The contents of the closed box in each of the following descriptions (located be

Question

The contents of the closed box in each of the following descriptions (located below) represent a system, and the arrows in the descriptions are to show the changes to the system during some process. The lengths of arrows represent the relative magnitudes of q and w.

Which of the following descriptions is endothermic?

For which of these descriptions, if any, is E < 0?

For which description, if any, does the system experience a net gain in internal energy?

Q=heat

W=Work

Box 1 has Q (short arrow) leaving box and W (long arrow) entering box.

Box 2 has just W (long arrow) entering box.

Box 3 has Q (long arrow) entering box and W (short arrow) leaving box.

Explanation / Answer

We know that

E = Q+ W (1st Law of Thermodynamics)

1) Box 1 has Q (short arrow) leaving box and W (long arrow) entering box.

Q Short arrow leaving box so heat is leaving the system (small amount as arrow is small), Q <0

W Long arrow entering box wo work is done upon the system (large amount) W > 0

so overall all E = -Q+ W (however W> Q ) so  E > 0

2) Box 2 has just W (long arrow) entering box.

So Q = 0

W : upon the system W >0

so E = 0 + W   E >0

3) Box 3 has Q (long arrow) entering box and W (short arrow) leaving box.

Q Long arrow entering box so heat is entering the system, Q > 0

W short arrow leaving box wo work is done by the system W < 0

so overall all E = Q - W (however W < Q ) so  E > 0

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