1. Thermodynamics is most directly concerned with the relation among which physical ideas?
ⓐ. Light rays, mirrors, and lenses
ⓑ. Electric charge, magnetic poles, and current
ⓒ. Heat, work, temperature, and energy transfer
ⓓ. Atomic number, valency, and chemical bonding
Correct Answer: Heat, work, temperature, and energy transfer
Explanation: Thermodynamics studies how energy is transferred as heat and work and how these transfers affect the state of a system. Temperature is central because heat transfer is linked with temperature difference. Work appears when energy crosses the boundary through macroscopic mechanical effects, such as a piston moving. Thermodynamic analysis does not begin by tracking every molecule separately; it mainly uses measurable quantities like \(P\), \(V\), and \(T\). Heat and work are both energy-transfer ideas, so they are measured in \(\text{J}\).
2. A gas sample is described by its pressure \(P\), volume \(V\), temperature \(T\), and amount \(n\), without listing the position and velocity of each molecule. This is an example of a
ⓐ. macroscopic description
ⓑ. nuclear description
ⓒ. microscopic description
ⓓ. chemical-bond description
Correct Answer: macroscopic description
Explanation: A macroscopic description uses bulk measurable variables such as \(P\), \(V\), \(T\), and \(n\). It does not require the detailed motion of every molecule in the gas. Thermodynamics usually works with this macroscopic viewpoint because it describes the overall state of the system. A microscopic description belongs more naturally to kinetic theory, where molecular motion is considered directly. The same gas can have a thermodynamic description even when individual molecular details are not known.
3. A hot metal spoon is placed in colder water. The energy transfer from the spoon to the water takes place mainly because of
ⓐ. surface-area difference
ⓑ. shape difference
ⓒ. temperature difference
ⓓ. pressure difference
Correct Answer: temperature difference
Explanation: Heat is energy transferred because of a temperature difference. Since the spoon is hotter than the water, energy flows from the spoon to the water until thermal conditions become closer. Mass and shape may affect the rate or amount of energy involved, but they are not the basic cause of heat transfer. Colour is not the deciding thermodynamic condition in this situation. Heat should be understood as energy in transit across a boundary, not as a substance stored inside the spoon.
4. Gas in a cylinder pushes a light piston outward. In thermodynamic language, the work here is best described as
ⓐ. a measurement of temperature only
ⓑ. a count of the number of molecules in the gas
ⓒ. energy stored permanently as heat inside the gas
ⓓ. energy transfer through motion of the boundary
Correct Answer: energy transfer through motion of the boundary
Explanation: Work in thermodynamics is a mode of energy transfer caused by macroscopic force and displacement at the system boundary. In a piston-cylinder arrangement, the gas pushes the piston and the boundary moves outward. This outward motion allows the gas to transfer energy to the surroundings as work. Heat is a different mode of energy transfer and is caused by temperature difference. A moving boundary is the key feature that separates expansion work from simple temperature measurement.
5. Internal energy \(U\) of a gas refers mainly to
ⓐ. energy transferred as work
ⓑ. particle kinetic and potential energy
ⓒ. pressure-volume product only
ⓓ. energy crossing the system boundary as heat
Correct Answer: particle kinetic and potential energy
Explanation: Internal energy \(U\) is the energy associated with the microscopic motion and interactions of the particles of a system. It is not the same as heat, because heat describes energy transfer across a boundary due to temperature difference. It is also not the same as work, because work is energy transfer through macroscopic force and displacement. A gas can have internal energy even when no heat is currently entering or leaving it. The symbol \(U\) describes energy belonging to the state of the system, while \(Q\) and \(W\) describe transfers during a process.
6. Match the symbols with their usual basic meanings in thermodynamics.
| Symbol | Meaning |
|---|
| P. \(Q\) | 1. Work transfer |
| Q. \(W\) | 2. Temperature or thermal state variable |
| R. \(\Delta U\) | 3. Heat transfer |
| S. \(T\) | 4. Change in internal energy |
ⓐ. P-3, Q-1, R-2, S-4
ⓑ. P-3, Q-1, R-4, S-2
ⓒ. P-1, Q-3, R-4, S-2
ⓓ. P-4, Q-2, R-3, S-1
Correct Answer: P-3, Q-1, R-4, S-2
Explanation: The symbol \(Q\) is used for heat transfer, which occurs because of a temperature difference. The symbol \(W\) represents work transfer, usually associated with macroscopic boundary motion in a gas process. The symbol \(\Delta U\) means change in internal energy, not the absolute value of heat or work. The symbol \(T\) represents temperature, which describes the thermal state. Keeping these symbols separate is necessary because \(Q\), \(W\), and \(\Delta U\) do not mean the same physical thing.
