UPSC Geoscientist Archives

21. Which two competing effects determine the size of a star?

Which two competing effects determine the size of a star?

Nuclear fusion and electrostatic effects

Nuclear fusion and magnetic effects

Nuclear fusion and gravitational effects

Gravitational and electromagnetic effects

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The size of a stable star is determined by the balance between the inward force of gravity and the outward pressure generated by nuclear fusion in its core.

Gravity constantly pulls the star’s mass inward, tending to collapse it. Nuclear fusion reactions in the core, primarily converting hydrogen to helium, release enormous amounts of energy in the form of photons (radiation) and particles. This energy creates a strong outward pressure (radiation pressure and thermal pressure) that counteracts gravity. A star remains stable, maintaining a relatively constant size, when these two opposing forces are in equilibrium, known as hydrostatic equilibrium.

While other effects like magnetic fields can influence stellar activity and structure in specific regions or phases, the primary factors determining the overall size and stability of a star throughout its main sequence life are the balance between self-gravity and the pressure from nuclear fusion.

22. How do the scalar quantities differ from the vector quantities?

How do the scalar quantities differ from the vector quantities?

The only difference between the two quantities is that a scalar quantity includes magnitude only, whereas a vector quantity includes both magnitude as well as direction

Both the quantities include direction and magnitude. Scalar quantities can be combined using the rules of ordinary algebra, whereas vector quantities can be combined using the rules of vector algebra

A scalar quantity includes magnitude only, whereas a vector quantity includes both magnitude and direction, and both the quantities can be combined using the rules of ordinary algebra

A scalar quantity includes magnitude only and scalar quantities can be combined using the rules of ordinary algebra, whereas a vector quantity includes both magnitude and direction, and vector quantities can be combined using the rules of vector algebra

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Option D correctly describes the difference between scalar and vector quantities, including their definitions regarding magnitude and direction, as well as the rules for combining them.

Scalar quantities are physical quantities that have only magnitude (size). Examples include mass, temperature, time, and speed. They can be added, subtracted, multiplied, and divided using the rules of ordinary algebra. Vector quantities are physical quantities that have both magnitude and direction. Examples include displacement, velocity, acceleration, and force. They must be combined using the rules of vector algebra, which take direction into account (e.g., parallelogram law of addition).

Understanding the distinction between scalar and vector quantities is fundamental in physics as it determines how these quantities behave and are manipulated mathematically. The choice of algebra rules (ordinary vs. vector) is a crucial aspect of this distinction.

23. The centre of gravity of a system of rigid bodies coincides with their

The centre of gravity of a system of rigid bodies coincides with their centre of mass if and only if

their centre of mass is at their geometrical centre

the acceleration due to gravity is same throughout the system of bodies

the rigid bodies have same uniform mass densities

the rigid bodies are very large

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The centre of mass (CM) of a system is determined solely by the distribution of mass. The centre of gravity (CG) is the point where the net gravitational force acts on the system. The CG and CM coincide if and only if the gravitational acceleration (g) is uniform throughout the system. If g is constant, the weight of each part of the system is directly proportional to its mass (wᵢ = mᵢg), and the formula for the CG position becomes identical to that for the CM position. If g varies significantly across the system (e.g., a very large object or a system in a non-uniform gravitational field), the CG and CM will not coincide.

– CG and CM coincide when gravitational acceleration (g) is constant across the system.
– CM depends on mass distribution.
– CG depends on both mass distribution and the distribution of the gravitational field.

For most objects on Earth, the variation in g over the object’s size is negligible, so CG and CM are effectively at the same location. However, in theoretical physics or for extremely large systems (like a mountain), the slight variation in gravity could cause a separation between the two points.

24. Which one of the following statements is true for nuclear fission

Which one of the following statements is true for nuclear fission processes?

Light initial nuclides have lesser number of neutrons than protons.

Heavy initial nuclides have lesser number of neutrons than protons.

All initial nuclides have larger number of neutrons than protons.

All initial nuclides have equal number of protons and neutrons.

