THERMODYNAMICS & FLUID
MECHANICS EXAM QUESTIONS WITH
VERIFIED ANSWERS
How do you interpolate in the steam tables? - ANSWER-y = \frac{x-x1}{x2-x1} * (y2-y1)
+ y1
* \frac is percentage of the diff (y2-y1) you are away from y1
- interpolate between temperatures first
What is the dryness fraction? - ANSWER-x = m_vapour / (m_vapour + m_water)
- it is a system property
- aka. 'quality'
- can use x for properties but not for density
What types of work are there? - ANSWER-electrical = VI*Δt
mechanical = F * Δx
shaft(rotational) = (T/r)(2 π r n) = 2πnT
- where n is the number of revs
spring = 1/2 k (xf^2 - xi^2)
What is boundary work? - ANSWER-Work done by a system as it expands is the area
under the p-v curve of the process path. (p dv).
What is the 1st law of thermodynamics? For a thermodynamic cycle? - ANSWER-
Energy is always conserved.
A system undergoes a cycle, then the net energy transfer in into the system during the
cycle as heat is equal to the net energy transferred out as work.
What is the non-flow energy equation? - ANSWER-NFEE: q - w = Δu
(ignoring kinetic and potential energies)
* q and w are path dependent
* u is a property of the system
Full NFEE:
(q in + w in) - (q out + w out) = Δu + dke + dpe
What is Joule's law? - ANSWER-The internal energy depends only on temperature for
an ideal gas.
, What are specific heats? - ANSWER-c_v = energy required to raise substance by one
degree per one kg at constant volume
c_p = energy required to raise substance by one degree per one kg at constant
pressure
For ideal gas: Δu = c_v ΔT for any process (assuming constant c_v)
What is the purpose of enthalpy? - ANSWER-To incorporate boundary work and
change in internal energy in one term.
What is enthalpy? - ANSWER-It is the heat flow for an isobaric process: dq = dh for
isobaric process.
h = u + pv
dh = du + d(pv)
It is a system property.
For an isobaric nonflow process: dq = cp dT
cp = the specific heat at constant pressure
What are the specific heat relationships for an ideal gas? - ANSWER-dh = cp dT
cp = cv + R
γ = cp/cv
What is the work done for an isothermal process for an ideal gas? - ANSWER-w = RT
ln(v2/v1)
By joules law if dT = 0 => du=0
=> q=w
What is the formula for an adiabatic process of an ideal gas? - ANSWER-p v^γ = const.
where γ is the adiabatic index
* has exact value for monoatomic, diatomic, polyatomic gasses...
What is a polytropic process? - ANSWER-All processes follow p v^n = const.
isobaric n = 0
isothermal n = 1
adiabatic n = γ
isochoric n = inf
* have formulas for polytropic process to calculate work and heat transfer for values
except for n=1 (isothermal process) FOR AN IDEAL GAS
Can connect states using p v^n = const.
What is the steady flow energy equation and what approximations can we make? -
ANSWER-Total work done by system= P_s dt + p2v2dm - p1v1dm
(shaft work + pushing mass out - mass pushed in)
MECHANICS EXAM QUESTIONS WITH
VERIFIED ANSWERS
How do you interpolate in the steam tables? - ANSWER-y = \frac{x-x1}{x2-x1} * (y2-y1)
+ y1
* \frac is percentage of the diff (y2-y1) you are away from y1
- interpolate between temperatures first
What is the dryness fraction? - ANSWER-x = m_vapour / (m_vapour + m_water)
- it is a system property
- aka. 'quality'
- can use x for properties but not for density
What types of work are there? - ANSWER-electrical = VI*Δt
mechanical = F * Δx
shaft(rotational) = (T/r)(2 π r n) = 2πnT
- where n is the number of revs
spring = 1/2 k (xf^2 - xi^2)
What is boundary work? - ANSWER-Work done by a system as it expands is the area
under the p-v curve of the process path. (p dv).
What is the 1st law of thermodynamics? For a thermodynamic cycle? - ANSWER-
Energy is always conserved.
A system undergoes a cycle, then the net energy transfer in into the system during the
cycle as heat is equal to the net energy transferred out as work.
What is the non-flow energy equation? - ANSWER-NFEE: q - w = Δu
(ignoring kinetic and potential energies)
* q and w are path dependent
* u is a property of the system
Full NFEE:
(q in + w in) - (q out + w out) = Δu + dke + dpe
What is Joule's law? - ANSWER-The internal energy depends only on temperature for
an ideal gas.
, What are specific heats? - ANSWER-c_v = energy required to raise substance by one
degree per one kg at constant volume
c_p = energy required to raise substance by one degree per one kg at constant
pressure
For ideal gas: Δu = c_v ΔT for any process (assuming constant c_v)
What is the purpose of enthalpy? - ANSWER-To incorporate boundary work and
change in internal energy in one term.
What is enthalpy? - ANSWER-It is the heat flow for an isobaric process: dq = dh for
isobaric process.
h = u + pv
dh = du + d(pv)
It is a system property.
For an isobaric nonflow process: dq = cp dT
cp = the specific heat at constant pressure
What are the specific heat relationships for an ideal gas? - ANSWER-dh = cp dT
cp = cv + R
γ = cp/cv
What is the work done for an isothermal process for an ideal gas? - ANSWER-w = RT
ln(v2/v1)
By joules law if dT = 0 => du=0
=> q=w
What is the formula for an adiabatic process of an ideal gas? - ANSWER-p v^γ = const.
where γ is the adiabatic index
* has exact value for monoatomic, diatomic, polyatomic gasses...
What is a polytropic process? - ANSWER-All processes follow p v^n = const.
isobaric n = 0
isothermal n = 1
adiabatic n = γ
isochoric n = inf
* have formulas for polytropic process to calculate work and heat transfer for values
except for n=1 (isothermal process) FOR AN IDEAL GAS
Can connect states using p v^n = const.
What is the steady flow energy equation and what approximations can we make? -
ANSWER-Total work done by system= P_s dt + p2v2dm - p1v1dm
(shaft work + pushing mass out - mass pushed in)
