Question

When is work considered negative in thermodynamics?

Answer 1

It depends on textbook to textbook and how you write the first law of thermodynamics!

i.e law of conservation of energy.

suppose you said

delta U= q + w

where delta U is change in internal energy.

Here heat transfered TO the system (by surrounding) to taken positive, cuz it will increase the internal energy.

q transfered BY the system is negative.

And work done TO the system is taken as positive, cuz that will increase the energy.

w done BY the system is negative.

now let's say your book says

delta U = q-w

Now heat will follow the same convention, but convection of work will reverse.

i.e now if work is done ON the system is taken negative ( as negative times negative will give positive, which is how it should be since energy will increase) .

And work BY the system is positive (system loses energy, +ve * -ve = -ve)

now imagine if I write it to be as delta U= w-q

or delta U= -q-w

I leave this for you to think about. Just remember this

If heat is transfered TO the system, Energy should go up.

If work is done TO the system, energy should go up.

Adjust your signs so this is true.

Hope it helped, feel free to comment.