Physical Properties of Materials

41. Liquids and gases never show

Liquids and gases never show

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diamagnetic property
paramagnetic property
ferromagnetic property
electromagnetic property
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UPSC CDS-2 – 2016
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Ferromagnetism is a property of certain materials, like iron, nickel, and cobalt, where the magnetic moments of atoms are strongly aligned within domains, leading to a large, spontaneous magnetization. This domain structure and strong cooperative alignment are characteristic of solid states and do not occur in liquids or gases under normal conditions.
Ferromagnetism requires a specific long-range ordering of magnetic moments and formation of magnetic domains, which is typically a property of crystalline solids. Liquids and gases lack the necessary atomic structure for this phenomenon.
Liquids and gases can exhibit diamagnetic properties (weak repulsion from magnetic fields, present in all materials) and paramagnetic properties (weak attraction to magnetic fields due to unpaired electrons). Electromagnetic properties are general properties of matter involving interaction with electric and magnetic fields, which all materials possess. Ferromagnetism is a specific, strong magnetic property not found in liquid or gaseous states.

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42. In a bipolar junction transistor

In a bipolar junction transistor

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all the three regions (the emitter, the base and the collector) have equal concentrations of impurity
the emitter has the least concentration of impurity
the collector has the least concentration of impurity
the base has the least concentration of impurity
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UPSC CDS-2 – 2016
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In a bipolar junction transistor (BJT), the base region has the least concentration of impurity.
– A BJT consists of three semiconductor regions: the emitter, the base, and the collector, separated by two p-n junctions.
– These regions are doped with impurities (donors or acceptors) to create n-type and p-type semiconductors.
– The doping levels are designed specifically for the transistor’s operation:
– Emitter: Heavily doped (high concentration) to efficiently inject charge carriers (electrons or holes) into the base.
– Base: Lightly doped (low concentration) and made very thin to allow most injected carriers from the emitter to reach the collector and to minimize recombination within the base.
– Collector: Moderately doped (intermediate concentration, less than emitter but more than base) and typically larger in size to collect the carriers from the base and dissipate heat.
The doping profile and physical geometry of the emitter, base, and collector regions are crucial for the performance of a bipolar junction transistor, determining its gain, operating speed, and power handling capabilities.

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43. The spring constant of a spring depends on its

The spring constant of a spring depends on its

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length only
material only
length and its diameter
thickness, its diameter and its material
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UPSC CDS-2 – 2016
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The spring constant of a spring depends on its physical and material properties including its thickness, diameter, and material.
– The spring constant (k) quantifies the stiffness of a spring.
– For a helical spring, the spring constant is given by the formula k = (G * d⁴) / (8 * D³ * n), where G is the shear modulus of the material, d is the wire diameter (thickness), D is the coil diameter, and n is the number of active turns (related to length).
– Therefore, k depends on the material (shear modulus G), the wire thickness (d), the coil diameter (D), and the number of turns (n, which determines the overall length for a given coil diameter and pitch).
– Option D includes thickness (wire diameter), diameter (coil diameter), and material, which are the key determinants of the spring constant.
The spring constant is an intrinsic property of a specific spring configuration. Changing any of these physical parameters will change the spring constant. For example, a thicker wire makes the spring stiffer (higher k), a larger coil diameter or more turns make it less stiff (lower k), and using a material with a higher shear modulus makes it stiffer.

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44. Which one of the following is not a ferromagnetic material?

Which one of the following is not a ferromagnetic material?

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Cobalt
Iron
Silver
Ferric chloride
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UPSC CDS-1 – 2024
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The correct option is C) Silver.
Ferromagnetic materials are strongly attracted to magnetic fields and can be permanently magnetized. Common examples include iron, cobalt, nickel, and their alloys. Silver is a diamagnetic material, which means it is weakly repelled by a magnetic field. Ferric chloride is a paramagnetic material, which is weakly attracted by a magnetic field. Neither diamagnetic nor paramagnetic materials are classified as ferromagnetic.
Magnetic properties of materials are categorized into diamagnetism, paramagnetism, ferromagnetism, antiferromagnetism, and ferrimagnetism. The strength of the magnetic interaction and the behavior of magnetic dipoles within the material determine its category. Ferromagnetism is a cooperative phenomenon requiring a strong interaction between atomic magnetic moments.

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45. Which one of the following metals has both malleability and ductility

Which one of the following metals has both malleability and ductility properties ?

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Na
Au
Ce
Hg
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UPSC CDS-1 – 2021
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Gold (Au) is a well-known metal that exhibits both malleability and ductility to a very high degree. It can be hammered into extremely thin sheets (gold leaf) and drawn into very fine wires.
– Malleability is the ability of a material to be deformed under compression without fracturing (hammered into sheets).
– Ductility is the ability of a material to be deformed under tensile stress without fracturing (drawn into wires).
– These properties are characteristic of metals due to the nature of metallic bonding.
– Sodium (Na) is malleable and ductile but much softer and less durable than gold.
– Cerium (Ce) is a malleable metal, but not as famously malleable and ductile as gold.
– Mercury (Hg) is a liquid at room temperature and therefore does not exhibit these solid-state mechanical properties.
– Gold is considered the most malleable and ductile of all metals.

