.           What path does the electron follow in electric field when the electron is projected normally in the field?

Solution: when an electron is projected perpendicular to a uniform electric field it experiences an electric force in opposite direction of the field. Thus, when electron moves perpendicular to the electric field its trajectory is parabolic in nature.

114.           What property of the cathode rays indicates that they consist of electrons?

Solution: cathode are the stream of negatively charged particle which gets deflected by the electric and magnetic field points means it consists steam of electrons.

115.           Cathode rays cannot be regarded as electromagnetic waves, why?

Solution: cathode ray are the stream of negatively charged particle which gets deflected by the electric and the magnetic field but electromagnetic waves are chargeless particle which are not deflected by the electric and the magnetic field hinge cathode ray are not the electromagnetic waves.

116.           Lightning takes place at a higher altitude than at a lower altitude. Why?

Solution: the electric field set up in the atmosphere causes lightning discharge if the pressure is low. Therefore, due to the low pressure at high altitude the electric discharge take place and hence we found lightning mostly takes place at the higher altitude.

117.           Can photoelectric effect be explained on the basis of wave theory of light? justify your answer.

Solution: Wave nature of radiation cannot explain the photo electric effects because of :
(i) The immediate ejection of photo electrons 
(ii) The presence of threshold frequency for a metal surface.
(iii) The fact that kinetic energy of the emitted electrons is independent of the intensity of light and depends upon its frequency. 
Thus, the photoelectric effect cannot be explained on the basis of wave nature of light 

118.           What happens to the kinetic energy of photo electrons when intensity of light is doubled?

Solution: If the energy of a photon is greater than the work function of the material, the electron may escape from the metallic surface. Greater the intensity of the light at a particular frequency means greater the number of the photon’s incident per second and hence the greater the number of photoelectrons limited for second. Hence, there will be no effect on the kinetic energy of the photoelectron if the intensity of the light is doubled

119.           What is meant by stopping potential?

Solution: the minimum retarding potential at which photoelectric current becomes zero is called stopping potential. It is denoted as V0.

120.           What is the threshold frequency?

Solution: The minimum frequency of incident radiation which is just sufficient to eject an electron from the surface of the metal is known as threshold frequency.

121.           What are logic gates? Give a two-truth table for a two input NOR gate.

Solution: A logic gate is a digital circuit which has one or more input and one output. It follows the short and logical relationship between the input and the output voltage.

Symbol

Truth Table

2-input NOR Gate

B

A

Q

0

0

1

0

1

0

1

0

0

1

1

0

Boolean Expression Q = A+B

Read as A OR B gives NOT Q

122.           What is truth table? Write down the truth table for a two input NAND gate.

Solution: The table giving the different possible input and the corresponding output of a logic gate is called its truth table.

Symbol

Truth Table

2-input NAND Gate

B

A

Q

0

0

1

0

1

1

1

0

1

1

1

0

Boolean Expression Q = A.B

Read as A AND B gives NOT Q

123.           Why is x-ray radiation process called inverse photoelectric effect?

Solution: when the fast-moving electron strike on a high atomic weight substance X-ray is produced and when the radiation of a sufficient frequency incident on a metal surface electron are emitted that is photoelectric effect take places. It means fast moving electron produces X-ray and X-ray i.e., radiation produces photoelectron. Hence X-ray is the reverse phenomenon of photoelectric effect.

124.           A proton and an electron have the same de-Broglie wavelength. which one has greater kinetic energy explain

Solution: de-Broglie wavelength in terms of kinetic energy is represented as $E = \frac{{{h^2}}}{{2m{\lambda ^2}}}$.

Since, λ is same for both particle, E α $\frac{1}{m}$. As the mass of an electron is less than that of the photon show energy of the electron is greater than photon.

125.           The wave nature of particle is not observable in daily life, why?

Solution: From de Broglie equation, λ = $\frac{h}{mv}$, it is clear that the lighter the particle the greater the de Broglie wavelength and the faster the particle moves, The smaller is its de Broglie wavelength. The wave nature of the matter can be apparent if the wavelength of the wave associated with the moving matter is of the size of the matter. In the daily life, particles are heavier and they move with comparatively low speed hence this is not observable in our daily experiences.