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Nuclear fission involves the splitting of heavy atomic nuclei, such as isotopes of Uranium or Plutonium. For stable nuclei, as the atomic number (number of protons, Z) increases, the ratio of neutrons (N) to protons increases beyond 1 to compensate for the increasing electrostatic repulsion between protons. Heavy nuclei that undergo fission are located well above the line of stability on an N-Z chart and are typically neutron-rich. For example, Uranium-235 has 92 protons and 143 neutrons (N/Z ≈ 1.55), and Uranium-238 has 92 protons and 146 neutrons (N/Z ≈ 1.59). Thus, the heavy initial nuclides involved in fission processes have a larger number of neutrons than protons.

– Fission involves heavy nuclei.
– Heavy stable and fissionable nuclei have N > Z.
– The neutron-to-proton ratio increases with increasing atomic number for stable nuclei.

The fission process itself typically releases neutrons. This is because the fission fragments (the lighter nuclei produced) are more stable with a lower N/Z ratio than the initial heavy nucleus, so excess neutrons are emitted during the process, which can sustain a chain reaction.

25. Which one of the following statements is not correct for a freely fall

Which one of the following statements is not correct for a freely falling object?

It accelerates.

Its momentum keeps on changing.

Its motion is affected only by the gravity.

Its motion is affected both by the gravity as well as by the air resistance.

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In physics, a “freely falling object” is ideally defined as an object that is accelerating due to gravity only, with no other forces acting on it (specifically, negligible air resistance).
– A) It accelerates: Correct. It accelerates downwards due to gravity (acceleration due to gravity, g).
– B) Its momentum keeps on changing: Correct. As it accelerates, its velocity changes, and momentum (mass x velocity) therefore changes.
– C) Its motion is affected only by the gravity: Correct. This is the definition of free fall in an ideal scenario (e.g., vacuum).
– D) Its motion is affected both by the gravity as well as by the air resistance: This statement is not correct for a *freely falling object* as defined in physics. Free fall explicitly excludes significant air resistance. While real objects falling in an atmosphere experience both, the term “freely falling object” usually implies the ideal case where air resistance is negligible.

– Free fall is motion under the sole influence of gravity.
– Air resistance is typically ignored in the definition of free fall.
– A freely falling object accelerates and its momentum changes.

In practical scenarios, objects falling through air do experience air resistance, which opposes the motion and increases with speed. Eventually, a terminal velocity is reached when air resistance equals gravity. However, the term “freely falling” in the context of fundamental physics principles often refers to the ideal case without air resistance.

26. Who among the following is known to be the first one to consider the r

Who among the following is known to be the first one to consider the role of inertia in motion?

Galileo Galilei

Isaac Newton

Albert Einstein

Nicolaus Copernicus

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Galileo Galilei (1564-1642) is widely credited with being the first to formulate a clear concept of inertia. Through experiments and thought experiments, particularly concerning motion on inclined planes, he concluded that an object in motion would continue in motion with constant velocity in the absence of forces. This contradicted the prevailing Aristotelian view that force was required to maintain motion. While Isaac Newton later formalized this as his First Law of Motion (the law of inertia), Galileo’s work laid the essential groundwork and is considered the first significant step in understanding inertia.

– Inertia is the property of matter resisting changes in motion.
– Galileo’s work challenged Aristotelian physics regarding motion.
– Galileo’s experiments and thought experiments led him to the concept of inertia.
– Newton later formalized inertia as his First Law of Motion.

Before Galileo, the common belief based on Aristotle was that objects required a continuous force to stay in motion. Galileo’s understanding shifted this paradigm, proposing that motion persists unless acted upon by external forces like friction or gravity.

27. Which one of the following is the conservation law from which the Bern

Which one of the following is the conservation law from which the Bernoulli’s equation for fluid flow is derived?

Conservation of momentum

Conservation of volume

Conservation of mass

Conservation of energy

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Bernoulli’s equation is derived from the principle of conservation of energy applied to the flow of an inviscid (frictionless), incompressible fluid along a streamline. It states that the total mechanical energy of the fluid, which includes pressure energy, kinetic energy, and potential energy due to elevation, remains constant along a streamline in steady flow under the influence of gravity. The equation is essentially an expression of the work-energy theorem for a fluid element.

