Thermodynamics

Q.1 What is meant by the ‘state postulate’? Give an example.

Q.2 What is meant by ‘sensible energy’?

Q.3 Over the period of one minute, which of the following will add the most thermal
energy to your room? Explain.
a) A 600 W desktop computer
b) A 600 W resistance heater
c) A 600 W fan heater
d) A 600 g basketball falling off your desk (assume the vertical distance to the floor is
1m).

Q.4 What is the difference between an extensive and an intensive property? Give two
examples of each.

Q.5 A chemical substance is found to have a specific volume of 1.25×10
-3
m
3
/kg.
Determine its specific gravity.

Q.6 For combustion analysis, the lower heating value of pure carbon is the same as
the higher heating value. Why?

Q.7 The shopping centre escalator shown below is designed to carry 10 people at any
one time (assuming the mass of a person is 75 kg). It takes 20 seconds to travel from
level 1 to level 2 of the shopping centre. The salesperson from the company that
produces the escalator says it only uses 1.5 kW of power to run when it is fully loaded
with people. Based on the first law of thermodynamics, do you believe him? Do a
calculation to support your answer.

4.5 m
6 m

2503ENG Thermodynamics – Assignment
Q.8 The cylinder shown below has isothermal walls. For the purpose of analysis, the
system boundary is indicated by the dashed line. As the piston is moved, the volume
of the system changes such that it always contains all of the gas. Should this system
be described as an open system, a closed system or an isolated system? Give an
explanation for your answer.

Q.9 What is meant by an ‘isobaric process’?

Q.10 What is meant by an ‘isochoric process’?

Q.11 What is meant by an ‘adiabatic process’?

Q.12 On the ITS-90 temperature scale, how is the unit of 1 kelvin defined?

Q.13 110 cm of oil sits on top of some water in a column with the top open to the
atmosphere. The pressure gauge reads 62 kPa. What is the density of the oil?

Q.14 Determine the pressure difference in the air between points 1 and 3 in the figure
for Q. 15. Assume the air is at 25 °C and 101.3 kPa (at position 1) and the density of
the air between 1 and 3 is approximately constant. Comparing with the reading on the
gauge, does your answer verify that it is usually okay for engineers to neglect the
pressure change with depth in the gas?

Q.15 Express the absolute temperature 320.00 K in degrees Celsius (°C).

Isothermal
walls
5.500 m
1.100 m
1
3
water
oil
2

2503ENG Thermodynamics – Assignment
Q.16 The car tyre pressure is often set to 32 psi. Re-express this as an absolute
pressure in pascals if the barometric pressure is 101 kPa (abs).

Q.17 With the aid of a sketch, explain the greenhouse effect. What role does excess
CO2
gas in the atmosphere play in the greenhouse effect?

Q.18 An electric motor that has an efficiency of 85% delivers an output torque of 1.5
N.m at 500 rpm over a period of 5 minutes. Determine the electrical work done.

Q.19 A 60 kW (shaft output) electric car is powered by an electric motor. If the motor
has an average efficiency of 87%, determine the rate of heat supply by the motor to
the engine compartment at full load.

Q.20 The average velocity of air from a cooling fan in a large room is 5 m/s. The flow
rate of air through the fan is 4 m
3
/s. If the density of the air is 1.15 kg/m
3
, determine
the minimum power that needs to be supplied to the fan (noting that if the fan was
switched off, the air velocity would be zero).

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