7. Study the unit table and select the row that keeps the usual SI units consistent.
| Row | \(Q\) | \(W\) | \(P\) | \(V\) |
|---|
| P | \(\text{J}\) | \(\text{J}\) | \(\text{Pa}\) | \(\text{m}^3\) |
| Q | \(\text{K}\) | \(\text{J}\) | \(\text{Pa}\) | \(\text{m}\) |
| R | \(\text{J}\) | \(\text{K}\) | \(\text{m}^3\) | \(\text{Pa}\) |
| S | \(\text{Pa}\) | \(\text{m}^3\) | \(\text{J}\) | \(\text{K}\) |
ⓐ. Row Q
ⓑ. Row R
ⓒ. Row S
ⓓ. Row P
Correct Answer: Row P
Explanation: Heat \(Q\) and work \(W\) are both energy transfers, so their SI unit is \(\text{J}\). Pressure \(P\) is measured in \(\text{Pa}\), while volume \(V\) is measured in \(\text{m}^3\). Temperature is measured in \(\text{K}\), so it cannot replace the unit of heat or work. A volume unit must be cubic length, not just \(\text{m}\). This SI-unit separation is useful because thermodynamic formulas often combine \(P\), \(V\), \(Q\), and \(W\) in the same process.
8. A learner says, “A hot body contains a large amount of heat.” The most suitable thermodynamic response is:
ⓐ. Heat is energy transfer; internal energy is a state property.
ⓑ. Heat is measured in kelvin, while temperature is measured in joule.
ⓒ. Heat exists only when a gas boundary expands.
ⓓ. Heat and temperature name the same thermal property.
Correct Answer: Heat is energy transfer; internal energy is a state property.
Explanation: In thermodynamics, heat \(Q\) means energy transferred because of temperature difference. A body has internal energy \(U\), but heat is not treated as something simply stored inside it. Temperature \(T\) indicates thermal state and is measured in \(\text{K}\), not in \(\text{J}\). Heat can be transferred even when there is no gas expansion, such as when a hot object cools in contact with a colder object. The safer wording is that a hot body has internal energy and may transfer energy as heat to a colder body.
9. In a bicycle pump, the air inside often becomes warmer when it is compressed rapidly. A thermodynamic explanation is that
ⓐ. heat leaves the air and lowers its internal energy
ⓑ. pressure rise alone accounts for the warming
ⓒ. work done on the air raises its internal energy
ⓓ. compression stops molecular motion in the air
Correct Answer: work done on the air raises its internal energy
Explanation: During compression, the piston moves inward and does work on the air. This work is an energy transfer into the gas through the moving boundary. If the compression is rapid, there may be little time for much heat to escape during the process. The internal energy of the air can increase, and for a gas this is commonly observed as a rise in temperature. Pressure increase does not destroy energy; it is part of the changed thermodynamic state of the compressed air.
10. A pressure cooker is being discussed as a thermodynamic example. The steam inside it is most naturally described using quantities such as
ⓐ. temperature \(T\), magnetic field \(B\), and current \(I\)
ⓑ. volume \(V\), focal length \(f\), and refractive index \(n\)
ⓒ. pressure \(P\), charge \(q\), and current \(I\)
ⓓ. pressure \(P\), volume \(V\), and temperature \(T\)
Correct Answer: pressure \(P\), volume \(V\), and temperature \(T\)
Explanation: Steam inside a pressure cooker can be described using thermodynamic variables such as \(P\), \(V\), and \(T\). These variables tell us about the bulk state of the steam. Pressure is especially important in such a situation because the cooker is designed to operate at pressure above atmospheric pressure. Temperature is also important because heating changes the thermal state of the contents. The description is macroscopic because it avoids tracking every molecule of steam separately.
11. Heat \(Q\), work \(W\), and internal energy \(U\) share the feature that they are
ⓐ. path-independent state variables
ⓑ. temperature quantities measured in \(\text{K}\)
ⓒ. energy quantities measured in \(\text{J}\)
ⓓ. pressure quantities measured in \(\text{Pa}\)
Correct Answer: energy quantities measured in \(\text{J}\)
Explanation: Heat \(Q\) and work \(W\) are modes of energy transfer, while internal energy \(U\) is energy associated with the system. Because all three are energy-related quantities, they are measured in \(\text{J}\). They should not be confused with temperature \(T\), whose SI unit is \(\text{K}\). They are also not forms of pressure, since pressure has unit \(\text{Pa}\). The shared unit does not mean the meanings are identical; transfer quantities and state energy must still be separated.
