Heat and Thermodynamics

71. The surface of the concrete structure of a new construction is covered

The surface of the concrete structure of a new construction is covered with straw/gunny bags by wetting. This is done to :

[amp_mcq option1=”prevent fast evaporation, until hydration has proceeded well” option2=”protect the concrete structure from contamination by dust particles” option3=”prevent development of any fungus on the surface” option4=”give smoother and cleaner surface over cement structure” correct=”option1″]

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UPSC CDS-2 – 2024
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The curing of concrete involves maintaining adequate moisture and temperature for a period after placing to allow the cement to properly hydrate and gain strength. Covering the concrete surface with wet straw or gunny bags helps to prevent the rapid evaporation of water from the concrete mix. This ensures that enough water is available for the complete hydration of cement, leading to the development of the concrete’s intended strength, durability, and surface properties.
– Hydration is the chemical reaction between cement and water that hardens concrete.
– Proper curing requires maintaining moisture content.
– Rapid evaporation leads to incomplete hydration, reduced strength, increased permeability, and potential cracking.
Other curing methods include ponding (flooding the surface), spraying, using wet coverings like burlap, or applying curing compounds that form a membrane to seal in moisture. Curing should continue for a sufficient period, typically several days to weeks, depending on the type of cement, mix design, strength requirements, and environmental conditions.

72. Which one of the following is the correct relation between Celsius and

Which one of the following is the correct relation between Celsius and Fahrenheit temperature scales? (Symbols carry their usual meanings)

[amp_mcq option1=”TF = (5/9)TC + 32″ option2=”TF = (9/5)TC + 36″ option3=”TF = (5/9)TC + 36″ option4=”TF = (9/5)TC + 32″ correct=”option4″]

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UPSC CDS-2 – 2023
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The Celsius and Fahrenheit scales are two common temperature scales. The relationship between them is linear. On the Celsius scale, water freezes at 0°C and boils at 100°C. On the Fahrenheit scale, water freezes at 32°F and boils at 212°F.
The interval between the freezing and boiling points of water is 100 degrees on the Celsius scale and 180 degrees (212 – 32) on the Fahrenheit scale. Thus, 100°C corresponds to 180°F. This means a 1°C change is equivalent to a 1.8°F (180/100 = 9/5) change. Starting from the freezing point (0°C = 32°F), the formula to convert Celsius (TC) to Fahrenheit (TF) is TF = (9/5)TC + 32.
To convert Fahrenheit (TF) to Celsius (TC), the formula is TC = (5/9)(TF – 32). The two scales intersect at -40 degrees, where -40°C = -40°F.

73. People prefer to wear cotton clothes in summer season. This is due to

People prefer to wear cotton clothes in summer season. This is due to the fact that cotton clothes are

[amp_mcq option1=”good absorbers of water” option2=”good conveyors of heat” option3=”good radiators of heat” option4=”good absorbers of heat” correct=”option1″]

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UPSC CDS-2 – 2019
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People prefer to wear cotton clothes in summer primarily because cotton is a good absorber of water (sweat). The absorption of sweat from the body and its subsequent evaporation from the cotton fabric helps to cool the body through evaporative cooling. While cotton also allows air circulation (good conveyor of heat away from body) and radiates heat, its ability to manage sweat is the most significant reason for its preference in hot, humid conditions.
Cotton clothes are preferred in summer due to their good ability to absorb sweat, facilitating evaporative cooling.
Synthetic fabrics like polyester or nylon do not absorb sweat as well, trapping moisture against the skin and making the wearer feel hotter and uncomfortable in summer.

74. Rate of evaporation increases with

Rate of evaporation increases with

[amp_mcq option1=”an increase of surface area” option2=”an increase in humidity” option3=”a decrease in wind speed” option4=”a decrease of temperature” correct=”option1″]

This question was previously asked in
UPSC CDS-2 – 2019
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The rate of evaporation increases with an increase of surface area (A). Evaporation occurs at the surface of a liquid, where molecules with sufficient kinetic energy escape into the gaseous phase. A larger surface area exposes more molecules to the air, thus increasing the rate of evaporation. An increase in humidity (B) decreases the rate of evaporation as the air is already saturated with water vapor. A decrease in wind speed (C) decreases the rate of evaporation, as wind helps remove the saturated layer of air above the liquid surface, allowing more evaporation. A decrease in temperature (D) decreases the kinetic energy of molecules, leading to a lower rate of evaporation.
Factors that increase the rate of evaporation include increased surface area, increased temperature, increased wind speed, and decreased humidity.
Evaporation is a cooling process because the molecules with the highest kinetic energy escape, leaving behind molecules with lower average kinetic energy, resulting in a decrease in temperature of the remaining liquid.

