Thermodynamics Concepts

How do the energies of a flowing fluid and a fluid at rest compare? Name the specific forms of energy associated with each case

Flowing fluid has flow energy in addition to the kinetic, internal and potential energies in a static fluid.

When is the energy crossing the boundaries of a closed system heat and when is it work?

If it crosses the boundary by a temperature difference is heat, all other forms are work. Note: This means that a resistor inside of a system that heats up is doing work not transferring heat.

List the forms of energy that contribute to the internal energy of a system

Internal energy is made up of sensible, latent, chemical, and nuclear energies. Sensible is based on the kinetic energy of molecules.

Why does a bicyclist pick up speed on a downhill road even when he is not pedaling? Does this violate the conservation of energy principle?

It doesn't violate the conservation of energy because the bicyclist is gaining kinetic energy while losing potential energy.

What is the compressed liquid assumption? When finding the specific volume for a CL, do you use the vf value from the Temperature tables or the Pressure tables, why?

It is assumed that we cause use values at the saturated liquid state for volume on the temp table because the changes are minimal in the values on that table.

An ideal gas at a given state expands to a fixed final volume first at constant pressure and then at constant temperature. For which case is the work done greater?

It is greater at constant pressure.

What are the different mechanisms for transferring energy to or from a control volume?

It is heat transfer, work, or mass energy transfer.

On a hot summer day, a student turns his fan on when he leaves his room in the morning. When he returns in the evening, will the room be warmer or cooler than the neighboring rooms? Why? Assume all the doors and windows are kept closed.

It will be warmer because the fan is actually bringing energy into the system and is just moving the air around not actually removing heat from the system which is the room.

Define mass and volume flow rates. How are they related to each other?

Mass flow is the amount of mass leaving a system across a cross section per unit time. Volume flow is the amount of volume flowing per unit time.

Define a quality. What does a quality of 0.0 mean? What does a quality of 1.0 mean?

Mass of vapor/Mass total. It is a percentage often. 0.0 means its all liquid. 1.0 means its all vapor.

What is mechanical energy? How does it differ from thermal energy? What are the forms of mechanical energy of a fluid stream?

Mechanical energy can be converted to mechanical work completely and directly. It is in the forms of kinetic potential and flow energy.

Someone claims that the absolute pressure in a liquid of constant density doubles when the depth is doubled. Do you agree? Explain.

No because Patm is part of Pabs not just Pgage which is dependent on the depth.

Does the amount of mass entering a control volume have to be equal to the amount of mass leaving during an unsteady-flow process?

No it doesn't.

A health magazine reported that physicians measured 100 adults' blood pressure using two different arm positions: parallel to the body (along the side) and perpendicular to the body (straight out). Readings in the parallel position were up to 10 percent higher than those in the perpendicular position, regardless of whether the patient was standing, sitting, or lying down. Explain the possible cause for the difference.

The blood vessels are more restricted when the arm is parallel to the body than when the arm is perpendicular to the body. For a constant volume of blood to be discharged by the heart, the blood pressure must increase to overcome the increased resistance to flow.

Consider an air compressor operating steadily. How would you compare the volume flow rates of the air at the compressor inlet and exit?

The inlet would be greater than the exit because a compressor is compressing volume. which means its coming out at a smaller volume.

When analyzing the acceleration of gases as they flow through a nozzle, what would you choose as your system? What type of system is this?

The inside of the nozzle, and it is an open system.

A diffuser is an adiabatic device that decreases the kinetic energy of the fluid by slowing it down. What happens to this lost kinetic energy?

The lost energy is mostly converted to internal energy is the form of a rise of temperature of the fluid.

A large fraction of the thermal energy generated in the engine of a car is rejected to the air by the radiator through the circulating water. Should the radiator be analyzed as a closed system or as an open system? Explain

The radiator should be analyzed as an open system because mass is crossing system boundaries. The water is the mass crossing this boundary.

How are heat, internal energy, and thermal energy related to each other?

Thermal energy is the sensible and latent forms of internal energy, this is just referred to as heat normally.

