Measurement of Temperature and Heat, Change of State and Latent Heat, Modes of Heat Transfer
Measurement of Temperature and Heat, Change of State and Latent Heat, Modes of Heat Transfer MCQ Questions
7.
What is the SI unit of thermal conductivity?
C.
Watt per meter per Kelvin
ANSWER :
C. Watt per meter per Kelvin
Approach
To determine the SI unit of thermal conductivity, we need to recall its definition and the formula associated with heat transfer through conduction. Thermal conductivity quantifies a material's ability to conduct heat.
Step-by-step
Recall the formula for heat transfer by conduction: The rate of heat transfer (\(Q/t\)) through a material by conduction is given by Fourier's Law:
\[ \frac{Q}{t} = -k A \frac{dT}{dx} \] Where:
- \(Q\) is the amount of heat transferred (in Joules, J)
- \(t\) is the time taken (in seconds, s)
- \(Q/t\) is the rate of heat transfer or power (in Watts, W)
- \(k\) is the thermal conductivity of the material
- \(A\) is the cross-sectional area through which heat is transferred (in square meters, \(\text{m}^2\))
- \(dT\) is the temperature difference (in Kelvin, K)
- \(dx\) is the thickness of the material or distance over which the temperature difference occurs (in meters, m)
- The negative sign indicates that heat flows from higher to lower temperature.
Rearrange the formula to solve for \(k\):
\[ k = -\frac{(Q/t)}{A (dT/dx)} \]Substitute the SI units for each term:
- \(Q/t\) is power, so its unit is Watts (W).
- \(A\) is area, so its unit is square meters (\(\text{m}^2\)).
- \(dT/dx\) is temperature gradient, so its unit is Kelvin per meter (\(\text{K/m}\)).
Calculate the unit of \(k\):
\[ \text{Unit of } k = \frac{\text{W}}{\text{m}^2 \cdot (\text{K/m})} = \frac{\text{W}}{\text{m}^2 \cdot \text{K}} \cdot \text{m} = \frac{\text{W}}{\text{m} \cdot \text{K}} \] So, the SI unit of thermal conductivity is Watt per meter per Kelvin.
Correct Option:
C) Watt per meter per Kelvin (\(\text{W/(m}\cdot\text{K)}\)) is the correct SI unit for thermal conductivity, as derived from Fourier's Law of heat conduction.
Incorrect Options
A) Joule (J) is the SI unit of energy or work. It represents the amount of heat transferred, not the rate or the material's ability to conduct it.
B) Watt (W) is the SI unit of power, which is the rate of energy transfer (Joule per second). It represents the rate of heat transfer (\(Q/t\)), but not thermal conductivity itself.
D) Kelvin (K) is the SI unit of thermodynamic temperature. It represents temperature or temperature difference, but not thermal conductivity.
8.
What happens to the volume of a substance when it is heated?
Approach
This question asks about the effect of heating on the volume of a substance. We need to recall the general principle of thermal expansion, which describes how most materials change in size when their temperature changes.
Step-by-step
- Understanding Thermal Expansion: When a substance is heated, its constituent particles (atoms or molecules) gain kinetic energy.
- Increased Particle Motion: With increased kinetic energy, these particles vibrate more vigorously and move further apart from each other.
- Increased Inter-particle Distance: As the average distance between particles increases, the overall space occupied by the substance expands.
- Definition of Volume: Volume is the amount of space a substance occupies. Therefore, an increase in the average distance between particles leads to an increase in the substance's volume.
- General Rule: This phenomenon is known as thermal expansion and is a general characteristic of most substances (solids, liquids, and gases) when heated. There are a few exceptions, like water between \(0^\circ\text{C}\) and \(4^\circ\text{C}\), but the general rule is expansion.
Correct Option: C) It increases
When a substance is heated, its particles gain kinetic energy, vibrate more, and move further apart, causing the substance to expand and its volume to increase. This is known as thermal expansion.
Incorrect Options
- A) It decreases — This is generally incorrect. Volume usually decreases when a substance is cooled, not heated.
- B) It remains constant — This is incorrect. Most substances undergo a change in volume when heated or cooled. Only under very specific conditions or for theoretical ideal materials might volume remain constant.