75. The rate of evaporation of liquid does not depend upon

The rate of evaporation of liquid does not depend upon

[amp_mcq option1=”temperature” option2=”its surface area exposed to the atmosphere” option3=”its mass” option4=”humidity” correct=”option3″]

This question was previously asked in
UPSC CDS-2 – 2019
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The rate of evaporation of a liquid depends on factors such as temperature, surface area exposed to the atmosphere, humidity of the surrounding air, and air movement (wind). A higher temperature, larger surface area, lower humidity, and increased air movement all lead to a higher rate of evaporation. The mass of the liquid affects the total amount that can evaporate and how long the process takes, but it does not determine the *rate* at which evaporation occurs from the surface at any given moment under specific environmental conditions.
The rate of evaporation is a surface phenomenon influenced by factors affecting the energy and movement of molecules at the liquid-gas interface.
While total evaporation is limited by the mass, the speed at which it happens per unit area or per unit time is independent of the total volume or mass present, assuming sufficient liquid exists. For example, a thin film of water will evaporate at the same rate per unit area as a large puddle under the same conditions, until the film is gone.

76. In which of the following, heat loss is primarily not due to

In which of the following, heat loss is primarily not due to convection?

[amp_mcq option1=”Boiling water” option2=”Land and sea breeze” option3=”Circulation of air around blast furnace” option4=”Heating of glass surface of a bulb due to current in filament” correct=”option4″]

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UPSC CDS-2 – 2018
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In the heating of the glass surface of a bulb due to current in the filament, heat loss is primarily not due to convection.
Heat transfer occurs through conduction, convection, and radiation. Convection involves heat transfer through the movement of fluids (liquids or gases).
A) Boiling water: Heat is transferred throughout the water by convection currents.
B) Land and sea breeze: These are atmospheric movements caused by differential heating, a classic example of convection.
C) Circulation of air around blast furnace: Hot air rises and cooler air sinks, setting up convection currents around the furnace.
D) Heating of glass surface of a bulb due to current in filament: The filament of an incandescent bulb gets extremely hot due to the electric current. Heat is transferred from the filament to the surrounding space (including the glass envelope). If the bulb is evacuated, there is no medium for convection inside. Even if filled with an inert gas, the primary mode of heat transfer from the very hot filament across the gap to the glass envelope is thermal radiation. The glass surface then heats up by absorbing this radiation. While there might be some conduction through the support wires and subsequent convection/radiation from the outer glass surface, the initial transfer from the filament to the glass inside the bulb is significantly by radiation, not primarily convection.
Modern energy-efficient bulbs like LEDs produce less heat compared to incandescent bulbs. Incandescent bulbs convert only about 5-10% of electrical energy into light, with the rest being dissipated as heat, largely through radiation from the filament.

77. Which one of the following can extinguish fire more quickly?

Which one of the following can extinguish fire more quickly?

[amp_mcq option1=”Cold water” option2=”Boiling water” option3=”Hot water” option4=”Ice” correct=”option1″]

This question was previously asked in
UPSC CDS-2 – 2018
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Cold water can extinguish fire more quickly.
Water extinguishes fire primarily by cooling the burning material below its ignition temperature and by smothering (displacing oxygen) as it turns into steam. The cooling effect is due to water’s high specific heat capacity and high latent heat of vaporization. Cold water, compared to hot or boiling water, needs to absorb more heat to reach its boiling point (100°C) and then vaporize. This means a given mass of cold water can absorb a greater amount of heat from the fire, leading to more efficient cooling and quicker extinguishment. Boiling water is already at its boiling point and only absorbs heat through vaporization, while hot water requires less energy to reach boiling and vaporize compared to cold water. Ice must first absorb heat to melt before it can absorb further heat as liquid water and then vaporize, making its action potentially slower initially compared to liquid water directly applied.
Water is effective against fires involving solid materials (Class A fires), but it is not suitable for fires involving flammable liquids, gases, or electrical equipment, where it can spread the fire or pose an electrical hazard. Different types of fire extinguishers (e.g., foam, dry chemical, CO₂) are used for these other types of fires.

