Unit # 7 (Heat and Thermodynamics)
Exercise (Multiple Choice Questions)
1. a 2. d 3.a 4.c 5. c 6.b 7.b
8. a 9. b / c 10.c 11.a ? 12. c 13.d 14.a
15. c 16.b 17.c 18.b 19.b 20.a 21.b
Short Questions
8.1 In 60 oF walking on the ground weather feels comfortable, but swimming in the ocean feels very cold. Explain
Answer: At 60°F, air feels nice because our bodies stay warm. But in water at the same temperature, heat is pulled
away from us faster water being a better conductor than air, making it feel much colder. This is because water conducts
heat better than air, so we lose heat quickly while swimming.
8.2 Do you think low temperature is the only reason of snowfall on the mountain?
Answer: Snowfall on mountains apart from temperature also depends heavily on moisture. When warm, moist air rises
over mountains, it cools and condenses, forming clouds. As the air continues to rise, it cools further, leading to
precipitation in the form of snow. So, without sufficient moisture in the atmosphere, even low temperatures wouldn’t
result in snowfall.
8.3 Can the temperature of an isolated system change? Explain.
Answer: In an isolated system, the temperature remains constant because there’s no exchange of heat or matter with
the outside environment. This means the total energy of the system is conserved. Any internal processes, like energy
transfers between particles, don’t affect the overall temperature, as the system doesn’t gain or lose energy. Therefore,
while individual particles might have different energies, the average temperature of the entire system stays the same.
8.4 As a result of some process the internal energy of system is increased. How can one tell that the increase was
due to orderly macroscopic work W or due to the flow of disorderly microscopic energy Q?
Answer: To determine whether the increase in internal energy of a system is due to orderly macroscopic work (W) or
disorderly microscopic energy flow (Q), you can analyze the nature of the energy input:
1. Work (W): If the energy increase results from forces acting on the system in a structured manner, such as compression
or expansion against a piston, it’s due to work. This is typically measured as a mechanical action. There is a decrease in
system volume.
2. Heat (Q): If the energy increase comes from thermal interactions, like heat flowing into the system from a hotter
environment, it’s due to heat transfer. This is usually indicated by temperature changes or thermal conduction. The
temperature of the surrounding goes down.
By measuring the system’s temperature change and observing the type of energy transfer involved, you can identify
whether the increase in internal energy came from work or heat.
8.5 At what temperature the volume of the gas at 0 oC becomes double of its volume, while pressure remains
constant?
Answer: Using Charles law V/T=constant as P is constant
V1/T1=V2/T2 or V1/T1=2 (V1/T2)
T2=2T1 =2(237.15) =546.3K =273.15 oC
8.6 Work done on system puts energy into it. Work done by system removes energy from it. Give examples.
Answer: Work Done on a System:
1. Piston Compression: Work is done on the gas, increasing its internal energy and temperature (first law of
thermodynamics).
2. Heating a Gas: Adding heat to a gas while compressing it increases its energy.
Work Done by a System:
1. Expanding Gas: The gas does work on the piston, losing internal energy and cooling down.
2. Heat Engine: In a heat engine, work is done by the system, converting thermal energy to mechanical energy, decreasing
internal energy.
8.7 Should the internal energy of the system necessarily increase if heat is added to it?
Answer: No, the internal energy of a system doesn’t always increase when heat is added. According to the first law of
thermodynamics, the added heat can either increase internal energy or be used to do work by the system. For example, if
the system expands and does work on the surroundings, some of the heat goes into that, not just increasing internal
energy. Just as in isothermal process When T is constant internal energy also does not change.
8.8 Why the slope of adiabatic curve is steeper that the slope of isothermal curve?
Answer: The slope of the adiabatic curve is steeper than the isothermal curve because, in an adiabatic process, no
heat is exchanged with the surroundings. So, any work done by the system comes directly from its internal energy,
causing a faster drop in temperature and pressure. In contrast, in an isothermal process, heat is exchanged to keep the
temperature constant, making the pressure drop more gradual.
, 8.9 What are the limitations of first law of thermodynamics?
