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Unformatted text preview: FIRST LAW OF THERMODYNAMICS We have learnt that if a thermodynamic system has a conducting boundary (that is, it is not
isolated from its surroundings) and its temperature TI is different from that of the surroundings T2 , there will be an exchange of energy driven by the temperature difference and
this energy is called heat DQ. If T2 > TI DQ enters our system, if T2 <Tl , DQ leaves our system. For liquids and solids
DQ = chT [change of temperature] DQ = mL [change of phase] We have also learnt that we can use mechanical work to mimic the effects of heat. Indeed 4.18J
will produce same effect as transferring one calorie of DQ. [Expt On mechanical equivalent]. Next, consider a gas. If the piston moves by Ay work done is
AW = FAy = PAAy = PAV
For a ﬁnite change of volume ,. .. g and the work done WP2 is the area under the P vs. chrve.
w Two facts stand out:
1. In the process 1—2 the work done depends on the path (thermodynamic)
2. Because work can be used to mimic DQ, the quantity of heat exchange also depends
on the path AND NOTE: DQ and DWboth involve interaction of system with surrounding. [AT for former,
moving piston for latter] The beauty is that the algebraic sum of DQ and DW, that is,
iDQiDW IS INDEPENDENT OF THE PATH! Recall that whenever we have a change of energy independent of the path we can deﬁne a
potential so now we deﬁne a thermodynamic potential called the internal energy U and this takes account of all the changes of energy in a thermodynamic process. The conservation law for energy then asserts that
idUiDQiDWEO or equivalently dU +DQ + DW V ,
Path Independent Path Dependent this is the formal statement of the first law of thermodynamics. 13L; d —> Path independent change D —> Path dependent change
For solids/liquids DQ, DW go to change dU = chT or 1711.. For an ideal gas, U is a function of
temperature only. If gas is monatomic U W = gNkBT (Proved in class) If gas is diatomic U D A = g—NkBT . (Good near 300K) k3 =1.383><10"23J/K
Tis temperature in °K ...
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 Spring '10
 Shawhan
 Physics, Thermodynamics

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