UPSC NDA-2 Archives

11. When pure water boils vigorously, the bubbles that rise to the surface

When pure water boils vigorously, the bubbles that rise to the surface are composed primarily of

air

hydrogen

hydrogen and oxygen

water vapour

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When pure water boils vigorously, the process involves the phase transition of liquid water into gaseous water.

Boiling occurs when the vapour pressure of the liquid equals the surrounding atmospheric pressure, and the liquid turns into gas within the bulk of the liquid as well as at the surface. The bubbles formed during boiling are filled with the gaseous form of the liquid, which in this case is water vapour.

Air might be present as dissolved gas in water, and tiny air bubbles might be released upon heating before boiling, but the large, vigorous bubbles during boiling are predominantly water vapour. Water molecules (H₂O) decompose into hydrogen and oxygen only under extreme conditions (like electrolysis), not during simple boiling.

12. The ionization energy of hydrogen atom in the ground state is

The ionization energy of hydrogen atom in the ground state is

13.6 MeV

13.6 eV

13.6 Joule

Zero

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The ionization energy of a hydrogen atom is the minimum energy required to remove the electron from the ground state (n=1) and take it to the ionization limit (n=infinity).

The energy of an electron in the n-th energy level of a hydrogen atom is given by E_n = -13.6/n² eV. In the ground state (n=1), the energy is E₁ = -13.6/1² = -13.6 eV. At the ionization limit (n=infinity), the energy is E_infinity = -13.6/infinity² = 0 eV. The ionization energy is the difference between these energies: Ionization Energy = E_infinity – E₁ = 0 – (-13.6 eV) = +13.6 eV.

MeV (Mega electron Volt) and Joule are also units of energy, but 13.6 eV is the standard and correct value for the hydrogen atom’s ionization energy. 1 eV is approximately 1.602 x 10⁻¹⁹ J. 1 MeV is 10⁶ eV. 13.6 MeV is an extremely large amount of energy in this context. Zero ionization energy would mean no energy is required to remove the electron, which is incorrect for a bound electron.

13. The majority charge carriers in a p-type semiconductor are

The majority charge carriers in a p-type semiconductor are

free electrons

conduction electrons

ions

holes

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A p-type semiconductor is created by doping an intrinsic semiconductor (like silicon or germanium) with trivalent impurity atoms (like boron, gallium, or indium). Trivalent atoms have three valence electrons. When a trivalent atom substitutes a semiconductor atom (which has four valence electrons), there is a deficiency of one electron to form a complete covalent bond with the surrounding semiconductor atoms. This deficiency is called a “hole.” These holes can accept electrons from neighboring bonds and effectively move through the crystal lattice, acting as positive charge carriers. In a p-type semiconductor, the number of holes is much greater than the number of free electrons (which are present due to thermal generation), making holes the majority charge carriers.

– P-type semiconductors are created by doping with trivalent impurities.
– Trivalent impurities create electron deficiencies called holes.
– Holes act as positive charge carriers.
– In p-type semiconductors, holes are the majority charge carriers.

In contrast, n-type semiconductors are created by doping with pentavalent impurity atoms (like phosphorus, arsenic, or antimony). Pentavalent atoms have five valence electrons, one more than needed for covalent bonding with four neighbors. This extra electron is loosely bound and easily becomes a free electron, which acts as a negative charge carrier. In n-type semiconductors, free electrons are the majority charge carriers. Ions (the doping atoms fixed in the lattice) are not mobile charge carriers.

14. Which one of the following waves is used for detecting forgery in curr

Which one of the following waves is used for detecting forgery in currency notes ?

Ultraviolet waves

Infrared waves

Radio waves

Microwaves

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Many modern currency notes incorporate security features that are visible under ultraviolet (UV) light. These features often include fluorescent threads, watermarks, or specific inks that glow or change color when exposed to UV radiation. Checking currency notes under UV light is a common method used to detect counterfeits that lack these specialized security features.

– Ultraviolet (UV) light is used to reveal fluorescent security features in currency notes.
– These features are difficult to replicate by counterfeiters.

Infrared (IR) waves are also used in some advanced currency verification systems, as some inks have specific absorption or reflection properties in the IR spectrum. However, UV light inspection is a more common and basic method for detecting forgery in widely circulated currency. Radio waves and Microwaves are not typically used for visual security checks on currency notes.

15. The mirrors used as rear-view mirrors in vehicles are

The mirrors used as rear-view mirrors in vehicles are

concave

convex

cylindrical

plane

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Rear-view mirrors in vehicles are typically convex mirrors. Convex mirrors form virtual, erect, and diminished images of objects. The principal advantage of a convex mirror for this purpose is that it provides a wider field of view compared to a plane mirror of the same size. This allows the driver to see a larger area behind the vehicle, enhancing safety. While the image is smaller (diminished), this is an acceptable trade-off for the increased field of vision.

– Convex mirrors produce virtual, erect, and diminished images.
– Convex mirrors provide a wider field of view.

