thermodynamics - example with answer

[wanted90]

Hi

Here is a question with answer , in fact I don't understand the answer fully ..

please can explain to me the answer .. pleas anyone clear out the answer

 

and here in answer I saw that work is negative is that because work done by the system ?

 

The question : -

 

solution : -

 

[studiot]

What exactly did you not undestand about this?

[wanted90]

Why here work is become negitive not plus

 

Pleas clear out that

[studiot]

But the work is not negative.

 

This is just about sign conventions.

 

The expression of the First Law given is

 

dU = Q-W

 

so corrresponds to my first sign convention in your previous thread.

 

Work done by the system (aluminium block) is positive.

 

The quantity in brackets in the example is clearly positive and subtracted from the incoming heat, in acordance with this First Law convention.

 

Think of internal energy as like your bank account with receipts as the heat and payments as the work done. Both are positive.

 

The First Law of Banking says

 

Closing Balance = Opening Balance + receipts - payments.

 

Does this help?

Edited October 4, 2013 by studiot