Answer: The limitations of the first law of thermodynamics are:
1. It does not explain the direction of heat flow; it only deals with the quantity of energy.
2. It cannot determine how much of the energy is usable or how efficiently it can be converted to work. In actual heat
cannot be completely converted into work.
3. The first law does not give information about the spontaneity of processes.
4. It cannot explain why some energy transformations are irreversible. It does not tell whether the process is actually
possible or not.
8.10 What are the conditions of isothermal process?
Answer: The conditions for an isothermal process are:
1. Constant Temperature: The system's temperature remains unchanged throughout the process.
2. Slow Process: The process occurs slowly, allowing the system to stay in thermal equilibrium with its surroundings.
3. Perfectly Conducting Walls: The system's walls must be perfectly conducting to ensure efficient heat exchange with the
surroundings, maintaining constant temperature.
8.11 Why isothermal process are slow and adiabatic process are fast?
Answer: Isothermal processes are slow because the system needs time to exchange heat with the surroundings to
maintain a constant temperature. As the system does work, heat must flow in or out gradually to keep the temperature
steady, requiring the process to occur slowly. Adiabatic processes are fast because there is no heat exchange with the
surroundings. The system is thermally insulated, so any change in volume or pressure happens quickly, affecting the
temperature directly without the need for slow heat transfer.
8.12 Can a process be both isothermal and adiabatic? Explain.
Answer: No, a process cannot be both isothermal and adiabatic.
- In an isothermal process, the temperature remains constant, requiring heat exchange with the surroundings to balance
any work done by or on the system.
- In an adiabatic process, no heat is exchanged with the surroundings, meaning any work done directly changes the
system's temperature.
Since an isothermal process requires heat exchange and an adiabatic process does not, they are mutually exclusive.
8.13 Does the entropy increase for a Carnot engine for each cycle?
Answer: No, the entropy does not increase for a Carnot engine in each cycle. In a Carnot engine, the process is
completely reversible, which means that the total entropy change of the system and surroundings over a full cycle is zero.
While heat is transferred between the hot and cold reservoirs, the entropy increase in the cold reservoir is exactly balanced
by the entropy decrease in the hot reservoir, keeping the overall entropy unchanged.
8.14 Why in practical terms efficiency is defined as W/Q?
Answer: In practical terms, efficiency is defined as W/Q because it measures how effectively an engine converts input
energy into useful work. Here, W represents the useful work output, and Q is the heat input from the energy source. This
ratio indicates the proportion of energy converted into work compared to the total energy supplied, allowing for a
straightforward assessment of performance. A higher efficiency means more of the input energy is transformed into useful
work rather than being wasted as heat.
8.15 Does entropy of a system increase or decrease due to friction?
Answer: The entropy of a system increases due to friction. When friction occurs, it converts mechanical energy into
thermal energy, generating heat. This process disperses energy and increases disorder within the system, resulting in a rise
in entropy. Therefore, friction contributes to a net increase in the overall entropy of the system and its surroundings.
8.16 What are the similarities and dissimilarities in working of a refrigerator and air conditioner?
Answer: Similarities:
1. Both utilize a refrigeration cycle to transfer heat from a low-temperature area to a high-temperature .Both operate
on the same basic principle of refrigeration cycles, using a refrigerant to absorb and release heat.
2. Both remove heat from a designated area (inside the fridge or a room) and release it to the surroundings, thereby
cooling the specified area.
Dissimilarities: 1. Purpose: Refrigerators are designed to keep food and beverages cool, while air conditioners aim to
create a comfortable indoor environment.
2. Evaporation: In refrigerators, evaporation occurs inside the refrigerator compartment, while in air conditioners, the
evaporator is located indoors, and the condenser is outside.
