Heat and Thermodynamics

1. The temperature of a body decreases from 240 K to 210 K. The correspon

The temperature of a body decreases from 240 K to 210 K. The corresponding change (without sign) in the temperature in degree Fahrenheit is close to :

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54° F
30° F
17° F
44° F
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UPSC CISF-AC-EXE – 2023
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The temperature change from 240 K to 210 K is 30 K. The corresponding change in degree Fahrenheit is 54°F, which is option A.
A change in temperature on the Kelvin scale is equal to the same change in temperature on the Celsius scale (ΔT_K = ΔT_C). The relationship between a change in Celsius and a change in Fahrenheit is ΔT_F = (9/5) * ΔT_C.
Given the initial temperature T1 = 240 K and final temperature T2 = 210 K, the change in Kelvin is ΔT_K = T2 – T1 = 210 K – 240 K = -30 K. The question asks for the change *without sign*, so the magnitude of the change is 30 K. Since ΔT_K = ΔT_C, the change in Celsius is ΔT_C = 30°C. The corresponding change in Fahrenheit is ΔT_F = (9/5) * ΔT_C = (9/5) * 30 = 9 * 6 = 54°F.

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2. Gases can be liquefied by

Gases can be liquefied by

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reducing pressure and temperature.
applying pressure and reducing temperature.
reducing pressure and raising temperature.
applying pressure and raising temperature.
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UPSC CISF-AC-EXE – 2020
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Gases can be liquefied by increasing the intermolecular forces or reducing the kinetic energy of the gas particles so that they can come closer together and form a liquid state. Applying pressure forces the particles closer, increasing intermolecular interactions. Reducing temperature decreases the kinetic energy of the particles, making it easier for them to form liquid bonds. Liquefaction occurs when the temperature is at or below the critical temperature and sufficient pressure is applied. The most effective way is typically applying pressure and reducing temperature.
Liquefaction of gases requires increasing pressure and/or decreasing temperature to bring molecules closer and reduce their kinetic energy.
For every gas, there is a critical temperature above which it cannot be liquefied by pressure alone. Below the critical temperature, increasing pressure can cause liquefaction. Reducing the temperature makes liquefaction easier at lower pressures. Therefore, applying pressure and reducing temperature together is the standard method for liquefying gases.

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3. The transfer of thermal energy carries which of the following

The transfer of thermal energy carries which of the following phenomena?

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Conduction and convection only
Only conduction
Conduction, convection and radiation
Only radiation
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UPSC CISF-AC-EXE – 2020
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Thermal energy, or heat, can be transferred from one place to another through three fundamental mechanisms: conduction, convection, and radiation. Conduction is the transfer of heat through direct contact of particles, primarily in solids. Convection is the transfer of heat through the movement of fluids (liquids or gases). Radiation is the transfer of heat through electromagnetic waves, which does not require a medium and can occur through a vacuum.
The three main modes of thermal energy transfer are conduction, convection, and radiation.
Examples: Conduction transfers heat through a metal rod when one end is heated. Convection transfers heat in boiling water or rising hot air. Radiation transfers heat from the sun to the Earth, or from a fire to your hands. All three phenomena contribute to the transfer of thermal energy in different situations.

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4. A Kelvin thermometer and a Fahrenheit thermometer both give the same r

A Kelvin thermometer and a Fahrenheit thermometer both give the same reading for a certain sample. The corresponding Celsius temperature is about

301 °C
614 °C
276 °C
273 °C
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UPSC CISF-AC-EXE – 2019
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The corresponding Celsius temperature is about 301 °C.
We are given that a Kelvin thermometer and a Fahrenheit thermometer give the same reading for a certain sample. Let this reading be x.
So, the temperature in Kelvin (K) is x, and the temperature in Fahrenheit (F) is x.
We need to find the corresponding temperature in Celsius (C).

