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[wynnl]

For the following question, the solution manual sets du/dt (change of internal energy) = 0.  Moreover, it also set the change of total entropy to zero. Why is that? 

 

An electric motor under steady load draws 9.7 amperes at 110 volts; it delivers 0.93 kW of mechanical energy. The temperature of the surroundings is 300 K. What is the total rate of entropy generation Sg ? What is its origin ?

Edited by wynnl, 26 March 2013 - 09:21 PM.

[Shivshankar]

Hi wynnl,

 

The total entropy change is zero because entropy chnage of steady flow devices such a compressors, nozzles, pumps and heat exchangers is zero during steady state operation.

  

Since electric motor is example of closed system

 

ΔE = Q - W   ( ΔE = 0 for closed system) 

 

Q = W

 

http://en.wikipedia....Internal_energy

  

http://lorien.ncl.ac...vers/isothe.htm

 

http://www.tim-thomp...opy2.html#2lotc

 

http://en.wikipedia.org/wiki/Entropy

 

http://www.entropyla...ropyenergy.html

 

You have current (I) and voltage (V) given, so you should find power(P=VI) which will be electrical work (Welec) done ( which will be negative).You have been given mechanical energy which is shaft work done (Wshaft).You have temperature (T) and total work done by electrical and shaft.(Welec + Wshaft).

  

Now put this all data in you entropy balance equation which you know and find rate of entropy generation (Sgen).

 

ΔS = Σ Q/T + Sgen   

  

Where

 

ΔS = 0

 

Q = Welec + Wshaft

 

Entropy as a thermodynamic property has its origin  in the second law of thermodynamics.

 

I hope it helps.

 

Regards

Shivshankar

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Edited by Shivshankar, 02 May 2013 - 07:07 AM.