Physics Archives

91. Oil and water do NOT mix because of the property of

Oil and water do NOT mix because of the property of

[amp_mcq option1=”bulk modulus” option2=”shear strain” option3=”volume effect” option4=”surface tension” correct=”option4″]

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UPSC CAPF – 2020

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Oil and water do not mix due to the property of surface tension (more specifically, interfacial tension between the two liquids).

Water is a polar molecule with strong hydrogen bonds, giving it high surface tension. Oil is nonpolar with weaker intermolecular forces. They have different cohesive forces and weak adhesive forces between them, causing them to minimize contact area. This tendency to minimize interfacial area is a manifestation of interfacial tension, which is related to the surface tensions of the individual liquids and their interactions.

Bulk modulus is a measure of resistance to uniform compression. Shear strain is a measure of deformation under shear stress. Volume effect is not a standard term in this context.

92. An electron and a photon have same de Broglie wavelength. It implies t

An electron and a photon have same de Broglie wavelength. It implies that they have the same

[amp_mcq option1=”linear momentum” option2=”energy” option3=”speed” option4=”angular momentum” correct=”option1″]

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If an electron and a photon have the same de Broglie wavelength (λ), it implies that they have the same linear momentum.

The de Broglie wavelength is given by the formula λ = h/p, where h is Planck’s constant and p is the linear momentum. If λ is the same and h is constant, then p must be the same.

While their momentum is the same, their energy and speed are generally different. For a photon, energy E = pc and speed is c. For an electron with mass m and speed v, momentum p = mv (non-relativistic) and kinetic energy KE = p^2/(2m). Same momentum p does not mean same energy or speed due to the difference in their mass and the relativistic nature of the photon.

93. Which one of the following statements with regard to Newton’s third la

Which one of the following statements with regard to Newton’s third law of motion is NOT correct?

[amp_mcq option1=”Force never occurs singly in nature” option2=”When the earth pulls a stone downwards due to gravity, the stone exerts a force on the earth” option3=”There is ‘a cause-effect relation implied in the third law” option4=”There is no cause-effect relation implied in the third law” correct=”option3″]

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UPSC CAPF – 2020

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The statement “There is ‘a cause-effect relation implied in the third law” is NOT correct with regard to Newton’s third law of motion.

Newton’s third law states that for every action, there is an equal and opposite reaction. These action and reaction forces are a pair and occur simultaneously as part of a single interaction between two bodies. The law describes the nature of forces as interactions rather than implying one force causes the other in a sequential manner. There is no temporal delay or causal dependency between the action and reaction forces; they exist concurrently. Statement D, “There is no cause-effect relation implied in the third law”, is consistent with this understanding.

Statements A and B correctly describe aspects of the third law. A) Force never occurs singly in nature implies forces always come in pairs (action-reaction pairs). B) When the earth pulls a stone downwards due to gravity, the stone exerts a force on the earth is the classic example of an action-reaction pair according to the third law.

94. The radar used by police to check over-speeding vehicles works on the

The radar used by police to check over-speeding vehicles works on the principle of

[amp_mcq option1=”Raman effect” option2=”Induction effect” option3=”Doppler effect” option4=”Coulomb effect” correct=”option3″]

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The radar used by police to check over-speeding vehicles works on the principle of the Doppler effect.

The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. Police radar guns emit radio waves towards a moving vehicle. When these waves reflect off the vehicle, their frequency is shifted. If the vehicle is moving towards the radar, the reflected waves have a higher frequency; if moving away, they have a lower frequency. The radar unit measures this frequency shift and calculates the vehicle’s speed based on the magnitude of the shift.

The Raman effect involves inelastic scattering of photons. The Induction effect relates to the production of an electromotive force across an electrical conductor in a changing magnetic field. The Coulomb effect describes the electrostatic interaction between charged particles. None of these principles are the basis for how police speed radar operates.

95. Which one of the following statements regarding motion is correct?

Which one of the following statements regarding motion is correct?

[amp_mcq option1=”All the periodic motions are necessarily simple harmonic” option2=”All the simple harmonic motions are necessarily periodic motions” option3=”There is no co-relation between the simple harmonic motions and the periodicity of motion” option4=”The relation between the simple harmonic motion and periodic motion depends upon the mass of object undergoing the motion” correct=”option2″]

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All the simple harmonic motions are necessarily periodic motions.

Periodic motion is defined as any motion that repeats itself in a regular interval of time. Simple Harmonic Motion (SHM) is a special type of periodic motion where the restoring force is directly proportional to the displacement from the equilibrium position and acts towards the equilibrium. Since SHM repeats itself at regular intervals (its period), it is always a periodic motion. However, not all periodic motions are SHM (e.g., a simple pendulum with large amplitude is periodic but not SHM; a uniform circular motion is periodic but not oscillatory/SHM).

SHM is characterized by a sinusoidal variation of displacement, velocity, and acceleration with time. Examples include the oscillation of a mass on a spring (Hooke’s Law) or the small-angle oscillations of a simple pendulum.

96. Which one of the following statements regarding Fleming’s Rule is co

Which one of the following statements regarding Fleming’s Rule is correct?

