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Lecture 3 – The First Law (Ch. 1) Friday January 11 thPowerPoint Presentation

Lecture 3 – The First Law (Ch. 1) Friday January 11 th

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Internal energy The equivalence of work and heat The first law (conservation of energy) Functions of state Reversible work

Lecture 3 – The First Law (Ch. 1)

Friday January 11th

- Test of the clickers (HiTT remotes)
- I will not review the previous class
- Usually I will (certainly after Ch. 2)

Reading: All of chapter 1 (pages 1 - 23)

1st homework set due next Friday (18th).

Homework assignment available on web page.

Assigned problems: 2, 6, 8, 10, 12

Functions of state: internal energy U

Adiabatic

Joule’s paddle wheel

experiment

Work = -DUgrav

W = -(-mgh)

= mgh

Measured as a change in temperature, q

Gravitational energy is lost. 1st law is about conservation of energy. This energy goes into thermal (‘internal’) energy associated with the fluid.

Functions of state: internal energy U

Adiabatic

Joule’s paddle wheel

experiment

DUfluid= W = mgh

Measured as a change in temperature, q

!!!!!!!!!!!!!!!!!!!!!!!

Gravitational energy is lost. 1st law is about conservation of energy. This energy goes into thermal (‘internal’) energy associated with the fluid.

Functions of state: internal energy U

Stirring

Adiabatic

Rise in q

(temperature)

DU = W = torque × angular displacement = tdf

Functions of state: internal energy U

Electrical

work

Adiabatic

Rise in q

(temperature)

R

i

DU = W = i2R

Functions of state: internal energy U

Reversible

work

Force, F

Adiabatic

Rise in q

(temperature)

DU = W = Force × distance = -PDV

The First Law of Thermodynamics

These ideas lead to the first law of thermodynamics (a fundamental postulate):

DU = Q + W

or

dU = đQ+đW

“The change in internal energy of a system is equal to the heat supplied plus the work done on the system. Energy is conserved if the heat is taken into account.”

Note that đQand đW are notfunctions of state. However, dU is, i.e. the correct combination of đQ and đW which, by themselves are not functions of state, lead to the differential internal energy, dU, which is a function of state.

How to know if quantity is a function of state

Significant

heat flows in

How can U be state function, but not W?

Heat is involved (not adiabatic).

đQ + đW

U1

U2

How to know if quantity is a function of state

z

dS

y

x

dr

There is a mathematical basis.....

Consider the function F = f(x,y):

How to know if quantity is a function of state

In general, F is a state function if the differential dF is ‘exact’. dF (= Adx + Bdy) is exact if:

- See also:
- Appendix E
- PHY3513 notes
- Appendix A in Carter book

- In thermodynamics, all state variables are by definition exact. However, differential work and heat are not.

There is a mathematical basis.....

Consider the function F = f(x,y):

How to know if quantity is a function of state

This is by no means true for any function!

If integration does depend on path, then the differential is said to be ‘inexact’, i.e. it cannot be integrated unless a path is also specified. An example is the following:

đF = ydx - xdy.

Note: is a differential đF is inexact, this implies that it cannot be integrated to yield a function F.

Differentials satisfying the following condition are said to be ‘exact’:

This condition also guarantees that any integration of dF will not depend on the path of integration, i.e. only the limits of integration matter.

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