Concave mirrors can produce real or virtual, inverted or erect, magnified or diminished images depending on the object’s position. They are not suitable for providing a consistent wide-angle view of distant objects. Plane mirrors produce virtual, erect, and same-sized images, but their field of view is limited compared to convex mirrors of the same size.

16. Radioactivity is measured by

Radioactivity is measured by

GM Counter

Polarimeter

Calorimeter

Colorimeter

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Radioactivity is the spontaneous emission of radiation (alpha, beta, gamma rays) from unstable atomic nuclei. A Geiger-Muller counter (GM Counter) is a widely used instrument specifically designed to detect and measure ionizing radiation, which is characteristic of radioactive decay. It works by detecting the ionization caused by radiation passing through a gas-filled tube.

– Radioactivity involves the emission of ionizing radiation.
– A GM Counter is an instrument that detects ionizing radiation.

A Polarimeter measures the rotation of polarized light by optical active substances. A Calorimeter measures heat flow or thermal energy. A Colorimeter measures the absorbance or transmission of light by a solution at specific wavelengths to determine concentration or color properties. None of these are used to measure radioactivity. Other instruments for measuring radioactivity include scintillation counters and semiconductor detectors.

17. How long does light take to reach the Earth from the Sun ?

How long does light take to reach the Earth from the Sun ?

About 4 minutes

About 8 minutes

About 24 minutes

About 24 hours

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The average distance from the Earth to the Sun is approximately 149.6 million kilometers, or about 1.5 x 10^11 meters. The speed of light in a vacuum is approximately 3 x 10^8 meters per second. The time taken for light to travel this distance can be calculated using the formula time = distance / speed.
Time ≈ (1.5 x 10^11 m) / (3 x 10^8 m/s) = 0.5 x 10^3 seconds = 500 seconds.
Converting seconds to minutes: 500 seconds / 60 seconds/minute ≈ 8.33 minutes.
Among the given options, “About 8 minutes” is the closest and most accurate approximation for the time light takes to travel from the Sun to the Earth.

– Distance from Sun to Earth is approximately 1.5 x 10^11 m.
– Speed of light is approximately 3 x 10^8 m/s.
– Time = Distance / Speed.

This travel time is often used to define the Astronomical Unit (AU), where 1 AU is the average distance between the Earth and the Sun. Observing distant celestial objects means looking back in time, as the light we see was emitted minutes, years, or even billions of years ago, depending on the distance.

18. Electron emission from a metallic surface by application of light is k

Electron emission from a metallic surface by application of light is known as

Thermionic emission

Photoelectric emission

High field emission

Autoelectronic emission

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Electron emission from a metallic surface caused by the application of light is known as the photoelectric effect, and the emission itself is called photoelectric emission. When light of sufficient frequency strikes a metal surface, it can impart enough energy to electrons to overcome the work function (binding energy) and be ejected from the surface.

– Photoelectric emission is electron emission triggered by light.
– This phenomenon is the basis of the photoelectric effect.

Thermionic emission is electron emission due to heat. High field emission (or field emission or autoelectronic emission) is electron emission caused by a strong electric field. These are different mechanisms for electron emission from surfaces.

19. In a vacuum, a five-rupee coin, a feather of a sparrow bird and a mang

In a vacuum, a five-rupee coin, a feather of a sparrow bird and a mango are dropped simultaneously from the same height. The time taken by them to reach the bottom is t₁, t₂ and t₃ respectively. In this situation, we will observe that

”t1

”t3

”t1

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In a vacuum, there is no air resistance. According to the principles of physics, in the absence of air resistance, all objects dropped from the same height will fall with the same acceleration due to gravity, regardless of their mass, size, or shape. This means they will take the same amount of time to reach the ground. Therefore, the five-rupee coin, the feather, and the mango will all reach the bottom simultaneously, meaning $t_1 = t_2 = t_3$.

– In a vacuum, acceleration due to gravity is constant for all objects ($g$).
– Absence of air resistance.
– Time taken to fall from the same height is independent of mass.

This principle was first hypothesized by Galileo Galilei and later demonstrated convincingly in experiments, including the famous experiment conducted by astronaut David Scott on the Moon during the Apollo 15 mission, where a hammer and a feather were dropped simultaneously and hit the lunar surface at the same time. On Earth, air resistance significantly affects the fall time of objects, especially those with large surface area relative to mass like a feather.

20. The symbol of SI unit of inductance is H. It stands for

The symbol of SI unit of inductance is H. It stands for

Holm

Halogen

Henry

Hertz

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The SI unit of inductance is the Henry, symbolized by H. It is named after the American scientist Joseph Henry.

– H is the symbol for the SI unit of inductance.
– The unit is named after Joseph Henry.

Holm is not a standard physics unit. Halogen refers to a group of chemical elements (Fluorine, Chlorine, Bromine, Iodine, Astatine). Hertz (Hz) is the SI unit of frequency. Inductance is a measure of how much a component opposes changes in current flowing through it by storing energy in a magnetic field.