- D) It fluctuates — While there might be microscopic fluctuations in particle positions, the macroscopic volume change due to heating is a consistent increase, not a fluctuation.
9.
What is the relation between Celsius and Kelvin scales?
ANSWER :
C. K = °C + 273.15
Approach
This question asks for the correct conversion formula between the Celsius and Kelvin temperature scales. Understanding the definition of each scale and their reference points is key to deriving or recalling the relationship.
Step-by-step
- Understand the Celsius Scale: The Celsius scale is a centigrade scale, meaning there are 100 degrees between the freezing point and boiling point of water at standard atmospheric pressure. The freezing point of water is defined as \(0^\circ\text{C}\) and the boiling point as \(100^\circ\text{C}\).
- Understand the Kelvin Scale: The Kelvin scale is an absolute temperature scale. Its zero point, \(0\text{ K}\), is defined as absolute zero, the theoretical temperature at which all thermal motion ceases. The size of one Kelvin unit is equal to the size of one degree Celsius.
- Relate the Scales: The freezing point of water (\(0^\circ\text{C}\)) corresponds to \(273.15\text{ K}\) on the Kelvin scale. This means that to convert a temperature from Celsius to Kelvin, you need to add \(273.15\) to the Celsius value.
- Formulate the Conversion: If \(T_C\) represents the temperature in Celsius and \(T_K\) represents the temperature in Kelvin, the relationship is:\[ T_K = T_C + 273.15 \]
Correct Option:
C) K = °C + 273.15 — This formula correctly states that to convert a temperature from Celsius to Kelvin, you add 273.15 to the Celsius temperature. For example, \(0^\circ\text{C}\) is \(0 + 273.15 = 273.15\text{ K}\).
Incorrect Options
- A) °C = K - 273.15 — This is the correct formula for converting Kelvin to Celsius, not Celsius to Kelvin. It is the inverse of the correct relationship.
- B) K = °C - 273.15 — This formula would imply that \(0^\circ\text{C}\) is \(-273.15\text{ K}\), which is incorrect. Kelvin temperatures cannot be negative.
- D) °C = K + 273.15 — This formula is incorrect. It suggests that Kelvin is lower than Celsius by 273.15, which is the opposite of the actual relationship.
10.
The heat released when a gas condenses into a liquid is known as
A.
Latent heat of vaporization
B.
Latent heat of condensation
D.
Latent heat of sublimation
ANSWER :
B. Latent heat of condensation
Approach
This question tests understanding of phase changes and the associated latent heats. Latent heat refers to the energy absorbed or released during a phase transition without a change in temperature. We need to identify the specific term for the heat released when a gas changes into a liquid.
Step-by-step
- Understanding Phase Changes: Matter can exist in different phases: solid, liquid, and gas. Transitions between these phases involve the absorption or release of energy.
- Condensation Process: Condensation is the process where a gas changes into a liquid. For this to happen, the gas molecules must lose energy, typically in the form of heat, to slow down and come closer together to form a liquid.
- Latent Heat: The energy involved in a phase change at a constant temperature is called latent heat. It's "latent" because it doesn't cause a temperature change but rather a change in the state of matter.
- Identifying the Specific Latent Heat: When a gas condenses into a liquid, it releases heat. This specific amount of heat released per unit mass during condensation is known as the latent heat of condensation. It is numerically equal to the latent heat of vaporization, but the process is reversed (release vs. absorption).
Correct Option:
B) Latent heat of condensation is the correct term for the heat released when a gas condenses into a liquid. This process involves the release of energy as the gas molecules lose kinetic energy and form intermolecular bonds to become a liquid.
Incorrect Options
- A) Latent heat of vaporization: This is the heat absorbed when a liquid changes into a gas (vaporization or boiling). It's the opposite process of condensation, where heat is absorbed, not released.
- C) Latent heat of fusion: This is the heat absorbed when a solid changes into a liquid (melting) or released when a liquid changes into a solid (freezing). This process involves solid-liquid transitions, not gas-liquid.
- D) Latent heat of sublimation: This is the heat absorbed when a solid changes directly into a gas without passing through the liquid phase (sublimation) or released when a gas changes directly into a solid (deposition). This process does not involve the liquid phase as an intermediate step.
11.
