Thermodynamics Questions and Answers

Answer: B

The system undergoes a constant pressure process.

A free piston is held in place by its own weight and the atmosphere above it, both of which are constant. As the burner heats the gas, the gas expands and pushes the piston up, but the pressure needed to support the piston stays the same, so the heating is at constant pressure.

It is not constant volume, because the piston moves. It is not adiabatic, because heat is being added from the burner. And it is not isothermal: the temperature rises as heat is added at constant pressure, in proportion to the volume for an ideal gas.

Answer: C

The property is \(Z = pdv + vdp\).

A quantity is a thermodynamic property when its differential is exact, so that its change depends only on the end states. \(pdv + vdp = d(pv)\), the exact differential of the product pv, which is a property.

\(pdv\) alone is the boundary work, which depends on the path: the area under a p-v curve differs between paths joining the same two states. \(vdp\) is likewise path-dependent (it is the flow work term for an open system). \(pdv - vdp\) is not the differential of any function of state.

Answer: B

The macroscopic approach considers a certain quantity of matter without taking into account the events that happen at the molecular level. Properties such as pressure and temperature are measured for the system as a whole. The microscopic and statistical approaches consider the behaviour of the individual molecules, and 'stimulative' is not an approach of thermodynamics, so only the macroscopic approach fits.

The macroscopic approach is also called classical thermodynamics, and it deals with bulk properties such as pressure, volume and temperature rather than molecular motion.

Answer: B

The intensive property is density.

An intensive property does not depend on the size or extent of the system: if a system in equilibrium is divided into two parts, each part has the same value. Density is mass per unit volume, so halving the system halves both mass and volume and leaves density unchanged. Pressure and temperature are other examples.

Mass, energy and volume are extensive: they add up over the parts, so each half of the system has half the mass, half the energy and half the volume. Dividing an extensive property by mass gives an intensive one, which is why specific volume and specific energy are intensive while volume and energy are not.

Answer: D

Both A and R are correct.

A is right: the state postulate is the rule that says how many independent properties fix the state of a system. R is that rule as it applies to a simple compressible system, one with no significant electrical, magnetic, gravitational, motion or surface-tension effects: two independent intensive properties fix its state.

The word 'independent' matters. In a single-phase region, temperature and pressure are independent and together fix the state, which is why the question restricts itself to a single phase. During a phase change they are tied together and a second property such as specific volume is needed. Options marking either statement incorrect therefore miss the point of the restriction.

Answer: D

The extensive property is Total mass.

An extensive property depends on the size of the system: split the system in two and each half has half of it. Total mass, total volume and total energy behave this way.

Density, pressure and temperature are intensive: each half of a uniform system has the same density, pressure and temperature as the whole. A quick test is to ask whether the property adds up when two identical systems are combined; mass doubles, but pressure and temperature do not.

Answer: D

The correct statement is Temperature (T), pressure (P), and specific volume (v) are intensive properties.

An intensive property does not depend on the amount of substance. Temperature and pressure clearly do not, and specific volume is volume per unit mass, which makes it intensive even though volume itself is extensive.

The other statements are false. For an ideal gas, enthalpy depends on temperature only. U, S and H are state (point) functions, not path functions. Heat and work are the path functions: their values depend on the process, not just on the end states.

Answer: C

Specific volume is an intensive property.

Specific volume is volume per unit mass, \(v = V/m\). If a system is divided into two halves, each half has the same specific volume as the whole, so the value does not depend on how much matter there is. That is the test of an intensive property, like pressure and temperature.

Volume itself is extensive, because it doubles when the mass doubles; dividing an extensive property by mass always gives an intensive one. 'Intrinsic' is not one of the two classes of thermodynamic property, and specific volume certainly is a property, since it has a definite value in every state.

Answer: A

The tyre gauge reads gauge pressure.

A tyre pressure gauge measures how far the air pressure inside the tyre exceeds the surrounding atmospheric pressure. A flat tyre therefore reads zero, even though the air in it is at atmospheric pressure.

Absolute pressure is the gauge reading plus atmospheric pressure. Vacuum pressure is the amount by which a pressure falls below atmospheric, which does not apply to an inflated tyre, and atmospheric pressure is the reference the gauge subtracts, not what it shows.

Answer: A

Such a system is an open system.

In an open system, also called a control volume, both mass and energy cross the boundary. Turbines, compressors, pumps and nozzles are examples.

A closed system allows energy but not mass to cross, an isolated system allows neither, and adiabatic describes a process with no heat transfer, not a kind of boundary for mass.