Heat and Thermodynamics Archives

41. The specific latent heat of vaporization of a substance is the quantit

The specific latent heat of vaporization of a substance is the quantity of heat needed to change unit mass from

liquid to vapour with a change of temperature

liquid to vapour without a change of temperature

vapour to liquid without a change of temperature

vapour to liquid with a change of temperature

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The specific latent heat of vaporization is defined as the heat energy required to change the state of a unit mass of a substance from liquid to vapour at a constant temperature.

Latent heat is the heat energy absorbed or released during a phase transition (like melting, freezing, vaporization, condensation) at a constant temperature and pressure. Specific latent heat refers to the amount of heat per unit mass. Vaporization specifically refers to the transition from liquid to gas. This phase change occurs without a change in temperature, as the energy is used to break intermolecular bonds.

There are two main types of specific latent heat: specific latent heat of fusion (solid to liquid or liquid to solid) and specific latent heat of vaporization (liquid to gas or gas to liquid). Each substance has characteristic values for these latent heats.

42. What is the mass of a material, whose specific heat capacity is 400 J/

What is the mass of a material, whose specific heat capacity is 400 J/(kg °C) for a rise in temperature from 15 °C to 25 °C, when heat received is 20 kJ?

0.1 kg

1 kg

10 kg

5 kg

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UPSC NDA-1 – 2022

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The mass of the material can be calculated using the formula relating heat absorbed, specific heat capacity, mass, and temperature change.

The formula used is Q = mcΔT, where Q is the heat energy absorbed (20 kJ = 20,000 J), m is the mass, c is the specific heat capacity (400 J/(kg °C)), and ΔT is the change in temperature (25 °C – 15 °C = 10 °C). Rearranging the formula to find mass: m = Q / (cΔT). Plugging in the values: m = 20000 J / (400 J/(kg °C) * 10 °C) = 20000 / 4000 kg = 5 kg.

Specific heat capacity is a material property that quantifies the amount of heat required to raise the temperature of one unit mass of the substance by one degree Celsius (or Kelvin). The unit J/(kg °C) or J/(kg K) is commonly used. This calculation assumes no phase change occurs within the given temperature range.

43. Which one of the following is the lowest possible temperature ?

Which one of the following is the lowest possible temperature ?

0° Celsius

– 073° Celsius

– 173° Celsius

– 273° Celsius

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UPSC NDA-1 – 2021

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The lowest possible temperature is absolute zero, which is defined as 0 Kelvin. On the Celsius scale, 0 Kelvin corresponds to approximately -273.15° Celsius.

Absolute zero is the theoretical lowest possible temperature where particles have minimal vibrational motion. The Celsius scale is related to the Kelvin scale by the formula $T(K) = T(°C) + 273.15$. Therefore, 0 K = $T(°C) + 273.15$, which gives $T(°C) = -273.15$. Among the given options, -273° Celsius is the closest value to absolute zero.

The Fahrenheit scale also has a zero point, but it is not absolute zero. The relationship between Celsius and Fahrenheit is $T(°F) = T(°C) \times \frac{9}{5} + 32$. Absolute zero (-273.15°C) is approximately -459.67°F.

44. Which one of the following statements regarding a thermos flask is NOT

Which one of the following statements regarding a thermos flask is NOT correct?

The walls of flask are separated by vacuum and made of glass which is a poor conductor of heat

The glass walls themselves have shiny surfaces

The surface of inner wall radiates good amount of heat and the surface of outer wall absorbs some of heat that is radiated from the inner wall

The cork supports are poor conductors of heat

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UPSC NDA-1 – 2019

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The correct option is C) The surface of inner wall radiates good amount of heat and the surface of outer wall absorbs some of heat that is radiated from the inner wall. This statement is NOT correct regarding a thermos flask.

– A thermos flask is designed to minimize heat transfer by conduction, convection, and radiation.
– To minimize heat transfer by radiation, the surfaces of the inner and outer glass walls are made shiny (often silvered). Shiny surfaces are poor emitters and poor absorbers of thermal radiation.
– Statement A is correct: Vacuum between walls minimizes conduction and convection; glass is a poor conductor.
– Statement B is correct: The glass walls have shiny surfaces to reduce radiation.
– Statement D is correct: Cork or plastic supports minimize conduction through the neck.
– Statement C is incorrect because the shiny inner wall surface is designed to radiate a *poor* amount (low emissivity) of heat, not a “good amount”. While the outer wall would absorb some radiated heat, the primary mechanism to reduce radiative transfer is the low emissivity/absorptivity of the surfaces.

Heat transfer in a thermos flask is minimized through several features: vacuum (prevents conduction/convection), silvered/shiny surfaces (reduces radiation), poor conducting stopper (prevents conduction/convection), and supports made of insulating material (prevents conduction). The effectiveness relies on minimizing all three modes of heat transfer.