The heat required to convert a solid into a liquid is known as
B.
Latent heat of vaporization
C.
Latent heat of sublimation
D.
Latent heat of deposition
ANSWER :
A. Latent heat of fusion
Approach
This question asks about the specific term for the heat energy required to change a substance from a solid to a liquid state without a change in temperature. This concept is fundamental in thermodynamics and phase transitions.
Step-by-step
- When a substance changes its state from solid to liquid, it absorbs heat energy. This absorbed energy is used to overcome the intermolecular forces holding the particles in a fixed solid structure, allowing them to move more freely as a liquid.
- Crucially, during this phase change, the temperature of the substance remains constant until the entire solid has converted into a liquid. The heat absorbed during this process is called latent heat.
- The specific type of latent heat depends on the phase transition occurring. For a solid to liquid transition (melting), it is known as the latent heat of fusion.
Correct Option: A) Latent heat of fusion is the heat energy absorbed when a substance changes from a solid to a liquid state at a constant temperature (its melting point).
Incorrect Options
- B) Latent heat of vaporization refers to the heat energy absorbed when a substance changes from a liquid to a gaseous state at a constant temperature (its boiling point).
- C) Latent heat of sublimation refers to the heat energy absorbed when a substance changes directly from a solid to a gaseous state without passing through the liquid phase.
- D) Latent heat of deposition (or desublimation) refers to the heat released when a substance changes directly from a gaseous state to a solid state.
12.
What happens to the temperature of a substance when it undergoes a phase change?
ANSWER :
A. It remains constant
Approach
The question asks about the behavior of a substance's temperature during a phase change. This is a fundamental concept in thermodynamics and states of matter. During a phase change (like melting, freezing, boiling, or condensation), the energy supplied or removed is used to change the state of matter, not to increase or decrease its kinetic energy, which is related to temperature.
Step-by-step
- Understanding Phase Change: A phase change is a physical process where a substance changes from one state of matter (solid, liquid, gas) to another. Examples include melting (solid to liquid), boiling (liquid to gas), freezing (liquid to solid), and condensation (gas to liquid).
- Role of Latent Heat: During a phase change, the energy absorbed or released by the substance is known as latent heat. This energy is used to break or form intermolecular bonds, allowing the substance to change its physical state. For instance, during melting, heat energy is absorbed to break the bonds holding the solid structure together, converting it into a liquid. During boiling, heat is absorbed to overcome intermolecular forces and allow molecules to escape into the gaseous phase.
- Temperature and Kinetic Energy: Temperature is a measure of the average kinetic energy of the particles within a substance. When heat is added to a substance and it's not undergoing a phase change, this heat increases the kinetic energy of the particles, leading to a rise in temperature.
- Temperature during Phase Change: However, during a phase change, the heat energy supplied (e.g., during melting or boiling) or removed (e.g., during freezing or condensation) is entirely used to change the state of the substance. It does not increase the average kinetic energy of the particles. Therefore, the temperature of the substance remains constant throughout the phase change. For example, water boils at \(100^\circ \text{C}\) (at standard atmospheric pressure), and as long as there is liquid water being converted to steam, the temperature of the boiling water and the steam being formed remains \(100^\circ \text{C}\). Similarly, ice melts at \(0^\circ \text{C}\), and the temperature of the ice-water mixture remains \(0^\circ \text{C}\) until all the ice has melted.
Correct Option
A) It remains constant — During a phase change, the heat energy supplied or removed is used to alter the intermolecular forces and change the state of matter (latent heat), rather than to increase or decrease the kinetic energy of the molecules. Consequently, the temperature of the substance does not change until the entire substance has completed the phase transition.
Incorrect Options
- B) It increases — The temperature increases when heat is added to a substance that is in a single phase (e.g., heating liquid water from \(20^\circ \text{C}\) to \(90^\circ \text{C}\)), but not during a phase change itself.
- C) It decreases — The temperature decreases when heat is removed from a substance that is in a single phase (e.g., cooling liquid water from \(90^\circ \text{C}\) to \(20^\circ \text{C}\)), but not during a phase change itself.
- D) It fluctuates — The temperature remains stable and constant during a phase change, assuming a constant pressure and a pure substance. Fluctuations are not characteristic of a phase change under ideal conditions.