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46. The Hooke’s law is valid for

The Hooke’s law is valid for

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only proportional region of the stress-strain curve
entire stress-strain curve
entire elastic region of the stress-strain curve
elastic as well as plastic region of the stress-strain curve
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UPSC CDS-1 – 2019
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The correct answer is A. Hooke’s law states that the stress is directly proportional to the strain within the elastic limit of a material. More specifically, this linear relationship holds true only in the initial part of the elastic region, which is known as the proportional region. Beyond the proportional limit, the material may still behave elastically (return to its original shape upon unloading), but the stress-strain relationship becomes non-linear.
Hooke’s law (Stress ∝ Strain) is strictly valid only in the proportional region of the stress-strain curve.
The stress-strain curve for a ductile material typically shows several regions: proportional limit, elastic limit, yield point, ultimate tensile strength, and fracture point. The elastic limit is the maximum stress a material can withstand without permanent deformation. The proportional limit is the point up to which stress is directly proportional to strain. For many materials, these two points are very close, but Hooke’s law is defined by the proportionality.

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47. Why is argon gas used along with tungsten wire in an electric bulb?

Why is argon gas used along with tungsten wire in an electric bulb?

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To increase the life of the bulb
To reduce the consumption of electricity
To make the emitted light colored
To reduce the cost of the bulb
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UPSC CDS-1 – 2018
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Argon gas is used in electric bulbs along with a tungsten filament primarily to increase the life of the bulb. At the high operating temperature of the tungsten filament, tungsten atoms tend to evaporate or sublime. The presence of an inert gas like argon at a certain pressure within the bulb reduces the rate of evaporation of the tungsten filament, making it last longer.
– Tungsten filaments operate at very high temperatures (around 2500-3000°C) to produce light by incandescence.
– High temperatures cause the tungsten metal to sublime (evaporate) from the filament.
– Evaporated tungsten atoms deposit on the cooler glass bulb wall, causing blackening.
– Sublimation thins the filament over time, eventually causing it to break.
– Inert gases like argon, krypton, or xenon, when present at pressure, impede the movement of tungsten atoms away from the filament, reducing the evaporation rate and thus prolonging the filament’s life and preventing rapid blackening of the bulb.
Early incandescent bulbs were vacuum sealed. Adding inert gas was a significant improvement in bulb technology, leading to longer-lasting and brighter bulbs compared to vacuum bulbs. While the inert gas does cause a slight loss of energy through convection, this is offset by the ability to operate the filament at a slightly higher temperature for improved efficiency and light output, while still significantly increasing bulb lifespan.

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48. What is the maximum number of states of matter?

What is the maximum number of states of matter?

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Three
Four
Five
Variable
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UPSC CDS-1 – 2017
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The maximum number of states of matter is variable. While the most commonly known states are solid, liquid, gas, plasma, and Bose-Einstein condensate, the concept of “state of matter” or “phase” is broad. Under different conditions of temperature, pressure, magnetic field, etc., substances can exist in numerous distinct phases (e.g., superfluids, superconductors, various crystalline structures, liquid crystals, fermionic condensates, etc.). The number of possible states depends on the substance and the range of conditions considered, making it a variable quantity rather than a fixed small number.
Beyond the classical solid, liquid, gas, and plasma states, physics recognizes many other distinct states (phases) of matter. The number of these states is not fixed and can vary depending on the substance and the environmental conditions, as well as how a “state” is defined.
Phase diagrams illustrate the different phases of a substance under varying temperature and pressure. These diagrams can show multiple solid phases, indicating that even within one traditional state (solid), there can be different distinct states depending on the conditions.

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49. A parallel-plate capacitor, with air in between the plates, has capaci

A parallel-plate capacitor, with air in between the plates, has capacitance C. Now the space between the two plates of the capacitor is filled with a dielectric of dielectric constant 7. Then the value of the capacitance will become

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C
C/7
7C
14C
This question was previously asked in
UPSC CDS-1 – 2017
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The correct answer is C) 7C.
The capacitance of a parallel-plate capacitor is given by C = ε * A / d, where ε is the permittivity of the material between the plates, A is the area of the plates, and d is the distance between them. When there is air (or vacuum) between the plates, the permittivity is approximately ε₀, and the capacitance is C = ε₀ * A / d. When the space is filled with a dielectric material of dielectric constant κ, the permittivity changes to ε = κ * ε₀. Therefore, the new capacitance C’ becomes C’ = (κ * ε₀) * A / d = κ * (ε₀ * A / d). Given that the dielectric constant κ = 7, the new capacitance is C’ = 7 * C.
Introducing a dielectric material between the plates of a capacitor increases the electric field strength between the plates for the same voltage, or reduces the voltage across the plates for the same charge. This results in the capacitor being able to store more charge at the same voltage, effectively increasing its capacitance.

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50. Which one of the following statements about bar magnet is correct ?

Which one of the following statements about bar magnet is correct ?

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The pole strength of the north-pole of a bar magnet is larger than that of the south-pole
When a piece of bar magnet is bisected perpendicular to its axis, the north and south poles get separated
When a piece of bar magnet is bisected parallel to its axis, two new bar magnets are formed
The poles of a bar magnet are unequal in magnitude and opposite in nature
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UPSC CDS-1 – 2016
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When a piece of bar magnet is bisected parallel to its axis, two new bar magnets are formed.
Magnetic poles always exist in pairs. If you cut a bar magnet, you don’t separate the north and south poles; instead, each resulting piece becomes a new magnet with its own north and south pole. Cutting parallel to the axis divides the magnet into two thinner magnets, each retaining its original length and thus forming new poles on the cut surface.
The pole strength of a bar magnet’s north pole is equal in magnitude to that of its south pole, although they are opposite in nature. Cutting a magnet perpendicular to its axis results in shorter magnets, each with a full set of poles. Magnetic monopoles (isolated north or south poles) have not been observed in nature.

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