19.4: Heat Engines
Heat engines transform input heat into work in accordance with the laws of thermodynamics. For instance, as we learned in the previous chapter, increasing the temperature of a gas at constant volume will increase its pressure. This pressure can be transformed into a force that moves a piston.
The mechanics of various heat engines differ but their fundamentals are quite similar and involve the following steps:
 Heat is supplied to the engine from some source at a higher temperature
(Th) .  Some of this heat is transferred into mechanical energy through work done
(W) .  The rest of the input heat is transferred to some source at a lower temperature
(Tc) until the system is in its original state.
A single cycle of such an engine can be illustrated as follows:
In effect, such an engine allows us to 'siphon off' part of the heat flow between the heat source and the heat sink. The efficiency of such an engine is define as the ratio of net work performed to input heat; this is the fraction of heat energy converted to mechanical energy by the engine:
If the engine does not lose energy to its surroundings (of course, all real engines do), then this efficiency can be rewritten as
A Carnot Engine, the most efficient heat engine possible, has an efficiency equal to
where
Some Important Points
 In a practical heat engine, the change in internal energy must be zero over a complete cycle. Therefore, over a complete cycle
W=ΔQ .  The work done by a gas during a portion of a cycle =
PΔV , noteΔV can be positive or negative.
Gas Heat Engines
 When gas pressureforces are used to move an object then work is done on the object by the expanding gas. Work can be done on the gas in order to compress it.
 If you plot pressure on the vertical axis and volume on the horizontal axis (see
P−V diagrams in the last chapter), the work done in any complete cycle is the area enclosed by the graph. For a partial process, work is the area underneath the curve, orPΔV .
Question: A heat engine operates at a temperature of
a) What is the ideal efficiency of this engine?
b.) The engine drives a
c) If the engine is operating at
d) The fuel the engine uses is rated at
Answer:
a) We will plug the known values into the formula to get the ideal efficiency.
b) To find the power of the engine, we will use the power equation and plug in the known values.
c) First, we know that it is operating at
of
Thermodynamics and Heat Engines Problem Set
 Consider a molecule in a closed box. If the molecule collides with the side of the box, how is the force exerted by the molecule on the box related to the momentum of the molecule? Explain conceptually, in words rather than with equations.
 If the number of molecules is increased, how is the pressure on a particular area of the box affected? Explain conceptually, in words rather than with equations.
 The temperature of the box is related to the average speed of the molecules. Use momentum principles to relate temperature to pressure. Explain conceptually, in words rather than with equations.
 What would happen to the number of collisions if temperature and the number of molecules remained fixed, but the volume of the box increased? Explain conceptually, in words rather than with equations.
 Use the reasoning in the previous four questions to qualitatively derive the ideal gas law.
 Typical room temperature is about
300K . As you know, the air in the room contains bothO2 andN2 gases, with nitrogen the lower mass of the two. If the average kinetic energies of the oxygen and nitrogen gases are the same (since they are at the same temperature), which gas has a higher average speed?  Use the formula
P=F/A to argue why it is easier to pop a balloon with a needle than with a finger (pretend you don’t have long fingernails).  Take an empty plastic water bottle and suck all the air out of it with your mouth. The bottle crumples. Why, exactly, does it do this?
 You will notice that if you buy a large drink in a plastic cup, there will often be a small hole in the top of the cup, in addition to the hole that your straw fits through. Why is this small hole necessary for drinking?
 Suppose you were swimming in a lake of liquid water on a planet with a lower gravitational constant
g than Earth. Would the pressure10 meters under the surface be the same, higher, or lower, than for the equivalent depth under water on Earth? (You may assume that the density of the water is the same as for Earth.)  Why is it a good idea for Noreen to open her bag of chips before she drives to the top of a high mountain?
 Explain, using basic physics conservation laws, why the following conditions would cause the ideal gas law to be violated:
 There are strong intermolecular forces in the gas.
 The collisions between molecules in the gas are inelastic.
 The molecules are not spherical and can spin about their axes.
