Thermodynamics 1

Question 1

Define a closed system

Answer 1

A system whose mass remains constant but energy can be transferred to or from it.

Question 2

What is the change in enthalpy of a system at constant pressure equal to?

Answer 2

The heat addition.

Question 3

Define the non-flow energy equation (NFEE)

Answer 3

dU = Q + W
The difference in internal energy of a closed system is equal to the work done and the heat transfer on the system AKA the 1st Law.

Question 4

Under what condition is the change in internal energy of a system equal to it’s heat addition?

Answer 4

When it’s at constant pressure.

Question 5

Define the first law of thermodynamics

Answer 5

The change in internal energy of a closed system is equal to the sum of of its heat addition and work done by the surroundings. Net work delivered to surroundings is proportional to the heat taken.

Question 6

Define a system

Answer 6

A quantity of mass around which we draw a boundary to examine its property changed.

Question 7

Define an intensive propert

Answer 7

Properties that do not depend on the quantity of matter such as temperature and pressure.

Question 8

Define an extensive property

Answer 8

A property of the system that does depend on its quantity such as volume.

Question 9

Define a quasi-equilibrium process

Answer 9

A process in which only infinitesimally small temperature gradients and unbalanced forces exist.

Question 10

Define an adiabatic process

Answer 10

A process in which NO HEAT TRANSFER occurs. This means energy is only transferred by work.

Question 11

What is the polytropic index of an adiabatic system?

Answer 11

γ, where γ = Cp/Cv. It comes from the heat transfer being 0, dU= -PdV, the specific heat for constant volume equation and using R= Cp - Cv.

Question 12

Define internal energy

Answer 12

The sum of all microscopic energies in a body.

Question 13

What is an isochoric process?

Answer 13

One where the system is kept at constant VOLUME. Therefore no p-v work is done and all energy added converts to internal energy, raising temperature and pressure.

Question 14

What is an isobaric process?

Answer 14

One where the system is kept at a constant PRESSURE. So the heat addition here is the change in enthalpy.

Question 15

What is an isothermal process?

Answer 15

One where the system is kept at constant TEMPERATURE. The p-v graph for this is reciprocal so the to find the work done we integrate pressure from the ideal gas law with respect to volume.

Question 16

Define an isothermal system?

Answer 16

It is a REVERSIBLE ADIABATIC process - ENTROPY REMAINS CONSTANT because all work transfers are frictionless.

Question 17

In an isochoric process what is equal to the work done?

Answer 17

Isochoric = constant volume, so no pdV (moving boundary) work is done, leaving dQ = dU.

Question 18

in an isobaric process what is equal to the work done?

Answer 18

Isobaric = constant pressure, so the work done is a square pdV graph where W = -p(V2-V1).

Question 19

What is the work done in an isothermal process?

Answer 19

Using the pV=nRT graph it will be reciprocal, so integrate it wrt V to find the moving boundary work.

Question 20

Define a polytropic process?

Answer 20

One that obeys the relation pV^n=C. This covers all of the idealised processes (isobaric n=0, isothermal n=1, isochoric n=inf.)

Question 21

Define the second law of thermodynamics?

Answer 21

It is impossible to convert all heat addition to work on the surroundings in a system - so it must operate between at least 2 reservoirs.

Question 22

what is the function of a nozzle?

Answer 22

A nozzle decreases the fluid’s pressure to accelerate its flow (eg convergent pipe). At these very high speeds there is little time for it to transfer heat to/from surroundings so we always assume it to be adiabatic, and no work crosses the boundary either, meaning CHANGE IN VELOCITY = CHANGE IN ENTHALPY

Question 23

whats the function of a diffuser?

Answer 23

Just the opposite of a nozzle; increases the fluid’s pressure to decelerate its flow (eg divergent pipe). At these very high speeds there is little time for it to transfer heat to/from surroundings so we always assume it to be adiabatic, and no work crosses the boundary either, meaning CHANGE IN VELOCITY = CHANGE IN ENTHALPY

Question 24

whats the function of a turbine?

Answer 24

To extract work from a flow of fluid expanding from high pressure to low pressure. Its adiabatic and the change in velocity is negligible, but the fluid exerts torque on the shaft. So the work is equal to the change in enthalpy (negative cos work on surroundings).

Question 25

What is the function of a compressor?

Answer 25

The opposite of a turbine; work is done on the fluid to raise its pressure. Still we assume velocity doesn’t change because before and after there are nozzles/diffusers so that work is still equal to the change in enthalpy.

Question 26

What is the function of a throttle?

Answer 26

They restrict flow in an adiabatic steady flow process such that the inlet enthalpy is the same as the outlet enthalpy.

Question 27

What is the Kelvin-Plank statement?

Answer 27

It is impossible for any cyclical device to receive heat from a single reservoir and produce work (there must be a cold reservoir).

Question 28

Define the Clausius statement

Answer 28

Its impossible for a cyclical device to transfer heat from a cooler body to a hotter one without work being done on the system.