126.           What do you mean by matter waves?

Solution: according to the de Broglie the wave is associated with each moving particle which is known as matter waves. When the particles are in motion matter wave are produced. The velocity of matter waves depends upon the velocity of the particle in the motion.

127.           What do you mean by uncertainty principle?

Solution: Uncertainty principle states that it is impossible to determine precisely and simultaneously the value of both position and the momentum in atomic system.

128.           Point out the importance of de-Broglie wave.

Solution: The importance of the De Broglie wave are follows:

A matter particle will have a wavelength associated with it and only if it is in motion. The greater the momentum the shorter will be the wavelength.

Two different velocities are associated with a material particle in motion one refer to the mechanical motion of the particle and other refer to the propagation of the associated wave.

It justifies Bohr’s quantum condition of the hydrogen atom i.e., the stable state of the electron in the atom are governed by the integral rule.

Bohr’s postulate of atomic model is:

129.           State Bohr’s postulate of atomic model.

Solution: Bohr’s Postulates of atomic models are:

Electron revolves around the nucleus on certain permitted orbits of definite radii.

Electron can revolve around the nucleus in only those permissible orbits whose angular momentum is integral multiple of h/2π.

130.           If a radioactive nucleus Has a half-life of one year, will it be completely decayed at the end of two years explain.

Solution: No, it doesn’t mean that a particle with one year half life gets vanished after two years. On increasing time, the particle starts to decay, and along with decaying the half-life of undecayed materials also increases exponentially.

131.           How does a daughter nucleus differ from its parent when it emits i) an alpha particles ii) gamma rays.

Solution: In alpha decay, the mass of daughter nucleus mass gets decreased by four units and atomic number decreases by two units.

For gamma decay, there is no change in mass and charge or there is no difference in parent and daughter nucleus with respect to mass and charge.

132.           Nucleuses contain no electrons, yet it ejects them explain.

Solution: When a nucleus disintegrates, it emits β rays, known as β decay. Β-particles are nothing but they are electron.

133.           There are no electrons inside the nucleus, but they are emitted from an unstable nucleus, why?

Solution: A nucleus contains no electron but it can emit The nucleus ejects anti neutrinos which are kind of electrons. This process is mediated by the weak interaction. The neutron turns into a proton through the emission of a virtual W- boson. ... The virtual W- boson then decays into an electron and an anti-neutrino.

134.           How do Beta particles differ from electrons?

Solution: Electron and beta particles are essentially same, but the electron ejected by nucleus is said to be a beta particle.

135.           Higgs boson gives the complete picture of the universe. Comment on it.

Solution:

136.           What are the uses of radioisotopes?

Solution: Radioisotopes can be used in various field in today’s world. It can be used to make medicine for curing various disease, it can be used as the source of energy and also be used for carbon dating.

137.           What is the meaning of ionization energy of hydrogen atom is 13.6 eV?

Solution: The ionization potential of hydrogen is 13.6 electron volts (eV), which means that an electron in a hydrogen atom requires at least 13.6 eV of energy to be removed from the atom.

138.           What is the difference between an electron and a β -particle.

Solution:

Electron

β -particle

Electrons are present in the atom revolving around the nucleus.

β -particle are originated from atomic nuclei during beta decay.

It always has -1 charge.

It may have -1 or +1 charge.

139.           What are seismic waves?

Solution: The energy that travels under the surface layer of the earth and causes an earthquake is called seismic waves. They are also called as elastic waves because they cause deformation of the materials when they propagate.

140.           Differentiate between P-wave and S-wave.

Solution:

P-wave

S-wave

The first wave to arrive at seismic station is P-wave.

The second wave to arrive at seismic station is P-wave.

They are compression wave.

They are shear waves.

They can moves through solids and liquids.

They can only moves through solids.

 

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