Download presentation

Presentation is loading. Please wait.

1
Work In Simple Systems Physics 313 Professor Lee Carkner Lecture 7

2
Exam #1 Monday, March 29 th Covers: Lectures 1-9 Chapters 1-4 Format: About 10 multiple choice (~25% weight) About 4 problems (~75%weight) Equations provided Bring just pencil and calculator Worth 20% of final grade

3
Exercise 5 - Shake Work Find expression for P from equation of state and integrate P = 15TV -3.4 W = - 15TV -3.4 dV = -15T/-2.4V 2.4 W = (15)(265)/(2.4)(2) 2.4 - (15)(265)/(2.4)(3) 2.4 = Trying to add to internal energy

4
Work and Systems Thermodynamic systems are often designed to produce work … or to add work to a system Need to be able to compute the work Even between same two states, work will vary (depends on path)

5
Force and Temperature In general, work can be related as: dW = F dx Need a “force” term Need a “displacement” term Force term often depends on T Cannot compute work without understanding the heat transfer For simplicity we will often discuss isothermal systems

6
Hydrostatic Systems W = - P dV Can use ideal gas law, but need to limit T Examples: Isothermal: Isobaric:

7
Polytropic Process Often for compression and expansion of a gas, pressure and volume are related by: Where C and n are constants Called a polytropic process Example:

8
Stretched Wire W = dL how much energy does it take to cause a small increase in length? = k L

9
Surface W = dA how much energy does it take to cause a small increase in area? Integral of force over length, area or volume

11
Shaft Work When transmitting energy with a rotating shaft, work depends on the torque: T = Fr The displacement is related to the number of revolutions, n Work is then: We can also write power as Where (n/t) is the number of revolutions per second

12
Electrochemical Cell W = dZ how much energy does it take to cause a small movement of charge? The movement of charge produces a current: W = I dt Can measure current easier than charge

14
Dielectric Solid Can place a dielectric solid between the plates of a capacitor that produces a uniform electric field W = E dP how much energy does it take to cause a small alignment of induced dipoles? or else system is not in equilibrium

16
Paramagnetic Rod Induce the magnetic field by wrapping the material in wire and run a current Battery does work to move charge, induce a field and then induce small currents which produce magnetic dipoles W = 0 H dM how much energy does it take to cause a small alignment of induced magnetic dipoles?

18
Composite Systems Not just three dW = Y dX + Y’ dX’ + Y’’dX’’ … The plots of XY become multidimensional

21
Work -- General Case For a system specified by X, Y and Z, the work is the integral of one variable with respect to another Since dW = F dx, the two variables are related to the force and the displacement The displacement variable is extensive

Similar presentations

© 2019 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google