Engineering - Thermodynamics

Question 1

Define thermodynamics.

Answer 1

The study of the relationship between heat and other forms of energy.

Question 2

An ideal gas has no forces acting between the molecules ∴

Answer 2

∴ energy of molecules is entirely kinetic and depends only on the temp of the gas

Ideal-like gas if monoatomic, low T well below BP, low P

Question 3

Internal energy, U, of a system is sum of ?

How to increase U?

Answer 3

sum of total Ek of constituent particles and (unless it’s a near-ideal gas) total Ep of particles.

Can increase U by doing work on the system and/or by heating it.

Question 4

First law of thermodynamics:
ΔU = Q - W, where…

This is essentially..

Answer 4

where ΔU is increase in internal energy, Q is energy transferred to the system by heating, W is work done by the system.

..the principle of conservation of momentum.

Question 5

We can only calc ΔU not absolute value for U bc?

Answer 5

bc that would require knowledge of all Ek and Ep of every particle.

Question 6

Gas eqs further assumptions:
- need changes to occur very…

Answer 6

slowly - infinitely slowly - otherwise, in rapid changes, pressure or temp wouldn’t be uniform throughout the system.

Question 7

Gas eqs further assumptions:
- need the system to pass through ..

Answer 7

a series of eqm states. such a change is REVERSIBLE - a small change in conditions would make the change move in the other direction. the energy changes would also reverse.

Question 8

Gas eqs further assumptions:
- since we need to obey principle of conservation of energy…

Answer 8

there can be NO dissipative effects, such as friction or turbulence, which would transfer heat out of the system.it must be possible for the system to return to its exact initial conditions.

Question 9

Gas eqs further assumptions:
Note: changes in real engines aren’t ….

Answer 9

reversible, bc processes happen too quickly. but the theory of an ideal reversible heat engine is still very useful.

Question 10

Thermodynamic processes are sometimes called non-flow processes ie a process during which ….. Closed system/cyclic process. There are 4 diff non-flow thermodynamic processes; in each case there’s a certain restriction imposed on the system.

Answer 10

the fluid doesn’t move in or out of the system during the process that generates power.

Question 11

1) Isothermal processes:
- this is a process in which the system stays …

Answer 11

at the same temp

Question 12

1) Isothermal processes:
- temp constant ∴

Answer 12

∴ average Ek constant (N constant) ∴ total internal energy constant.
ΔU = 0

Question 13

1) Isothermal processes:
- ΔU = 0 ∴

Answer 13

∴ Q = W

Question 14

1) Isothermal processes:
- isothermal change in a system requires

(design of system)

Answer 14

gas to be kept in thin-walled vessels of excellent conducting material, surrounded by a constant-temp bath. Slow expansion and contraction so temp is constant —> process is reversible

Question 15

1) Isothermal processes:
For a reversible isothermal change change, Eq? graph?

Answer 15

pV = K (Boyle’s law)

curved graph where p1V1 = p2V2 etc

Question 16

2) Constant-pressure processes (work is done on/by the gas):
-consider constant force applied to piston –> compression of gas ∴

Eq?

Answer 16

∴ work is done on gas by force.

W = pΔV

Question 17

2) Constant-pressure processes (work is done on/by the gas):
-for a constant-pressure process , eq?

Answer 17

ΔU = Q - pΔV

Question 18

3) Constant-volume process:
-If volume of system held constant..

Answer 18

system does no work
(W = pΔV = 0) bc work don requires expansion/compression (ie ΔV)

Question 19

3) Constant-volume process:
-thermodynamics eq?

Answer 19

ΔU = Q
If heat absorbed by system, internal energy must increase.

Question 20

4) Adiabatic processes:
-this is an isolated system, where..

Answer 20

where no energy is able to transfer in or out by heating.

Question 21

4) Adiabatic processes:
- Q = 0 ∴

Answer 21

ΔU = -W

Question 22

4) Adiabatic processes:
-for an ideal gas, the internal energy is the total..

Answer 22

total molecular translational Ek. Real engines may be diatomic or polyatomic ∴ have rotational and vibrational Ek too. Each method by which a system can absorb energy is called a ‘degree of freedom’. Monoatomic gases can absorb E in 3 translational directions (x,y,z) ∴ have 3 degrees of freedom. Polyatomic can also rotate ∴ more. So any change in internal energy will be diff depending on atomicity of gas.

Question 23

4) Adiabatic processes:
-it can be shown that if gas is in other ways ideal (ie all U is Ek), when it undergoes reversible adiabatic change then eq?

graph?

Answer 23

pv^γ = K, where γ is a constant for the particular gas that is dependent on the degrees of freedom and hence the atomicity of the gas.

curve steeper than isothermal changes

(btw the γ is on the V)

Question 24

4) Adiabatic processes:
-a system with… would undergo an adiabatic change.
-a process that happens …. is also adiabatic.

Answer 24

.. perfect thermal insulation

…so rapidly that there’s insufficient time for heat to transfer in or out

Question 25

4) Adiabatic processes:
-another form of the adiabatic eq: p1V1^γ = p2V2^γ combined with
pV = nRT
for fixed mass of gas TV^γ-1 =k ∴

Answer 25

T1V1^γ-1 = T2V2^γ-1

This allows the final temp to be calculated in an adiabatic expansion or contraction.

Question 26

Summary of non-flow processes:

process
restriction
1st law of thermodynamics
ideal gas eq

Answer 26

check image of table on phone

isothermal: ΔU=0, Q=W, pV=K
adiabatic: Q=0, ΔU=-W, pV^γ=K
Constant V: W=0, ΔU=Q, p/T=K
Constant p: W=pΔV, ΔU=Q-pΔV, V/T=K

Question 27

Specific heat capacities:
For a gas, which can change significantly in volume on heating or cooling, defining SHC purely on unit temp rise causes problems. Instead two cases are considered:

Answer 27

processes at constant volume
processes at constant pressure

Question 28

Specific heat capacities:
Two specific heat capacities result:
- Cv =
- Cp =

Answer 28

Cv = the energy required to produce unit temp rise in unit mass of gas at constant volume.
Cp = the energy required to produce unit temp rise in unit mass of gas at constant pressure.

Question 29

Specific heat capacities:
-γ = (in terms of the above)

Answer 29

γ = Cp/Cv

Question 30

Specific heat capacities:
-The rapid expansion and contraction of air when sound waves pass through it is a near-adiabatic process. Speed of sound, c =?

Answer 30

c = sqrt γp/ρ

where p = pressure and ρ = density of gas

Question 31

Determining work done from a p-V diagram:
- A p-V diagram for a thermodynamic process that has an arrow to show the direction of the change is known as …

Answer 31

an indicator diagram!

Question 32

Determining work done from a p-V diagram:
- To estimate the area under graph (W)…

Answer 32

count graph squares (half filled or more are inc, less than half filled are ignored).

Question 33

Cyclic processes:
-A cyclic process is one in which ..

Answer 33

the system undergoes two or more consecutive changes such that the final state is the same as the initial state. Thus one cycle is repeated continuously.

Question 34

Work done per cycle = ?

Answer 34

area of loop