# First Law of Thermodynamics Class Exercises

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27 Οκτ 2013 (πριν από 4 χρόνια και 6 μήνες)

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First Law of Thermodynamics Class Exercises

AP Physics

1. We’ve seen that an object’s internal energy can change with energy being transferred in or out as
heat. Is there any other way that an object’s internal energy can change?

2. Can yo
u think of other examples of object’s internal energy changing without heat?

3. What is the First Law of Thermodynamics?

4. What is a heat engine? How does one work?

5. a) The flame in a Hero’s steam engine puts in 12,300 J of heat and
the internal energy of the system
increases by 10,700 J. How much work is done by the steam engine?

b) What is the efficiency of this engine?

6. a) As a balloon’s internal energy rises by 320 J, it expands and does 25 J of work. How much heat
w

b) What is the efficiency of the balloon as a heat engine?

7. In a fire syringe, 8,000 J of work are done on the system and the system gives off 2,800 J of heat.
How much does the internal energy of the system change?

T
he most common, and most easily understood heat engines
are systems of gases enclosed in a movable piston.

8. What do we mean by the state of a system?

9. What do we need to know to describe the state of a sample of gas enclosed in a piston?

10. What do we mean by a process that occurs to a system?

11. Consider having a syringe full of compressed gas lift a
weight for us. How can we get the greatest amount of work
performed by the syringe?

12. What does it mean for a
proce
ss to be reversible? Why do
we focus on reversible processes?

Now let’s look at some special types of processes that affect thermodynamic systems. Remember that
our focus is on gases in pistons:

13. Define an
I
SOBARIC

process:

14. A syste
m composed of 0.005 kg of helium in a 20 cm diameter cylinder expands isobarically at
101.3 kPa pushing the piston from 6 cm above the base to 18 cm above the base as the internal energy
increases by 17656.2 J (and the temperature rises from room temperatu
re to 700 °C). How much heat
occurs to the air during this process?

15. A system made from a 6 cm diameter cylinder capped by a moveable piston contains 0.007 kg of
argon gas at 205 kPa (its in a hyperbaric chamber). The piston is 1 m above the ba
se when it taken from
a bath of boiling water. The piston is placed in a bath of ice water and slides isobarically until it is 73
cm above the base. The ice water bath absorbs 520.5 J of heat. How much does the argon’s internal
energy change?

16.

Define an
I
SOMETRIC

process:

17. A sample of 0.01 kg of air is heated isometrically from 20°C to 300 °C. How much heat happens to
the air during the process? [At constant volume, c
V

= 784.26 J/(kg K)]

18. A sample of 0.5 kg of hydrogen

gas that starts at 70 °C is cooled isometrically in a bath of liquid
nitrogen by removing 1544832 J as heat. What temperature is the gas at the end of the process?

[c
V

= 10221.4 J/(kg K)]

19. Define an
I
SOTHERMAL

process:

20. You do 1500

J of work compressing a piston in a water bath that keeps it at a constant temperature.
How much heat occurs to the piston during the process?

21. A piston absorbs 300 J of energy as heat as it expands isothermally. How much work does the
piston d
o?

22. Define an
A
DIABATIC

process:

23. You do 2.5 J of work to adiabatically compress a fire syringe containing 4.814 x 10
-
6

kg of air at
room temperature. How hot does the air get at the end of the process?

24. In a car engine the
hot air inside the piston is at 2300 K when it begins to expand adiabatically. The
air does 861.4 J of work as it expands. How hot is the air at the end of the expansion?