12. The product \(P\Delta V\) uses pressure in \(\text{Pa}\) and volume change in \(\text{m}^3\). Its unit reduces to ______.
ⓐ. \(\text{J K}^{-1}\)
ⓑ. \(\text{K}\)
ⓒ. \(\text{J}\)
ⓓ. \(\text{Pa m}^{-3}\)
Correct Answer: \(\text{J}\)
Explanation: \( \textbf{Quantity involved:} \) The expression is \(P\Delta V\).
\( \textbf{Pressure unit:} \) \(1\,\text{Pa}=1\,\text{N m}^{-2}\).
\( \textbf{Volume-change unit:} \) \(\Delta V\) is measured in \(\text{m}^3\).
\( \textbf{Unit multiplication:} \)
\[
\text{Pa}\,\text{m}^3=(\text{N m}^{-2})(\text{m}^3)
\]
\( \textbf{Power simplification:} \)
\[
(\text{N m}^{-2})(\text{m}^3)=\text{N m}
\]
\( \textbf{Work unit link:} \) \(1\,\text{N m}=1\,\text{J}\).
\( \textbf{Result:} \) The unit of \(P\Delta V\) is \(\text{J}\).
\( \textbf{Physical meaning:} \) This is why pressure-volume area can represent work in thermodynamics.
\( \textbf{Final answer:} \) The missing unit is \(\text{J}\).
13. Read the situation below.
A gas enclosed in a cylinder is placed on a heater. The piston moves outward slowly while the gas expands against the outside pressure.
The most suitable description of the energy transfers is:
ⓐ. heat leaves the gas and work is done on it
ⓑ. heat is blocked and no boundary work occurs
ⓒ. heat enters the gas and the gas does work
ⓓ. internal energy becomes zero during expansion
Correct Answer: heat enters the gas and the gas does work
Explanation: The heater supplies energy to the gas because there is thermal interaction between the heater and the gas. As the piston moves outward, the gas pushes the boundary and transfers energy to the surroundings as work. These are two different transfer modes: heat due to temperature difference and work due to boundary displacement. The piston motion does not prevent heat transfer; both can occur in the same process. Internal energy need not become zero during expansion, because it depends on the state of the gas rather than on expansion alone.
14. A \(P\)-\(V\) graph is drawn with pressure \(P\) on the vertical axis and volume \(V\) on the horizontal axis. For a small strip, the area has the form \(P\Delta V\). What does this area represent in a volume-change process?
ⓐ. boundary work during the volume change
ⓑ. absolute temperature during the process
ⓒ. amount of gas in the sample
ⓓ. density change of the gas
Correct Answer: boundary work during the volume change
Explanation: On a \(P\)-\(V\) graph, a small rectangular strip under the curve has area \(P\Delta V\). The unit of this product is \(\text{Pa}\,\text{m}^3=\text{J}\), so it has the unit of work. During expansion, the volume increases and the gas does work on the surroundings under the usual school convention. During compression, the volume decreases and the work done by the gas is negative. The graph area is therefore linked with boundary work, not with temperature or amount of gas directly.
15. Consider the following statements.
I. Thermodynamics can describe a gas using variables such as \(P\), \(V\), and \(T\).
II. Thermodynamics always requires the velocity of every molecule to be known.
III. Molecular motion is considered more directly in the microscopic viewpoint.
ⓐ. I and III only
ⓑ. II and III only
ⓒ. I, II, and III
ⓓ. I only
Correct Answer: I and III only
Explanation: Statement I is true because thermodynamics uses macroscopic variables such as \(P\), \(V\), and \(T\). Statement II is not true because thermodynamics does not normally require the velocity of every molecule. Statement III is true because molecular-level motion belongs to the microscopic viewpoint, especially kinetic theory. A thermodynamic description can still be useful even when molecular details are unknown. This is why thermodynamics is powerful for gases, engines, refrigerators, and many bulk thermal processes.
16. A gas pushes a piston at a constant pressure of \(1.5\times10^5\,\text{Pa}\). Its volume increases from \(2.0\,\text{L}\) to \(5.0\,\text{L}\). What is the work done by the gas?
ⓐ. \(45\,\text{J}\)
ⓑ. \(450\,\text{J}\)
ⓒ. \(4.5\times10^5\,\text{J}\)
ⓓ. \(-450\,\text{J}\)
Correct Answer: \(450\,\text{J}\)
Explanation: \( \textbf{Given:} \) \(P=1.5\times10^5\,\text{Pa}\).
