# Difference between Isothermal and Adiabatic?

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## Main Question or Discussion Point

So for Isothermal, there is no change of T so hence delta U is 0. However, if there isn't a change of temperature, how is there a change of Q? I thought heat was the flow of energy of different temperatures. This confuses me for adiabatic as well. With no heat, how is there a temperature change. Also, apparently adiabatic processes, the change of temperature must be negative. However, why? For a second, I thought it may be because there's no added heat but there shouldn't be heat leaving either.
I feel like this is all tangled up in my head and I would appreciate some clarification!

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Chestermiller
Mentor
Do you think that work being done by the gas or on the gas has anything to do with this?

I did think about that. So if Q is 0 all the internal energy would go to work. However, I thought temperature change was required for there to be a change of internal energy. Then, I ended up back to, how is there no heat when there is a temperature change when heat is involved when there's a change of temperature. Even if I think that all the energy went to work, because of what I believed in the previous statement, it doesn't make sense to me.

Chestermiller
Mentor
I did think about that. So if Q is 0 all the internal energy would go to work. However, I thought temperature change was required for there to be a change of internal energy.
Yes. That's correct for an ideal gas.
Then, I ended up back to, how is there no heat when there is a temperature change when heat is involved when there's a change of temperature. Even if I think that all the energy went to work, because of what I believed in the previous statement, it doesn't make sense to me.
Heat is not the only thing that can cause a change in internal energy and, along with it, a temperature change. Work can also cause a change in internal energy (even without heat), and, along with it, a temperature change. This is the whole idea behind the first law of thermodynamics $\Delta U=Q-W$, which is basically a statement of conservation of energy. Both Q (heat) and W (work) can cause the internal energy (and temperature) to change. Joule proved this experimentally when he did mechanical work on a liquid and its temperature increased. This basically showed that doing work on a system is equivalent to adding heat to the system.