78. Joule-Thomson process is extremely useful and economical for attaining

Joule-Thomson process is extremely useful and economical for attaining low temperature. The process can be categorized as

[amp_mcq option1=”isobaric process” option2=”isoenthalpic process” option3=”adiabatic process” option4=”isochoric process” correct=”option2″]

This question was previously asked in
UPSC CDS-2 – 2017
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The Joule-Thomson process (also known as the Joule-Kelvin effect) is an isoenthalpic process, meaning that the enthalpy of the gas or liquid remains constant during the expansion through a valve or porous plug.
In the Joule-Thomson process, a temperature change occurs when a real gas or liquid expands or is compressed adiabatically while flowing through a valve or throttling device. The process is performed at constant enthalpy.
For most gases (except hydrogen and helium at room temperature), the Joule-Thomson effect causes cooling upon expansion from high to low pressure, provided the initial temperature is below the gas’s inversion temperature. This effect is crucial in refrigeration, air conditioning, and the liquefaction of gases.

79. On a day when I am in hurry to go to office, I have a fixed quantity o

On a day when I am in hurry to go to office, I have a fixed quantity of rice which was just cooked and kept in a bowl. In order to cool it quickly, which one of the following is the best option?

[amp_mcq option1=”Let it be kept on the table in a room where there is no fan, no air conditioner” option2=”Let it be kept in a room with AC set at a temperature around 23 °C and a ceiling fan (or table fan) operating at slow speed” option3=”Let it be kept in a bowl of water (at room temperature) and operating a ceiling fan (or table fan) at full speed” option4=”Let it be kept in a bowl of water at room temperature only” correct=”option3″]

This question was previously asked in
UPSC CDS-1 – 2024
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The correct option is C) Let it be kept in a bowl of water (at room temperature) and operating a ceiling fan (or table fan) at full speed.
Rapid cooling is achieved by maximizing the rate of heat transfer from the rice to the surroundings. Putting the bowl in a water bath (option C and D) allows for efficient heat transfer by conduction from the rice through the bowl to the water. Operating a fan (option B and C) increases the rate of convection of air over the surface, enhancing cooling. The combination of placing the bowl in water and using a fan is the most effective because the water bath provides good conductive heat transfer, and the fan significantly increases the rate of evaporation from the water’s surface (which cools the water and thus the rice) and also enhances convective cooling of the rice itself.
Evaporation is a very efficient cooling process as it requires a significant amount of latent heat to change water from liquid to gas phase, drawing heat from the surroundings. Convection is heat transfer through the movement of fluids (air or water); forced convection (with a fan) is faster than natural convection. Conduction is heat transfer through direct contact. Option C utilizes conduction, evaporation, and forced convection for maximum cooling speed.

80. If $x$ is the temperature of a system in Kelvin and $y$ is the tempera

If $x$ is the temperature of a system in Kelvin and $y$ is the temperature of the system in ${}^\circ\text{C}$, then the correct relation between them is

[amp_mcq option1=”$x = 273 – y$” option2=”$x = 273 + y$” option3=”$x = 173 + y$” option4=”$x = 173 – y$” correct=”option2″]

This question was previously asked in
UPSC CDS-1 – 2020
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The Kelvin and Celsius scales are two common temperature scales. The relationship between a temperature in Celsius ($y$) and the corresponding temperature in Kelvin ($x$) is given by the formula $x = y + 273.15$. For most practical purposes and in common examinations, this is often simplified to $x = y + 273$.
The Kelvin scale is an absolute temperature scale, with 0 K being absolute zero. The Celsius scale is a relative scale where 0°C is the freezing point of water and 100°C is the boiling point of water (at standard atmospheric pressure). A change of 1 degree Celsius is equal to a change of 1 Kelvin.
The formula $x = y + 273$ implies that a temperature of 0°C is equivalent to 273 K, and a temperature of 100°C is equivalent to 373 K. Absolute zero, which is -273.15°C, is equivalent to 0 K.
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