 The molecules have nonzero volume.
To the right is a graph of the pressure and volume of a gas in a container that has an adjustable volume. The lid of the container can be raised or lowered, and various manipulations of the container change the properties of the gas within. The points
a,b, andc represent different stages of the gas as the container undergoes changes (for instance, the lid is raised or lowered, heat is added or taken away, etc.) The arrows represent the flow of time. Use the graph to answer the following questions.  Consider the change the gas undergoes as it transitions from point
b to pointc . What type of process is this? adiabatic
 isothermal
 isobaric
 isochoric
 entropic
 Consider the change the gas undergoes as it transitions from point
c to pointa . What type of process is this? adiabatic
 isothermal
 isobaric
 isochoric
 none of the above
 Consider the change the gas undergoes as it transitions from point
a to pointb . Which of the following best describes the type of process shown? isothermal
 isobaric
 isochoric
 How would an isothermal process be graphed on
a P−V diagram?  Write a scenario for what you would do to the container to make the gas within undergo the cycle described above.
 Why is it so cold when you get out of the shower wet, but not as cold if you dry off first before getting out of the shower? _____________________________________________________________
 Antonio is heating water on the stove to boil eggs for a picnic. How much heat is required to raise the temperature of his 10.0kg vat of water from
20∘C to100∘C ?  Amy wishes to measure the specific heat capacity of a piece of metal. She places the 75g piece of metal in a pan of boiling water, then drops it into a styrofoam cup holding 50 g of water at
22∘C . The metal and water come to an equilibrium temperature of25∘C . Calculate: The heat gained by the water
 The heat lost by the metal
 The specific heat of the metal
 John wishes to heat a cup of water to make some ramen for lunch. His insulated cup holds 200 g of water at
20∘C . He has an immersion heater rated at 1000 W (1000 J/s) to heat the water. How many JOULES of heat are required to heat the water to
100∘C ?  How long will it take to do this with a 1000W heater?
 Convert your answer in part b to minutes.
 How many JOULES of heat are required to heat the water to
 You put a 20g cylinder of aluminum
(c=0.2 cal/g/∘C) in the freezer(T=−10∘C) . You then drop the aluminum cylinder into a cup of water at20∘C . After some time they come to a common temperature of12∘C . How much water was in the cup?  Emily is testing her baby’s bath water and finds that it is too cold, so she adds some hot water from a kettle on the stove. If Emily adds 2.00 kg of water at
80.0∘C to 20.0 kg of bath water at27.0∘C , what is the final temperature of the bath water?  You are trying to find the specific heat of a metal. You heated a metal in an oven to
250∘C . Then you dropped the hot metal immediately into a cup of cold water. To the right is a graph of the temperature of the water versus time that you took in the lab. The mass of the metal is 10g and the mass of the water is 100g. Recall that water has a specific heat of1 cal/g∘C .  How much heat is required to melt a 20 g cube of ice if
 the ice cube is initially at
0∘C  the ice cube is initially at
−20∘C (be sure to use the specific heat of ice)
 the ice cube is initially at
 A certain alcohol has a specific heat of
0.57 cal/g∘C and a melting point of−114∘C . You have a 150 g cup of liquid alcohol at22∘C and then you drop a 10 g frozen piece of alcohol at−114∘C into it. After some time the alcohol cube has melted and the cup has come to a common temperature of7∘C . a. What is the latent heat of fusion (i.e. the ‘L ’ in theQ=mL equation) for this alcohol? b. Make a sketch of the graph of the alcohol’s temperature vs. time c. Make a sketch of the graph of the water’s temperature vs. time  Calculate the average speed of
N2 molecules at room temperature(300K) . (You remember from your chemistry class how to calculate the mass (inkg ) of anN2 molecule, right?)  How high would the temperature of a sample of
O2 gas molecules have to be so that the average speed of the molecules would be10 % the speed of light?  How much pressure are you exerting on the floor when you stand on one foot? (You will need to estimate the area of your foot in square meters.)
 Calculate the amount of force exerted on a
2cm×2cm patch of your skin due to atmospheric pressure(P0=101,000Pa) . Why doesn’t your skin burst under this force?  Use the ideal gas law to estimate the number of gas molecules that fit in a typical classroom.
 Assuming that the pressure of the atmosphere decreases exponentially as you rise in elevation according to the formula
