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ThermodynamicsThermodynamics

The study of The study of energy energy

conversions conversions between heat and between heat and

other forms.other forms.

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Pressure: force / area Pressure: force / area

Units of Pressure: N/mUnits of Pressure: N/m22 Also known as Pascals (Pa) Also known as Pascals (Pa)

33Work = F(Work = F(ΔΔh) = (F/A) (h) = (F/A) (ΔΔh*A) = P (h*A) = P (ΔΔV)V)

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If a piston compresses its If a piston compresses its volume by .03 mvolume by .03 m33 at a at a

constant pressure of 10constant pressure of 1044 Pa, Pa, A.) How much work does it A.) How much work does it

dodo??

B.) Does the gas gain or lose B.) Does the gas gain or lose

energy?energy?

Work = P (Work = P (ΔΔV) = 10V) = 1044 (.03) = 300 J (.03) = 300 J

A Gas being compressed gains A Gas being compressed gains energyenergy

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Do Now: Do Now:

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Identify the type of process:Identify the type of process:1.1.Compressed air expands when Compressed air expands when released from airhornreleased from airhorn2.2.A sealed 2L bottle of Pepsi A sealed 2L bottle of Pepsi warms in a room.warms in a room.3.3.The piston in a diesel engine The piston in a diesel engine compresses quickly and elevates compresses quickly and elevates the Temperature enough to ignite the Temperature enough to ignite the fuel.the fuel.4.4.The water in a cooking pot The water in a cooking pot expands slightly as it heats up expands slightly as it heats up before beginning to boil. before beginning to boil.

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The First LawThe First Law

A total of 135 J of work is done on a gaseous A total of 135 J of work is done on a gaseous refrigerant as it is compressed. If internal energy refrigerant as it is compressed. If internal energy increases by 114 J, what is the total amount of increases by 114 J, what is the total amount of energy transferred as heat?energy transferred as heat?

W = -135 J W = -135 J

ΔU = +114 J ΔU = +114 J

ΔU = Q - WΔU = Q - W

+114 J = Q – (-135 J)+114 J = Q – (-135 J)

Q = -21 JQ = -21 J

Energy removed as Energy removed as heatheat

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The First LawThe First Law

If the internal energy of a gas decreases by 3500 J If the internal energy of a gas decreases by 3500 J in a isovolumetric process, how much heat is in a isovolumetric process, how much heat is transferred? Is heat transferred to or from the gas?transferred? Is heat transferred to or from the gas?

W = 0 W = 0

ΔU = -3500 J ΔU = -3500 J

ΔU = Q - WΔU = Q - W

-3500 J = Q – 0-3500 J = Q – 0

Q = -3500 JQ = -3500 J

Energy removed as Energy removed as heatheat

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Part 2Part 2

If the internal energy of a gas decreases by 3500 J If the internal energy of a gas decreases by 3500 J in a isovolumetric process, by how much does the in a isovolumetric process, by how much does the temperature of the gas change if there is 2.0 kg of temperature of the gas change if there is 2.0 kg of gas and the specific heat is 350 J/kggas and the specific heat is 350 J/kgooC ?C ?

Cp = 350 Cp = 350

Q = -3500 J Q = -3500 J

Q = CQ = CppM∆TM∆T

-3500 J = (350)(2.0)∆T-3500 J = (350)(2.0)∆T

∆∆T = -5.0 T = -5.0 ooCC

Temperature Temperature decreasesdecreases

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The First LawThe First Law

A gas is trapped in a small metal cylinder with a A gas is trapped in a small metal cylinder with a movable piston and is submerged in a large amount movable piston and is submerged in a large amount of ice water. The initial temperature of the gas is of ice water. The initial temperature of the gas is 0°C. A total of 1200 J of work is done on the gas by 0°C. A total of 1200 J of work is done on the gas by a force that a force that slowlyslowly pushes the piston inward. pushes the piston inward.

How much energy is transferred as heat How much energy is transferred as heat between the gas and the ice water?between the gas and the ice water?

ΔU = 0 J ΔU = 0 J

0 = Q – W0 = Q – WQ = WQ = W

““slow” process slow” process means it has time means it has time to stay in thermal to stay in thermal equilibrium with equilibrium with the surrounding the surrounding ice water.ice water.

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Cyclic ProcessesCyclic Processes

The process is cyclic, so the working The process is cyclic, so the working fluid returns to its initial fluid returns to its initial temperature and pressure.temperature and pressure.

U = 0 so Qnet = Wnet

-Heat engines take heat from a substance at a high temperature, use that energy to do some work, and then expel heat from the substance at a lower temperature.

-Internal combustion engines are heat engines.

-Hot gases do work on the piston and then are expelled to a lower-temperature environment.

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Heat Engines Heat Engines

-Heat Q-Heat Qhh: Enters : Enters

engine (burning engine (burning fuel) fuel)

-Heat Q-Heat Qcc: Leaves : Leaves

engine (exhaust)engine (exhaust)

-The difference is -The difference is used to do workused to do work

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Do now: Explain why a cold air-conditioned Do now: Explain why a cold air-conditioned room feels colder if you enter it from room feels colder if you enter it from

outside on a summer day and warmer if you outside on a summer day and warmer if you enter it from a walk-in freezer?enter it from a walk-in freezer?

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22ndnd Law of Thermodynamics – No cyclic Law of Thermodynamics – No cyclic process can be 100% efficient at process can be 100% efficient at

converting heat to energy.converting heat to energy.

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Engine Type eff (calculated maximum values)

eff (measured values)

Steam engine 0.29 0.17

Steam turbine 0.40 0.30

Gasoline engine 0.60 0.25

Diesel engine 0.56 0.35

These are the maximum possible These are the maximum possible efficiencies based on how much heat is efficiencies based on how much heat is

“wasted” as exhaust. Friction and “wasted” as exhaust. Friction and conduction of heat reduce the efficiency conduction of heat reduce the efficiency

even further.even further.

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Efficiency Problem Efficiency Problem In one cycle, an engine absorbs 4,540 J of In one cycle, an engine absorbs 4,540 J of

heat from a high temperature reservoir heat from a high temperature reservoir and expels 1,430 J of heat as exhaust to a and expels 1,430 J of heat as exhaust to a cold temperature reservoir. What is the cold temperature reservoir. What is the

efficiency of the engine?efficiency of the engine?

During the cycle, the engine’s piston During the cycle, the engine’s piston compresses the gas by a volume of .06 mcompresses the gas by a volume of .06 m33. .

What is the average pressure during the What is the average pressure during the cycle? cycle?

(hint: how much work is done?) (hint: how much work is done?)

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A typical diesel engine has an ideal A typical diesel engine has an ideal efficiency of 56%. If it releases 2,500 efficiency of 56%. If it releases 2,500 J of energy as exhaust per cycle, how J of energy as exhaust per cycle, how

much work will it do per cycle?much work will it do per cycle?

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