[amp_mcq option1=”Fleming’s left hand rule gives direction of force on a current-carrying conductor in a magnetic field” option2=”Fleming’s right hand rule gives direction of force on a current-carrying conductor in a magnetic field” option3=”Both the left-hand and right-hand rule can be used for finding direction of force on a current-carrying conductor in a magnetic field” option4=”Fleming’s rules has nothing to do with magnetic field” correct=”option1″]

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Fleming’s left hand rule gives direction of force on a current-carrying conductor in a magnetic field.

Fleming’s Left-Hand Rule is used to determine the direction of the force (thrust or motion) on a current-carrying conductor when placed in an external magnetic field. It relates the directions of the magnetic field, the electric current, and the resulting force. The thumb represents the direction of the force, the forefinger represents the direction of the magnetic field, and the middle finger represents the direction of the current.

Fleming’s Right-Hand Rule is used to determine the direction of induced current when a conductor moves in a magnetic field. It is used in generators. The Left-Hand Rule is primarily used in motors.

97. Two magnetic field lines produced by the same source

Two magnetic field lines produced by the same source

[amp_mcq option1=”never intersect” option2=”can originate from same point” option3=”can terminate at same point” option4=”can intersect depending on situation” correct=”option1″]

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UPSC CAPF – 2020

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Two magnetic field lines produced by the same source never intersect.

Magnetic field lines represent the direction of the magnetic field at different points. If two magnetic field lines were to intersect at a point, it would mean that the magnetic field at that point has two different directions simultaneously, which is impossible.

Magnetic field lines are continuous curves forming closed loops (outside the magnet from North to South, inside from South to North). They are denser where the magnetic field is stronger. They originate from the North pole and terminate at the South pole outside the magnet.

98. Which one of the following statements for an object falling freely und

Which one of the following statements for an object falling freely under the influence of gravity is correct?

[amp_mcq option1=”Zero acceleration always implies zero velocity” option2=”Zero acceleration has no relation with the velocity of the object” option3=”Zero velocity at any instant necessarily means zero acceleration at that instant” option4=”Acceleration is constant all throughout the free fall” correct=”option4″]

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Acceleration is constant all throughout the free fall.

An object falling freely under the influence of gravity experiences a constant acceleration equal to the acceleration due to gravity (g), assuming air resistance is negligible. This acceleration acts downwards.

Zero acceleration implies constant velocity, not necessarily zero velocity (A is incorrect). Zero acceleration means there is no net force acting on the object; velocity could be constant and non-zero (B is incorrect). An object thrown upwards momentarily has zero velocity at its peak, but the acceleration due to gravity is still acting on it at that instant (C is incorrect).

99. Which one of the following phenomena CANNOT be exhibited by sound wave

Which one of the following phenomena CANNOT be exhibited by sound waves ?

[amp_mcq option1=”Reflection” option2=”Refraction” option3=”Interference” option4=”Polarisation” correct=”option4″]

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UPSC CAPF – 2019

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Sound waves are longitudinal waves, meaning the oscillations of the medium particles are parallel to the direction of wave propagation. Polarisation is a phenomenon exhibited only by transverse waves, where oscillations occur perpendicular to the direction of propagation, allowing them to be restricted to a specific plane. Since sound waves in air are longitudinal, they cannot be polarised.

The key principle here is understanding the difference between longitudinal and transverse waves and which wave phenomena apply to each. Polarisation is a characteristic property of transverse waves.

Sound waves can exhibit reflection (e.g., echoes), refraction (bending of sound waves when passing from one medium to another or through variations in temperature/density within a medium), and interference (superposition of waves resulting in constructive or destructive effects). These phenomena are common to both longitudinal and transverse waves under appropriate conditions. Sound waves can be transverse in solid materials, but typically when discussing ‘sound waves’ in the context of common phenomena, it refers to sound in air or liquids, which is longitudinal.

100. When a beam of white light is allowed to fall on a prism, the maximum

When a beam of white light is allowed to fall on a prism, the maximum deviation of the emerging light occurs for

[amp_mcq option1=”red light” option2=”violet light” option3=”yellow light” option4=”blue light” correct=”option2″]

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UPSC CAPF – 2019

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The correct answer is B) violet light.

When a beam of white light passes through a prism, it undergoes dispersion, splitting into its constituent colours (spectrum: Violet, Indigo, Blue, Green, Yellow, Orange, Red – VIBGYOR). This happens because the refractive index of the prism material is different for different wavelengths (colours) of light.

* Refraction is the bending of light as it passes from one medium to another. The amount of bending depends on the refractive index of the medium and the angle of incidence.
* For visible light passing through a prism, the refractive index of the prism material is higher for shorter wavelengths and lower for longer wavelengths.
* Violet light has the shortest wavelength among the colours in the visible spectrum.
* Red light has the longest wavelength among the colours in the visible spectrum.
* Since the refractive index is highest for violet light, it is bent the most (deviated the maximum) when passing through the prism.
* Conversely, since the refractive index is lowest for red light, it is bent the least (deviated the minimum).

The order of deviation from maximum to minimum is typically Violet > Indigo > Blue > Green > Yellow > Orange > Red.

This phenomenon of dispersion is described by Cauchy’s equation, which relates the refractive index of a material to the wavelength of light. This effect is responsible for the formation of rainbows, where raindrops act like tiny prisms, dispersing sunlight.