8.17 What is meant by reverse entropy? And what is the significance of entropy? Explain.
Answer: Reverse entropy refers to processes that seem to decrease disorder locally but do not violate the second law
of thermodynamics, as the total entropy of the universe still increases. It is a theoretical situation contradicting the second
law of thermodynamics. The significance of entropy includes:
1. It sets limitations on efficiency of heat engines and refrigerators.
2. It also links the concept of available energy .Lower entropy more the useful energy
3. It helps predict feasibility of reactions and processes.
Exercise (Multiple Choice Questions)
1. a 2. d 3.a 4.c 5. c 6.b 7.b
8. a 9. b / c 10.c 11.a ? 12. c 13.d 14.a
15. c 16.b 17.c 18.b 19.b 20.a 21.b
Short Questions
8.1 In 60 oF walking on the ground weather feels comfortable, but swimming in the ocean feels very cold. Explain
Answer: At 60°F, air feels nice because our bodies stay warm. But in water at the same temperature, heat is pulled
away from us faster water being a better conductor than air, making it feel much colder. This is because water conducts
heat better than air, so we lose heat quickly while swimming.
8.2 Do you think low temperature is the only reason of snowfall on the mountain?
Answer: Snowfall on mountains apart from temperature also depends heavily on moisture. When warm, moist air rises
over mountains, it cools and condenses, forming clouds. As the air continues to rise, it cools further, leading to
precipitation in the form of snow. So, without sufficient moisture in the atmosphere, even low temperatures wouldn’t
result in snowfall.
8.3 Can the temperature of an isolated system change? Explain.
Answer: In an isolated system, the temperature remains constant because there’s no exchange of heat or matter with
the outside environment. This means the total energy of the system is conserved. Any internal processes, like energy
transfers between particles, don’t affect the overall temperature, as the system doesn’t gain or lose energy. Therefore,
while individual particles might have different energies, the average temperature of the entire system stays the same.
8.4 As a result of some process the internal energy of system is increased. How can one tell that the increase was
due to orderly macroscopic work W or due to the flow of disorderly microscopic energy Q?
Answer: To determine whether the increase in internal energy of a system is due to orderly macroscopic work (W) or
disorderly microscopic energy flow (Q), you can analyze the nature of the energy input:
1. Work (W): If the energy increase results from forces acting on the system in a structured manner, such as compression
or expansion against a piston, it’s due to work. This is typically measured as a mechanical action. There is a decrease in
system volume.
2. Heat (Q): If the energy increase comes from thermal interactions, like heat flowing into the system from a hotter
environment, it’s due to heat transfer. This is usually indicated by temperature changes or thermal conduction. The
temperature of the surrounding goes down.
By measuring the system’s temperature change and observing the type of energy transfer involved, you can identify
whether the increase in internal energy came from work or heat.
8.5 At what temperature the volume of the gas at 0 oC becomes double of its volume, while pressure remains
constant?
Answer: Using Charles law V/T=constant as P is constant
V1/T1=V2/T2 or V1/T1=2 (V1/T2)
T2=2T1 =2(237.15) =546.3K =273.15 oC
8.6 Work done on system puts energy into it. Work done by system removes energy from it. Give examples.
Answer: Work Done on a System:
1. Piston Compression: Work is done on the gas, increasing its internal energy and temperature (first law of
thermodynamics).
2. Heating a Gas: Adding heat to a gas while compressing it increases its energy.
Work Done by a System:
1. Expanding Gas: The gas does work on the piston, losing internal energy and cooling down.
2. Heat Engine: In a heat engine, work is done by the system, converting thermal energy to mechanical energy, decreasing
internal energy.
8.7 Should the internal energy of the system necessarily increase if heat is added to it?
Answer: No, the internal energy of a system doesn’t always increase when heat is added. According to the first law of
thermodynamics, the added heat can either increase internal energy or be used to do work by the system. For example, if
the system expands and does work on the surroundings, some of the heat goes into that, not just increasing internal
energy. Just as in isothermal process When T is constant internal energy also does not change.
8.8 Why the slope of adiabatic curve is steeper that the slope of isothermal curve?
Answer: The slope of the adiabatic curve is steeper than the isothermal curve because, in an adiabatic process, no
heat is exchanged with the surroundings. So, any work done by the system comes directly from its internal energy,
causing a faster drop in temperature and pressure. In contrast, in an isothermal process, heat is exchanged to keep the
temperature constant, making the pressure drop more gradual.
, 8.9 What are the limitations of first law of thermodynamics?