P=P0e−ha , whereP0 is the atmospheric pressure at sea level(101,000Pa) ,h is the altitude in km and a is the scale height of the atmosphere(a≈8.4km) . Use this formula to determine the change in pressure as you go from San Francisco to Lake Tahoe, which is at an elevation approximately
2km above sea level.  If you rise to half the scale height of Earth’s atmosphere, by how much does the pressure decrease?
 If the pressure is half as much as on sea level, what is your elevation?
 Use this formula to determine the change in pressure as you go from San Francisco to Lake Tahoe, which is at an elevation approximately
 At Noah’s Ark University the following experiment was conducted by a professor of Intelligent Design (formerly Creation Science). A rock was dropped from the roof of the Creation Science lab and, with expensive equipment, was observed to gain
100J of internal energy. Dr. Dumb explained to his students that the law of conservation of energy required that if he put100J of heat into the rock, the rock would then rise to the top of the building. When this did not occur, the professor declared the law of conservation of energy invalid. Was the law of conservation of energy violated in this experiment, as was suggested? Explain.
 If the law wasn’t violated, then why didn’t the rock rise?
 An instructor has an ideal monatomic helium gas sample in a closed container with a volume of
0.01m3 , a temperature of412K , and a pressure of474kPa . Approximately how many gas atoms are there in the container?
 Calculate the mass of the individual gas atoms.
 Calculate the speed of a typical gas atom in the container.
 The container is heated to
647K . What is the new gas pressure?  While keeping the sample at constant temperature, enough gas is allowed to escape to decrease the pressure by half. How many gas atoms are there now?
 Is this number half the number from part (a)? Why or why not?
 The closed container is now compressed isothermally so that the pressure rises to its original pressure. What is the new volume of the container?
 Sketch this process on a PV diagram.
 Sketch cubes with volumes corresponding to the old and new volumes.
 A famous and picturesque dam,
80m high, releases24,000kg of water a second. The water turns a turbine that generates electricity. What is the dam’s maximum power output? Assume that all the gravitational potential energy of the water is converted into electrical energy.
 If the turbine only operates at
30 % efficiency, what is the power output?  How many Joules of heat are exhausted into the atmosphere due to the plant’s inefficiency?
 A heat engine operates at a temperature of
650K . The work output is used to drive a pile driver, which is a machine that picks things up and drops them. Heat is then exhausted into the atmosphere, which has a temperature of300K . What is the ideal efficiency of this engine?
 The engine drives a
1200kg weight by lifting it50m in2.5sec . What is the engine’s power output?  If the engine is operating at
50 % of ideal efficiency, how much power is being consumed?  How much power is exhausted?
 The fuel the engine uses is rated at
2.7×106J/kg . How many kg of fuel are used in one hour?
 Calculate the ideal efficiencies of the following scifi heat engines:
 A nuclear power plant on the moon. The ambient temperature on the moon is
15K . Heat input from radioactive decay heats the working steam to a temperature of975K .  A heat exchanger in a secret underground lake. The exchanger operates between the bottom of a lake, where the temperature is
4C , and the top, where the temperature is13C .  A refrigerator in your dorm room at Mars University. The interior temperature is
282K ; the back of the fridge heats up to320K .
 A nuclear power plant on the moon. The ambient temperature on the moon is
 How much external work can be done by a gas when it expands from
0.003m3 to0.04m3 in volume under a constant pressure of400kPa ? Can you give a practical example of such work?  In the above problem, recalculate the work done if the pressure linearly decreases from
400kPa to250kPa under the same expansion. Hint: use aPV diagram and find the area under the line.  One mole
(N=6.02×1023) of an ideal gas is moved through the following states as part of a heat engine. The engine moves from state A to state B to state C, and then back again. Use the Table (below) to answer the following questions: Draw a PV diagram.
 Determine the temperatures in states A, B, and C and then fill out the table.
 Determine the type of process the system undergoes when transitioning from A to B and from B to C. (That is, decide for each if it is isobaric, isochoric, isothermal, or adiabatic.)
 During which transitions, if any, is the gas doing work on the outside world? During which transitions, if any, is work being done on the gas?
 What is the amount of net work being done by this gas?
State 
Volume 
Pressure 
Temperature 