\( \textbf{Initial volume:} \) \(V_i=2.0\,\text{L}\).
\( \textbf{Final volume:} \) \(V_f=5.0\,\text{L}\).
\( \textbf{Unit conversion:} \) \(1\,\text{L}=10^{-3}\,\text{m}^3\).
\( \textbf{Volume change:} \)
\[
\Delta V=(5.0-2.0)\times10^{-3}\,\text{m}^3
\]
\[
\Delta V=3.0\times10^{-3}\,\text{m}^3
\]
\( \textbf{Work relation at constant pressure:} \)
\[
W=P\Delta V
\]
\( \textbf{Substitution:} \)
\[
W=(1.5\times10^5)(3.0\times10^{-3})\,\text{J}
\]
\( \textbf{Calculation:} \)
\[
W=4.5\times10^2\,\text{J}=450\,\text{J}
\]
\( \textbf{Sign meaning:} \) The gas expands, so work done by the gas is positive.
\( \textbf{Final answer:} \) The work done by the gas is \(450\,\text{J}\).
17. Assertion: When a hot cup is placed in a cooler room, heat transfer is from the cup to the room.
Reason: Heat transfer across a boundary occurs due to temperature difference and naturally proceeds from higher temperature to lower temperature in ordinary thermal contact.
ⓐ. Assertion is true, but Reason is false
ⓑ. Assertion is false, but Reason is true
ⓒ. Both Assertion and Reason are true, and Reason explains Assertion
ⓓ. Both Assertion and Reason are true, but Reason does not explain Assertion
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: The cup is hotter than the room, so energy is transferred from the cup to the surroundings as heat. The Reason states the cause of that transfer: a temperature difference exists across the boundary. In ordinary thermal contact, energy transfer as heat is from the higher-temperature body to the lower-temperature body. The cup does not transfer heat because it has greater mass or a different shape; the thermal condition is the deciding factor. The Reason therefore directly explains the Assertion. When temperatures become equal, the net heat transfer between the cup and room stops.
18. A gas sample is described in two records.
Record P: \(P=1.0\times10^5\,\text{Pa}\), \(V=2.0\times10^{-3}\,\text{m}^3\), and \(T=300\,\text{K}\).
Record Q: positions and speeds of all molecules at one instant.
For a thermodynamic description, the primary record is
ⓐ. Record Q, because thermodynamics requires every molecular speed
ⓑ. both records equally, because macroscopic and microscopic descriptions are identical
ⓒ. Record P, because it uses bulk variables of the gas
ⓓ. neither record, because pressure and temperature are not physical quantities
Correct Answer: Record P, because it uses bulk variables of the gas
Explanation: Record P uses pressure \(P\), volume \(V\), and temperature \(T\), which are macroscopic thermodynamic variables. These quantities describe the bulk state of the gas without requiring molecular-level detail. Record Q is a microscopic description because it deals with individual molecular positions and speeds. Thermodynamics can connect with microscopic ideas, but it does not usually begin by tracking every molecule. The two viewpoints are related, yet they are not the same kind of description.
19. The table compares two basic thermodynamic devices.
| Device | Basic purpose |
|---|
| Heat engine | Converts part of absorbed heat into useful P |
| Refrigerator | Uses external Q to transfer heat from cold region to hot region |
The entries P and Q are respectively
ⓐ. temperature and pressure
ⓑ. work and work
ⓒ. volume and heat
ⓓ. pressure and volume
Correct Answer: work and work
Explanation: A heat engine is a cyclic device that converts part of the heat it absorbs into work output. It cannot be understood simply as a device that creates temperature or pressure. A refrigerator uses external work input to transfer heat from a colder region to a hotter region. This makes it different from spontaneous heat flow from hot to cold. In both devices, work is central, but its role is opposite: output in a heat engine and input in a refrigerator.
20. For an ideal gas, the usual starting relation among \(P\), \(V\), \(n\), \(R\), and \(T\) is
ⓐ. \(P+V=nRT\)
ⓑ. \(PT=nRV\)
ⓒ. \(PV=\frac{R}{nT}\)
ⓓ. \(PV=nRT\)
Correct Answer: \(PV=nRT\)
Explanation: The ideal gas equation connects pressure \(P\), volume \(V\), number of moles \(n\), gas constant \(R\), and absolute temperature \(T\). Its standard form is \(PV=nRT\). The temperature used in this equation must be measured on the absolute scale, so the unit is \(\text{K}\). The equation is a macroscopic relation because it links measurable bulk properties of the gas. It becomes a useful starting point for later thermodynamic processes involving gases.