45. Thermal capacity of a body depends on the

Thermal capacity of a body depends on the

mass of the body only

mass and shape of the body only

density of the body

mass, shape and temperature of the body

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UPSC NDA-1 – 2018

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The correct answer is A.

Thermal capacity (or heat capacity) of a body is the amount of heat energy required to raise its temperature by one degree Celsius (or Kelvin). It is an extensive property, meaning it is proportional to the amount of substance. Specifically, Thermal Capacity (C) = mass (m) × specific heat capacity (c). The specific heat capacity (c) is an intensive property that depends on the material of the body.
Looking at the options:
A) mass of the body only: While incomplete as it doesn’t mention the material (specific heat capacity), mass is a fundamental determinant.
B) mass and shape of the body only: Shape does not affect thermal capacity. Incorrect.
C) density of the body: Density is mass per unit volume. Thermal capacity depends on total mass, not density directly unless volume is fixed and material is implied. Incorrect.
D) mass, shape and temperature of the body: Shape is incorrect. While specific heat capacity can vary with temperature, “temperature of the body” in this context is ambiguous and shape is explicitly wrong. Incorrect.
Given the options, A is the best fit as mass is a primary factor, even though the material’s specific heat capacity is also crucial. The question asks what it “depends on” among the options provided. Mass is the only consistently correct factor listed without incorrect additions in options B, C, and D. This suggests the question focuses on the extensive nature of thermal capacity.

The specific heat capacity ‘c’ is a property of the material and represents the heat capacity per unit mass. The thermal capacity ‘C’ of a body is the total heat capacity for that specific body, which is the product of its mass and the specific heat capacity of the material it’s made from.

46. Which of the following statements about specific heat of a body is/are

Which of the following statements about specific heat of a body is/are correct ?

1. It depends upon mass and shape of the body
2. It is independent of mass and shape of the body
3. It depends only upon the temperature of the body

Select the correct answer using the code given below :

1 only

2 and 3

1 and 3

2 only

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UPSC NDA-1 – 2018

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The correct answer is (D) 2 only. Specific heat capacity is an intrinsic property of the material itself. It is defined as the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius (or Kelvin). Therefore, it is independent of the mass and the shape of the *specific body* made from that substance. While specific heat capacity can vary slightly with temperature and pressure for a given substance, statement 3 claiming it depends *only* upon the temperature is inaccurate, as the fundamental material composition is the primary determinant. Statement 1 is definitively incorrect. Statement 2 correctly states that it is independent of mass and shape of the body.

Specific heat capacity is an intensive property of a substance, meaning it does not depend on the amount (mass) or form (shape) of the substance.

Heat capacity (C), in contrast to specific heat capacity (c), *does* depend on the mass of the body (C = mc). Specific heat capacity is a fundamental property used to compare how different substances store thermal energy. For example, water has a high specific heat capacity compared to metals.

47. Which of the following statements about latent heat for a given substa

Which of the following statements about latent heat for a given substance is/are correct ?

1. It is fixed at a given temperature.
2. It depends upon the temperature and volume.
3. It is independent of temperature and volume.
4. It depends on the temperature but independent of volume.

Select the correct answer using the code given below :

1 and 3

4 only

1 and 4

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UPSC NDA-1 – 2018

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Statement 1 is correct: For a specific substance at a given pressure, the latent heat associated with a phase transition (like melting or boiling) is fixed at the transition temperature. Statement 4 is also considered correct in the context that the value of latent heat is characteristic of the specific transition temperature, though the phrasing “depends on the temperature” can be ambiguous.

– Latent heat is the energy absorbed or released during a phase change at constant temperature and pressure.
– Statement 1 accurately reflects that for a specific phase transition at standard conditions, the latent heat value is constant at the transition temperature.
– While the phase change occurs *at* a constant temperature, the *value* of the latent heat can technically vary with the pressure, which in turn affects the transition temperature. Statement 4 might be interpreting “depends on the temperature” as being specific to the transition temperature value, and “independent of volume” in the sense that the volume of the substance isn’t a variable determining the latent heat value itself, unlike say, specific heat capacity which can be defined at constant volume.

Statement 2 is incorrect as volume is not a primary variable defining latent heat. Statement 3 is incorrect as latent heat is fundamentally linked to the transition temperature. Given the options, 1 and 4 are the most plausible correct statements, despite the awkward phrasing of 4. This suggests that the question setter considered both 1 and 4 as correct representations, with 4 perhaps indicating dependence on the *specific transition temperature*.

48. Which one of the following statements is correct ?

Which one of the following statements is correct ?

Any energy transfer that does not involve temperature difference in some way is not heat

Any energy transfer always requires a temperature difference

On heating the length and volume of the object remain exactly the same

Whenever there is a temperature difference, heat is the only way of energy transfer

This question was previously asked in

UPSC NDA-1 – 2018

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This question is about the definition and characteristics of heat and energy transfer in physics. Heat is a specific mode of energy transfer.

