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Physics 101: Lecture 27, Pg 1

Thermodynamics


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First Law of ThermodynamicsEnergy Conservation

D

U =Q- W

Heat flowinto system

Increase in internalenergy of system

Work done by system

V

P

1 2

3

V1 V2

P1

P3Equivalent ways of writing 1st Law:

Q = DU + W

The change in internal energy of a system (DU) is equal to the heat

flow into the system (Q) minus the work done by the system (W)


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Signs Example

You are heating some soup in a pan on thestove. To keep it from burning, you also stirthe soup. Apply the 1stlaw of

thermodynamics to the soup. What is the signof (A=Positive B= Zero C=Negative)

1) Q

2) W3) DU


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Work Done bya System ACT

M

MDy

W = p DV :For constant Pressure

The work done by the gas as it contracts isA) Positive B) Zero C) Negative


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Thermodynamic Systems andP-V Diagrams

ideal gas law: PV = nRT

for nfixed, Pand Vdetermine state of system

T = PV/nR

U = (3/2)nRT = (3/2)PV

Examples:

which point has highest T?

which point has lowest U?

to change the system from Cto B,

energy must be (Added to : Removed from) system

V

P

A B

C

V1 V2

P1

P3


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First Law of ThermodynamicsIsobaric Example

V

P1 2

V1 V2

P2 molesof monatomic ideal gas is taken

from state 1to state 2at constant pressure

p=1000 Pa, where V1 =2m3and V2 =3m

3. Find

T1, T2, DU, W, Q.


7/16Physics 101: Lecture 27, Pg 7

First Law of ThermodynamicsIsochoric Example

2 molesof monatomic ideal gas is takenfrom state 1to state 2at constant volume

V=2m3, where T1=120Kand T2 =180K. Find

Q.

V

P

2

1

V

P2

P1



Homework Problem:Thermo I

V

P 1 2

34

V

P

1 2

34

V

P

1 2

34

V

P

1 2

34

V

1 2

34

P

Wtot= ??



PV ACTs

Shown in the picture below are the pressure versus volume graphs for

two thermal processes, in each case moving a system from state Atostate Balong the straight line shown. In which case is the work done

by the system the biggest?

1. Case 1

2. Case 23. Same

A

B4

2

3 9 V(m3)

Case 1

A

B

4

2

3 9 V(m3)

P(atm)

Case 2

P(atm)



PV ACT 2


two thermal processes, in each case moving a system from state Atostate Balong the straight line shown. In which case is the change ininternal energy of the system the biggest?

1. Case 1

2. Case 23. Same

A

B4

2

3 9 V(m3)

Case 1

A

B

4

2

3 9 V(m3)

P(atm)

Case 2

P(atm)



PV ACT3


two thermal processes, in each case moving a system from state Atostate Balong the straight line shown. In which case is the heat addedto the system the biggest?

1. Case 1

2. Case 23. Same

A

B4

2

3 9 V(m3)

Case 1

A

B

4

2

3 9 V(m3)

P(atm)

Case 2

P(atm)



Q= DU + W

Heat flow

into system


Work done by system

V

P

1 2

3

V1 V2

P1

P3

Which part of cycle has largest change in internal energy, U ?

Which part of cycle involves the least work W ?

What is change in internal energy for full cycle?

What is net heat into system for full cycle (positive or negative)?

Some questions:

First Law Questions



Special PV Cases

Constant Pressure (isobaric)

Constant Volume

Constant Temp DU = 0

Adiabatic Q=0

V

PW = PDV (>0)

1 2

34

DV > 0

V

PW = PDV = 0

1 2

34

DV = 0



Preflights 3 - 5Consider a hypothetical device that takes 1000 J of heatfrom a hot reservoir at 300K, ejects 200 J of heat to a cold

reservoir at 100K, and produces 800 J of work.

Does this device violate the first law of thermodynamics ?

1. Yes

2. No



Reversible?

Most physics processes are reversible,you could play movie backwards and stilllooks fine. (drop ball vs throw ball up)

Exceptions:

Non-conservative forces (friction)

Heat Flow:

Heat never flows spontaneously from cold to hot


16/16Physics 101: Lecture 27 Pg 16

Summary:

1st Law of Thermodynamics: Energy Conservation

Q= DU + W

Heat flowinto system


Work done by system

V

P

point on p-V plot completely specifiesstate of system (pV = nRT)

work doneis area under curve

Udepends only on T (U = 3nRT/2 = 3pV/2)

for a complete cycle DU=0 Q=W

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