Why are throttling devices commonly used in refrigeration and air-conditioning applications?

They are cheap and can cause a large temperature drop which is what we are trying to get out of those systems.

An office worker claims that a cup of cold coffee on his table warmed up to 80°C by picking up energy from the surrounding air, which is at 25°C. Is there any truth to his claim? Does this process violate any thermodynamic laws?

This claim is not true. It violates the second law. Also it could be thought about by thermal equilibrium.

A room is heated a s a result of solar radiation coming through the windows. Is this a heat or work interaction for the room?

This is a heat interaction since the change happens because of a temperature difference between the sun (outside the system) and the room.

A room is heated by an iron that is left plugged in. Is this a heat or work interaction? Take the entire room, including the iron, as the system.

This is a work interaction because electrical work is going into the system, it is just being converted into heat within the system not crossing the boundary.

An insulated room is heated by burning candles. Is this a heat or work interaction? Take the entire room, including the candles, as the system.

This is actually neither because it is not crossing the boundary. Its just a conversion within the system already.

A gas in a piston-cylinder device is compressed, and as a result its temperature rises. Is this a heat or work interaction?

This is work because it was caused by the boundary being moved and not by a temperature difference from the surroundings.

What is total energy? Identify the different forms of energy that constitute total energy

Total energy is a sum of all the energy within a system. it consists of kinetic potential and internal. It is divided into macroscopic energy or microscopic energy. Macro is kinetic and potential. Micro is related to the molecular structure of a system and they are independent of outside reference frames, which is internal energy.

Lifting a weight to a height of 20 m takes 20 s for one crane and 10 s for another. Is there any difference in the amount of work done on the weight by each crane?

Work is force over a distance which means that the work is the same because the force is the same and distance is the same. Power is different however because of the time difference.

Will the temperature of air rise as it is compressed by an adiabatic compressor? Why?

Yeah because work is being added even if heat isn't.

Is the relation Δ𝐻𝐻 = 𝑚𝑚𝑐𝑐𝑝𝑝,𝑎𝑎𝑎𝑎𝑎𝑎Δ𝑇𝑇 restricted to constant- pressure processes only, or can it be used for any kind of process of an ideal gas?

cp is a property so it can be used for any kind of process of an ideal gas.

Is the relation restricted to constant-volume processes only, or can it be used for any kind of process of an ideal gas?

cv is a property so it can be used for any kind of process of an ideal gas.

A fixed mass of an ideal gas is heated from 50 to 80°C (a) at constant volume and (b) at constant pressure. For which case do you think the energy required will be greater? Why?

it will be greater at constant pressure because cp is always greater than cv.

Would you expect the temperature of air to drop as it undergoes a steady-flow throttling process? Explain.

no because h1 = h2 in a throttling valve and h is a function of temperature for an ideal gas such as air.

Is it possible to compress an ideal gas isothermally in an adiabatic piston-cylinder device? Explain.

no because ideal gas internal energy is based on temperature differences so it makes work = zero in the first law break down.

The kinetic energy of a fluid increases as it is accelerated in an adiabatic nozzle. Where does this energy come from?

the kinetic energy comes from a conversion of internal energy. This is shown by the fluid getting colder.

A fixed mass of an ideal gas is heated from 50 to 80°C at a constant volume of (a) 1 m3 and (b) 3 m3 . For which case do you think the energy required will be greater? Why?

they should be the same because it is a function of temperature and not of volume.

A fixed mass of an ideal gas is heated from 50 to 80°C at a constant pressure of (a) 1 atm and (b) 3 atm. For which case do you think the energy required will be greater? Why? The energy required is 𝑚𝑚𝑐𝑐𝑝𝑝,𝑎𝑎𝑎𝑎𝑎𝑎Δ𝑇𝑇, which will be the same in both cases. This is because the cp of an ideal gas does not vary with pressure (The required energy is not a function of pressure).

they will be the same because it is only a function of temperature and not pressure.

When is the flow through a control volume steady?

when there is not changes of volume and different times.