Let’s analyze each statement:
A) Any energy transfer that does not involve temperature difference in some way is not heat: This statement is correct. Heat is defined as energy transferred from a hotter object to a colder object due to a temperature difference. Energy can also be transferred as work (e.g., mechanical work), which does not require a temperature difference. Therefore, energy transfer that doesn’t involve a temperature difference is not heat.
B) Any energy transfer always requires a temperature difference: This is incorrect. Energy can be transferred as work (mechanical, electrical, etc.), which does not require a temperature difference.
C) On heating the length and volume of the object remain exactly the same: This is incorrect. When most objects are heated, they undergo thermal expansion, meaning their length, area, and volume increase (though some exceptions exist like water between 0°C and 4°C).
D) Whenever there is a temperature difference, heat is the only way of energy transfer: This is incorrect. While a temperature difference causes heat transfer (conduction, convection, radiation), other forms of energy transfer, such as work done on or by the system, can also occur simultaneously, even if there is a temperature difference.

The First Law of Thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system ($\Delta U = Q – W$). This equation clearly distinguishes between heat (Q) and work (W) as two primary modes of energy transfer. Heat is driven by temperature difference, while work is driven by forces and displacement.

49. Why is it difficult to measure the coefficient of expansion of a liqui

Why is it difficult to measure the coefficient of expansion of a liquid than solid ?

Liquids tend to evaporate at all temperatures

Liquids conduct more heat

Liquids expand too much when heated

Their containers also expand when heated

This question was previously asked in

UPSC NDA-1 – 2017

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The question asks why it is difficult to measure the coefficient of expansion of a liquid compared to a solid.

When measuring the volume expansion of a liquid, the liquid must be held in a container. When the liquid is heated, the container also gets heated and expands. The observed increase in the volume of the liquid (apparent expansion) is the difference between the actual increase in volume of the liquid (real expansion) and the increase in volume of the container.
$V_{observed} = V_{real, liquid} – V_{expansion, container}$
To find the real coefficient of volume expansion of the liquid, one needs to account for the expansion of the container, which itself has a coefficient of volume expansion (or linear expansion, from which volume expansion can be derived). This adds complexity to the measurement process compared to measuring the expansion of a solid rod or block, where the change in length or volume is directly measured.
Option D correctly identifies this key difficulty: the container’s expansion must be factored in.

Liquids have two coefficients of volume expansion: the coefficient of apparent expansion and the coefficient of real expansion. The coefficient of real expansion of the liquid is equal to the coefficient of apparent expansion of the liquid plus the coefficient of volume expansion of the container material. While liquids generally expand more than solids, the main difficulty in measurement lies in the experimental setup requiring a container that also expands. Evaporation (option A) can also make measurements difficult, especially over long periods or at higher temperatures, but the fundamental challenge inherent to volume expansion measurement of liquids involves the container.

50. 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. What would be the corresponding reading in a Celsius thermometer?

574

301

273

232

This question was previously asked in

UPSC NDA-1 – 2017

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The question states that a Kelvin thermometer and a Fahrenheit thermometer give the same reading for a certain sample and asks for the corresponding reading in Celsius.

Let the reading on both the Kelvin and Fahrenheit scales be $x$.
The conversion formula from Celsius ($T_C$) to Kelvin ($T_K$) is $T_K = T_C + 273.15$. For many problems, 273 is used as an approximation, but using 273.15 gives a more precise answer.
The conversion formula from Celsius ($T_C$) to Fahrenheit ($T_F$) is $T_F = \frac{9}{5} T_C + 32$.
We are given $T_K = T_F = x$. So, we have two equations:
1) $x = T_C + 273.15$
2) $x = \frac{9}{5} T_C + 32$
Set the two expressions for $x$ equal to each other:
$T_C + 273.15 = \frac{9}{5} T_C + 32$
$273.15 – 32 = \frac{9}{5} T_C – T_C$
$241.15 = (\frac{9}{5} – 1) T_C$
$241.15 = (\frac{9-5}{5}) T_C$
$241.15 = \frac{4}{5} T_C$
$T_C = \frac{241.15 \times 5}{4} = \frac{1205.75}{4} = 301.4375$
The question asks for the reading in a Celsius thermometer. The calculated value is approximately 301.44. Among the given options, 301 is the closest value.

If we use the approximation $T_K = T_C + 273$, the calculation becomes:
$T_C + 273 = \frac{9}{5} T_C + 32$
$273 – 32 = \frac{4}{5} T_C$
$241 = \frac{4}{5} T_C$
$T_C = \frac{241 \times 5}{4} = \frac{1205}{4} = 301.25$
This value is also very close to 301. This confirms that 301 is the most likely intended answer, allowing for slight rounding or the use of an approximation in the original question setting.