– Bernoulli’s equation relates pressure, velocity, and elevation of a fluid.
– It is a form of the conservation of energy applied to fluid flow.
– Assumptions include inviscid, incompressible, and steady flow along a streamline.

The continuity equation, which represents conservation of mass, is often used alongside Bernoulli’s equation in fluid dynamics problems, relating the velocity of the fluid to the cross-sectional area of the flow. Conservation of momentum is related to forces and acceleration, as described by Newton’s laws, and is fundamental to the more general Navier-Stokes equations, which can reduce to Bernoulli’s equation under specific conditions.

28. The magnetization in ferromagnetic materials

The magnetization in ferromagnetic materials

can never vanish

is always opposite to the direction of applied magnetic field

is always in the direction perpendicular to the applied magnetic field

may not vanish even when the applied magnetic field is reduced to zero

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Ferromagnetic materials exhibit hysteresis. When a magnetic field is applied, they become strongly magnetized. When the applied magnetic field is removed, a significant amount of magnetization often remains, even if the external field is reduced to zero. This retained magnetization is called remanent magnetization or remanence. This property allows ferromagnetic materials to be used to create permanent magnets.

– Ferromagnetic materials show hysteresis in their magnetization curve.
– Remanence is the residual magnetization left after the applied field is removed.
– This property is essential for creating permanent magnets.

Magnetization in ferromagnetic materials is caused by the alignment of magnetic domains. Applied fields cause domains oriented with the field to grow and those opposed to shrink. Even after the external field is removed, some of this alignment persists due to the strong internal interactions between domains.

29. Which of the following can be used to determine the polarity of an unm

Which of the following can be used to determine the polarity of an unmarked bar magnet?

An electroscope

Iron filings

A compass

Another unmarked bar magnet

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A compass is essentially a small magnet (needle) that is free to rotate and align itself with the Earth’s magnetic field. The red or North-seeking pole of the compass needle points towards the Earth’s magnetic North pole. If you bring an unmarked bar magnet near the compass, the compass needle will be either attracted or repelled by the poles of the bar magnet. By observing which pole of the compass (whose polarity is known) is attracted or repelled by each end of the unmarked magnet, you can determine the polarity of the unmarked magnet. Like poles repel, and opposite poles attract.

– A compass needle has known magnetic poles.
– Magnetic poles interact (like repel, opposite attract).
– Using a compass, one can identify the poles of an unknown magnet by observing attraction/repulsion.

An electroscope detects electric charge, not magnetism. Iron filings visualize magnetic field lines but don’t directly label poles. Bringing two unmarked magnets together allows observation of attraction/repulsion, but without knowing the polarity of at least one, you cannot label the poles definitively, although you can confirm they are magnets and identify pairs of like and unlike poles.

30. Which one of the following important greenhouse gases is not conside

Which one of the following important greenhouse gases is not considered to be largely changed in its amount due to human activities?

Water vapour

Carbon dioxide

Chlorofluorocarbon

Methane

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Water vapor (H₂O) is the most abundant greenhouse gas in the atmosphere. However, its concentration is primarily controlled by temperature and the water cycle (evaporation, condensation, precipitation). While the amount of water vapor *changes* in response to global warming caused by other greenhouse gases (acting as a positive feedback loop, where warmer air holds more water vapor), human activities do not directly and significantly change the *global* atmospheric concentration of water vapor through emissions in the way they do for carbon dioxide, methane, and chlorofluorocarbons. Carbon dioxide (from burning fossil fuels, deforestation), methane (from agriculture, fossil fuels), and chlorofluorocarbons (synthetic chemicals) have seen their atmospheric concentrations dramatically increased primarily due to human activities.

– Water vapor concentration is mainly controlled by temperature and natural processes.
– CO₂, CH₄, and CFCs concentrations are significantly increased by direct human emissions.
– Water vapor acts largely as a feedback to changes initiated by other greenhouse gases.

Water vapor contributes significantly to the natural greenhouse effect. Its concentration in the atmosphere varies geographically and temporally, but the overall global atmospheric burden is not primarily driven by direct human emissions like the other listed gases.