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The conversion formulas between these scales are:
1. Fahrenheit to Celsius: C = (F – 32) * 5/9
2. Kelvin to Celsius: C = K – 273.15 (or often approximated as C = K – 273)

Let’s use the exact conversion K = C + 273.15 and F = (9/5)C + 32.
Since K = x and F = x, we have:
x = C + 273.15 (Equation 1)
x = (9/5)C + 32 (Equation 2)

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Equating the right sides of Equation 1 and Equation 2:
C + 273.15 = (9/5)C + 32

Rearrange the terms to solve for C:
273.15 – 32 = (9/5)C – C
241.15 = (9/5 – 5/5)C
241.15 = (4/5)C

C = (241.15 * 5) / 4
C = 1205.75 / 4
C = 301.4375

Rounding to the nearest whole number or considering the options, the corresponding Celsius temperature is about 301 °C.

The question asks for “about” the Celsius temperature, indicating an approximation is acceptable. Using the approximation C = K – 273 would yield C = 301.4375 – 0.15 ≈ 301.2875, still close to 301 °C. The unusual point where Kelvin and Fahrenheit scales read the same is 301.4375 in both K and °F.

5. Which one of the following statements about the Principle of Calorimet

Which one of the following statements about the Principle of Calorimetry is correct ?

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It is always valid.
It is valid when temperature is constant.
It is valid only when there is no change of state.
It is valid only under equilibrium condition.
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UPSC CISF-AC-EXE – 2018
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The Principle of Calorimetry is based on the law of conservation of energy. It states that in an isolated system, the total amount of heat lost by the hot bodies is equal to the total amount of heat gained by the cold bodies. This principle is fundamentally valid whenever heat exchange occurs in an isolated system, including processes involving changes of state (by accounting for latent heat) and processes involving temperature changes. Options B, C, and D state conditions under which the principle is *only* valid, which are incorrect limitations. The principle is valid even when temperature is not constant (during temperature change), when there is a change of state (by including latent heat), and it describes the process of heat exchange *towards* equilibrium, not only at equilibrium. Therefore, “It is always valid” (interpreted as valid in an isolated system for which it is defined) is the most accurate statement among the choices, as the other options describe false limitations.
The Principle of Calorimetry is based on energy conservation in thermal interactions and applies in isolated systems, irrespective of whether temperature changes or phase changes occur.
In practical calorimetry experiments, efforts are made to create an isolated system to minimize heat exchange with the surroundings and ensure the principle holds true for the components within the calorimeter.

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6. The phenomenon of change of a liquid into vapours at any temperature i

The phenomenon of change of a liquid into vapours at any temperature is known as evaporation, which takes place

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at its boiling point
above its boiling point
below its boiling point
at room temperature
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UPSC CISF-AC-EXE – 2017
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Evaporation is the process by which a liquid changes into a gas or vapor. This process occurs at the surface of the liquid. Unlike boiling, which happens at a specific temperature (the boiling point) throughout the bulk of the liquid when its vapor pressure equals the surrounding pressure, evaporation can happen at *any* temperature where the liquid exists. Molecules at the surface gain enough kinetic energy to overcome the intermolecular forces holding them in the liquid phase and escape into the gas phase. This process occurs *below* the boiling point of the liquid.
– Evaporation is a surface phenomenon.
– It occurs when liquid molecules gain enough energy to escape into the gas phase.
– Evaporation can take place at any temperature where the liquid is present.
– Boiling is different; it occurs throughout the liquid at a specific temperature (boiling point) where vapor pressure equals external pressure.
Factors affecting the rate of evaporation include temperature (higher temperature, faster evaporation), surface area (larger area, faster evaporation), humidity (lower humidity, faster evaporation), and wind speed (higher wind speed, faster evaporation). Evaporation is a cooling process because the molecules with the highest kinetic energy escape from the liquid surface.

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7. In a pressure cooker, the temperature at which the food is cooked depe

In a pressure cooker, the temperature at which the food is cooked depends mainly upon which of the following?

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  1. Area of the hole in the lid
  2. Temperature of the flame
  3. Weight of the lid

Select the correct answer using the code given below.