Answer: The limitations of the first law of thermodynamics are:
1. It does not explain the direction of heat flow; it only deals with the quantity of energy.
2. It cannot determine how much of the energy is usable or how efficiently it can be converted to work. In actual heat
cannot be completely converted into work.
3. The first law does not give information about the spontaneity of processes.
4. It cannot explain why some energy transformations are irreversible. It does not tell whether the process is actually
possible or not.
8.10 What are the conditions of isothermal process?
Answer: The conditions for an isothermal process are:
1. Constant Temperature: The system's temperature remains unchanged throughout the process.
2. Slow Process: The process occurs slowly, allowing the system to stay in thermal equilibrium with its surroundings.
3. Perfectly Conducting Walls: The system's walls must be perfectly conducting to ensure efficient heat exchange with the
surroundings, maintaining constant temperature.
8.11 Why isothermal process are slow and adiabatic process are fast?
Answer: Isothermal processes are slow because the system needs time to exchange heat with the surroundings to
maintain a constant temperature. As the system does work, heat must flow in or out gradually to keep the temperature
steady, requiring the process to occur slowly. Adiabatic processes are fast because there is no heat exchange with the
surroundings. The system is thermally insulated, so any change in volume or pressure happens quickly, affecting the
temperature directly without the need for slow heat transfer.
8.12 Can a process be both isothermal and adiabatic? Explain.
Answer: No, a process cannot be both isothermal and adiabatic.
- In an isothermal process, the temperature remains constant, requiring heat exchange with the surroundings to balance
any work done by or on the system.
- In an adiabatic process, no heat is exchanged with the surroundings, meaning any work done directly changes the
system's temperature.
Since an isothermal process requires heat exchange and an adiabatic process does not, they are mutually exclusive.
8.13 Does the entropy increase for a Carnot engine for each cycle?
Answer: No, the entropy does not increase for a Carnot engine in each cycle. In a Carnot engine, the process is
completely reversible, which means that the total entropy change of the system and surroundings over a full cycle is zero.
While heat is transferred between the hot and cold reservoirs, the entropy increase in the cold reservoir is exactly balanced
by the entropy decrease in the hot reservoir, keeping the overall entropy unchanged.
8.14 Why in practical terms efficiency is defined as W/Q?
Answer: In practical terms, efficiency is defined as W/Q because it measures how effectively an engine converts input
energy into useful work. Here, W represents the useful work output, and Q is the heat input from the energy source. This
ratio indicates the proportion of energy converted into work compared to the total energy supplied, allowing for a
straightforward assessment of performance. A higher efficiency means more of the input energy is transformed into useful
work rather than being wasted as heat.
8.15 Does entropy of a system increase or decrease due to friction?
Answer: The entropy of a system increases due to friction. When friction occurs, it converts mechanical energy into
thermal energy, generating heat. This process disperses energy and increases disorder within the system, resulting in a rise
in entropy. Therefore, friction contributes to a net increase in the overall entropy of the system and its surroundings.
8.16 What are the similarities and dissimilarities in working of a refrigerator and air conditioner?
Answer: Similarities:
1. Both utilize a refrigeration cycle to transfer heat from a low-temperature area to a high-temperature .Both operate
on the same basic principle of refrigeration cycles, using a refrigerant to absorb and release heat.
2. Both remove heat from a designated area (inside the fridge or a room) and release it to the surroundings, thereby
cooling the specified area.
Dissimilarities: 1. Purpose: Refrigerators are designed to keep food and beverages cool, while air conditioners aim to
create a comfortable indoor environment.
2. Evaporation: In refrigerators, evaporation occurs inside the refrigerator compartment, while in air conditioners, the
evaporator is located indoors, and the condenser is outside.
8.17 What is meant by reverse entropy? And what is the significance of entropy? Explain.
Answer: Reverse entropy refers to processes that seem to decrease disorder locally but do not violate the second law
of thermodynamics, as the total entropy of the universe still increases. It is a theoretical situation contradicting the second
law of thermodynamics. The significance of entropy includes:
1. It sets limitations on efficiency of heat engines and refrigerators.
2. It also links the concept of available energy .Lower entropy more the useful energy
3. It helps predict feasibility of reactions and processes.
