wanted90 Posted October 4, 2013 Share Posted October 4, 2013 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 : - Link to comment Share on other sites More sharing options...
studiot Posted October 4, 2013 Share Posted October 4, 2013 What exactly did you not undestand about this? Link to comment Share on other sites More sharing options...
wanted90 Posted October 4, 2013 Author Share Posted October 4, 2013 Why here work is become negitive not plus Pleas clear out that Link to comment Share on other sites More sharing options...
studiot Posted October 4, 2013 Share Posted October 4, 2013 (edited) 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 Link to comment Share on other sites More sharing options...
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