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1 and 2 only
2 and 3 only
1 and 3 only
1, 2 and 3
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UPSC IAS – 2021
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The temperature at which food is cooked in a pressure cooker depends mainly upon the area of the hole in the lid and the weight of the lid.
– A pressure cooker works by creating a sealed environment that traps steam, increasing the internal pressure.
– The boiling point of water increases with pressure. At standard atmospheric pressure (1 atm), water boils at 100°C (212°F). In a pressure cooker, the pressure can rise to about 2 atm (or more), raising the boiling point to around 120-125°C (248-257°F).
– The maximum pressure inside the cooker is regulated by a pressure release valve, which typically consists of a small hole or vent in the lid covered by a weight or spring mechanism.
– The pressure at which steam is released depends directly on the weight placed on the vent and inversely on the area of the vent hole under the weight. A heavier weight or a smaller hole area will result in higher pressure and thus a higher cooking temperature.
– The temperature of the flame affects the *rate* at which the water heats up and turns into steam, and thus how quickly the desired pressure is reached. However, once the pressure regulator starts venting steam, the temperature inside stabilizes at a level determined by the pressure, not the flame temperature (as long as the heat input is sufficient to maintain that pressure).
– Therefore, the main factors determining the cooking temperature are the weight of the lid (specifically the pressure regulator) and the area of the hole it covers, which together regulate the internal pressure.
Cooking at a higher temperature under pressure significantly reduces cooking time compared to boiling at atmospheric pressure. Different pressure cookers and their regulators are designed to operate at specific pressures, typically “low” or “high” pressure settings, corresponding to different weights or spring tensions.

8. Which one of the following heat transfer mechanism does NOT require a

Which one of the following heat transfer mechanism does NOT require a medium ?

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Conduction
Convection
Radiation
Collision
This question was previously asked in
UPSC CAPF – 2024
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Heat transfer occurs through three primary mechanisms: conduction, convection, and radiation. Conduction is the transfer of heat through direct contact and molecular vibrations within a material; it requires a medium (solid, liquid, or gas). Convection is the transfer of heat through the bulk movement of a fluid (liquid or gas); it requires a medium that can flow. Radiation is the transfer of heat through electromagnetic waves, such as infrared radiation. These waves can travel through a vacuum (like space) and do not require a physical medium to transfer energy. Collision is not a distinct primary heat transfer mechanism; energy transfer through molecular collisions is the basis of conduction and part of convection at the molecular level. Therefore, radiation is the only mechanism listed that does not require a medium.
Conduction and convection require a medium for heat transfer, while radiation does not. Heat from the Sun reaches Earth through radiation across the vacuum of space.
Examples: Conduction heats the handle of a metal spoon in hot soup. Convection heats water in a pot as warmer water rises and cooler water sinks. Radiation is felt as warmth from a fire or the sun. All three mechanisms can occur simultaneously in many situations, but one often dominates depending on the conditions.

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9. Latent heat corresponds to the change in heat at constant

Latent heat corresponds to the change in heat at constant

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temperature only
volume only
pressure only
temperature, volume and pressure
This question was previously asked in
UPSC CAPF – 2020
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Latent heat corresponds to the change in heat required for a substance to undergo a phase transition (like melting, boiling, or condensation) at a constant temperature.
During a phase change at constant pressure, the temperature remains constant while heat energy is absorbed (for melting/boiling/sublimation) or released (for freezing/condensation/deposition). This energy is used to change the state (break or form intermolecular bonds) rather than increase the kinetic energy of the molecules, which would result in a temperature change.
For typical phase transitions under standard conditions, pressure is also constant. However, the defining characteristic of latent heat is that it is the energy involved in a phase change *without* a change in temperature. Volume typically changes during a phase transition.

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10. At triple point the substance co-exists in 1. Liquid phase 2. Solid

At triple point the substance co-exists in

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  • 1. Liquid phase
  • 2. Solid phase
  • 3. Vapour phase

Select the correct answer using the code given below :

1 only
1 and 2 only
2 and 3 only
1, 2 and 3
This question was previously asked in
UPSC CAPF – 2020
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At the triple point of a substance, the solid, liquid, and vapour (gaseous) phases of that substance coexist in thermodynamic equilibrium.
The triple point is a unique specific temperature and pressure for a substance, distinct from the melting point (solid-liquid equilibrium) or boiling point (liquid-vapour equilibrium) which vary with pressure.
For water, the triple point is at 0.01°C (273.16 K) and 611.657 pascals (about 0.006 atm). This specific point is used to define the Kelvin temperature scale.

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