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# PHYSICS 231 INTRODUCTORY PHYSICS I - PowerPoint PPT Presentation

PHYSICS 231 INTRODUCTORY PHYSICS I. Lecture 18. Chapter 12. The Laws of Thermodynamics. Principles of Thermodynamics. Energy is conserved FIRST LAW OF THERMODYNAMICS Examples: Engines (Heat -> Mechanical Energy) Friction (Mechanical Energy -> Heat) All processes must increase entropy

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Lecture 18

### Chapter 12

The Laws of Thermodynamics

• Energy is conserved

• FIRST LAW OF THERMODYNAMICS

• Examples:

• Engines (Heat -> Mechanical Energy)

• Friction (Mechanical Energy -> Heat)

• All processes must increase entropy

• SECOND LAW OF THERMODYNAMICS

• Entropy is measure of disorder

• Engines can not be 100% efficient

• Change temperature

• Change state of matter

• Can also change U by doing work on the gas

Change of

Internal Energy U

Work done on the gas

First Law of Thermodynamics

Note:

(Work done by the gas) = - (Work done on the gas)

Add heat => Increase Int. Energy & Gas does work

A cylinder of radius 5 cm is kept at pressure with a piston of mass 75 kg.

a) What is the pressure inside the cylinder?

b) If the gas expands such that the cylinder rises 12.0 cm, what work was done by the gas?

c) What amount of the work went into changing the gravitational PE of the piston?

d) Where did the rest of the work go?

1.950x105 Pa

183.8 J

88.3 J

Compressing the outside air

A massive copper piston traps an ideal gas as shown to the right. The piston is allowed to freely slide up and down and equilibrate with the outside air.

The pressure inside thecylinder is _________ thepressure outside.

a) Greater than

b) Less than

c) Equal to

A massive copper piston traps an ideal gas as shown to the right. The piston is allowed to freely slide up and down and equilibrate with the outside air.

The temperature inside the cylinder is __________ the temperatureoutside.

a) Greater than

b) Less than

c) Equal to

A massive copper piston traps an ideal gas as shown to the right. The piston is allowed to freely slide up and down and equilibrate with the outside air.

If the gas is heated by a steady

flame, and the piston rises to a new equilibrium position, the new pressure will be _________ than

the previous pressure.

a) Greater than

b) Less than

c) Equal to

V

V

V

Some Vocabulary

P

• Isobaric

• P = constant

• Isovolumetric

• V = constant

• W = 0

• Isothermal

• T = constant

• U = 0 (ideal gas)

• Q = 0

P

P

P

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, isothermally compressing the gas to

half its original volume (b)

Pb is _______ Pa

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, isothermally compressing the gas to

half its original volume (b)

Tb is ________ Ta

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, isothermally compressing the gas to

half its original volume (b)

Wab is ________ 0

a) Greater than

b) Less than

c) Equal to

Vocabulary: Wab is work done by gas between a and b

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, isothermally compressing the gas to

half its original volume (b)

Ub is ________ Ua

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, isothermally compressing the gas to

half its original volume (b)

Qab is ________ 0

a) Greater than

b) Less than

c) Equal to

Vocabulary: Qab is heat added to gas between a and b

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, adiabatically compressing the gas to

half its original volume (b)

Pb is _______ Pa

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, adiabatically compressing the gas to

half its original volume (b)

Wab is ______ 0

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, adiabatically compressing the gas to

half its original volume (b)

Qab is _______ 0

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, adiabatically compressing the gas to

half its original volume (b)

Ub is _______ Ua

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

Weight is slowly added to the piston, adiabatically compressing the gas to

half its original volume (b)

Tb is _______ Ta

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

The gas is cooled, isobarically compressing the gas to half its original volume (b)

Pb is _______ Pa

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

The gas is cooled, isobarically compressing the gas to half its original volume (b)

Wab is _______ 0

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

The gas is cooled, isobarically compressing the gas to half its original volume (b)

Tb is _______ Ta

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

The gas is cooled, isobarically compressing the gas to half its original volume (b)

Ub is _______ Ua

a) Greater than

b) Less than

c) Equal to

Outside Air: Room T, Atm. P

A massive piston traps an amount of

Helium gas as shown. The piston freely slides up and down.

The system initially equilibrates at

room temperature (a)

The gas is cooled, isobarically compressing the gas to half its original volume (b)

Qab is _______ 0

a) Greater than

b) Less than

c) Equal to

P

Path moves to right:

Wby the gas = Area under curve

P

V

Path moves to left:

Wby the gas = - Area under curve

V

(Won the gas = - Wby the gas)

WA->B = Area

WB->A = - Area

Work from closed cycles

Consider cycle A -> B -> A

(work done by gas)

Consider cycle A -> B -> A

WA->B->A= Area

(work done by gas)

WB->A = - Area

WA->B = Area

Work from closed cycles

Reverse the cycle, make it counter clockwise

(work done by gas)

Reverse the cycle, make it counter clockwise

WA->B->A= - Area

(work done by gas)

in closed cycles

a) What amount of work is performed by the gas in the cycle IAFI?

b) How much heat was inserted into the gas in the cycle IAFI?

c) What amount of work is performed by the gas in the cycle IBFI?

W=3.04x105 J

Q = 3.04x105 J

V (m3)

W = -3.04x105 J

Consider a monotonic ideal gas.

a) What work was done by the gas from A to B?

b) What heat was added to the gas between A and B?

c) What work was done by the gas from B to C?

d) What heat was added to the gas beween B and C?

e) What work was done by the gas from C to A?

f) What heat was added to the gas from C to A?

P (kPa)

A

75

20,000 J

50

20,000

B

25

-10,000 J

C

V (m3)

-25,000 J

0.2

0.4

0.6

0

15,000 J

Take solutions from last problem and find:

a) Net work done by gas in the cycle

b) Amount of heat added to gas

WAB + WBC + WCA = 10,000 J

QAB + QBC + QCA = 10,000 J

This does NOT mean that the engine is 100% efficient!

C

P

Consider an ideal gas undergoing the trajectory through the PV diagram.

In going from A to B to C, the work done BY the gas is _______ 0.

B

A

V

>

<

=

C

P

In going from A to B to C, the change of the internal energy of the gas is _______ 0.

B

A

V

>

<

=

C

D

P

In going from A to B to C, the amount of heat added to the gas is _______ 0.

B

A

V

>

<

=

C

D

P

In going from A to B to C to D to A, the work done BY the gas is _______ 0.

B

A

V

>

<

=

C

D

P

In going from A to B to C to D to A, the change of the internal energy of the gas is _______ 0.

B

A

V

>

<

=

C

D

P

In going from A to B to C to D to A, the heat added to the gas is _______ 0.

B

A

V

>

<

=