Work Physics 313 Professor Lee Carkner Lecture 6.
Post on 22-Dec-2015
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Exercise #4 Stretched Wire Change in tension with temperature
(d/dT)L = -(d/dL)T(dL/dT)= -(AY/L)(L)
Frequency of guitar string f = ()½(1/) = (/)½(1/2L) f = (8.8/0.00005)½[1/(2)(0.8)] =
Change in tension (d/d)L = -aAY = -(2X10-5)(0.0001)(2.1X106)(20) = -0.084 f = [(8.8-0.084)/0.00005) ]½[1/(2)(0.8)] = 260.95
Hz Difference ~ 1 Hz
Work
Work is force times displacement
In thermodynamics we will only consider external work
Internal work involves one part of the system acting on another
Measured in joules
Sign Conventions
Work by the system is negative
Work done on the system is positive
Note that this is the opposite of the engineering convention (e.g. Halliday and Resnick)
Does the system gain or lose energy?
Work and Hydrostatic Systems
Work is not a property of the system
Work is a transfer of energy due to a volume change
Work, Pressure and Volume
dW = F dx
dW = -P dV
If dV is positive (increase in V) then W is negative (work by the system)
Total Work
To find the total work, integrate dW between the initial and final states:
Need to know P as a function of V
W depends on both the change of
volume and how the volume changed
Isothermal Process
Final expression for work in terms of constants and Vi and Vf
PV = nRT
W = - (nRT/V) dVW = -nRT (1/V) dV
PV Diagram
The process by which the volume changes is a line or curve connecting the two points
For different processes, different curves,
different amounts of work Even if the initial and final points are the same
Closed Cycle
If the same path is traveled in both directions, W=0
Cyclic processes are important for engines Repeat the same process over and over,
extract work each cycle
Path Dependence What are the paths? Isothermal: keep constant T (add
or subtract heat)
Isobaric: constant P (add or subtract heat) horizontal
choric: keep constant volume (rigid container, W=0)
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