A 



B 



C 


 A sample of gas is used to drive a piston and do work. Here’s how it works:
 The gas starts out at standard atmospheric pressure and temperature. The lid of the gas container is locked by a pin.
 The gas pressure is increased isochorically through a spigot to twice that of atmospheric pressure.
 The locking pin is removed and the gas is allowed to expand isobarically to twice its volume, lifting up a weight. The spigot continues to add gas to the cylinder during this process to keep the pressure constant.
 Once the expansion has finished, the spigot is released, the highpressure gas is allowed to escape, and the sample settles back to
1atm .  Finally, the lid of the container is pushed back down. As the volume decreases, gas is allowed to escape through the spigot, maintaining a pressure of
1atm . At the end, the pin is locked again and the process restarts.
 Draw the above steps on a
P−V diagram.  Calculate the highest and lowest temperatures of the gas.
 A heat engine operates through
4 cycles according to thePV diagram sketched below. Starting at the top left vertex they are labeled clockwise as follows: a, b, c, and d. From
a−b the work is75J and the change in internal energy is100J ; find the net heat.  From the ac the change in internal energy is
−20J . Find the net heat from bc.  From cd the work is
−40J . Find the net heat from cda.  Find the net work over the complete
4 cycles.  The change in internal energy from bcd is
−180J . Find: the net heat from cd
 the change in internal energy from da
 the net heat from da
 From
 A
0.1 sample mole of an ideal gas is taken from state A by an isochoric process to state B then to state C by an isobaric process. It goes from state C to D by a process that is linear on aPV diagram, and then it goes back to state A by an isobaric process. The volumes and pressures of the states are given in the Table (below); use this data to complete the following: Find the temperature of the
4 states  Draw a
PV diagram of the process  Find the work done in each of the four processes
 Find the net work of the engine through a complete cycle
 If
75J of heat is exhausted in DA and AB and CD are adiabatic, how much heat is inputted in BC?  What is the efficiency of the engine?
 Find the temperature of the
state 
Volume in 
Pressure in 

A 


B 


C 


D 


Answers to Selected Problems
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 800,000 cal or 3360 kJ
 150 cal (630 J)
 same as a!

0.027 cal/g∘C (0.11 J/g∘C)
 67,000 J
 67.2 s
 1.1 min
 11.0 g

31.8∘C 
0.44 cal/g∘C  1600 cal (6720 J)
 1800 cal (7560 J)
 59.3 cal/g

517m/s 
1.15×1012K  .

40N 
≈ 1028 molecules
21,000Pa  Decreases to
61,000Pa 
5.8km }}
 No
 allowed by highly improbable state. More likely states are more disordered.

 a.
8.34×1023 b.6.64×10−27kg c.1600m/s d.744kPa e.4.2×1020 or0.0007moles g.0.00785m3 
1.9MW 
0.56MW 
1.3Mw

54 % 
240kW 
890kW 
590kW 
630kg

98 % 
4 % 
12 %


14800J 
12,000J  b.
720K,300K,600K c. isochoric; isobaric d.C toA;B−C e.0.018J  b.
300K,1200K 
1753J 
−120J 
80J 
35J 
−100J,80J,80J
