Conservation

1 / 10

# Conservation - PowerPoint PPT Presentation

Conservation. Mechanical work involves a force acting through a distance. Work can involve a change in internal energy. Temperature may change Temperature did not cause the work. Work Expanded. Work is an energy transfer that is not due to a temperature difference. .

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about 'Conservation' - helga

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

### Conservation

Mechanical work involves a force acting through a distance.

Work can involve a change in internal energy.

Temperature may change

Temperature did not cause the work

Work Expanded

Work is an energy transfer that is not due to a temperature difference.

Heat is an energy transfer that is due to a temperature difference.

First Law of Thermodynamics
• Internal energy can only change by heat transferred to and work done by a system.
• Heat lost is –
• Work by a system is +
• Work on a system is -

When work is defined as positive on a system it is DU = Q + W.

PV Diagram
• The ideal gas law links pressure, volume, and temperature.
• PV = nRT
• For fixed quantity of gas, any two of the three determine the third.
• Graph of pressure vs volume describes the system.

P

V

Slow Motion
• During rapid change the macroscopic state variables are not well defined.
• On a PV diagram it is assumed that the system changes slowly.
• This is a quasi-static change.
Measuring Work
• The mechanical work can be measured on the PV diagram.
• The work done is the area under the PV curve describing the system.

P

V

work done

Constant Temperature
• Changes often involve holding one variable constant.
• Moving a piston slowly changes pressure and volume.
• Temperature remains the same.

Walls and top are insulating

Base is kept at a constant temperature

Isothermal Process
• For an ideal gas at constant temperature DT = 0, P is inversely proportional to V.
• P = nRT(1/V)
• The work done by an ideal gas can be found with calculus.

P

V

Internal Energy Conserved
• An ideal gas only changes internal energy with temperature change.
• DU = 0
• From the first law, work done equals heat flow.
• DU = Q – W
• Q = W

Work done by gas

Heat flow into gas

A scuba diver is swimming at a depth of 25 m where the pressure is 3.5 atm. The exhaled air forms bubbles 8.0 mm in radius. The bubbles stay at 300 K as they rise.

How much work is done by each bubble as it expands reaching the surface?

The ideal gas law is used to find the amount of gas.

P and V are related.

This is an isothermal process.

W = 0.95 J.

Underwater

next