i see the problem now......

In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid.

so delta Q = 0.

1st law thermodynamic is delta U = delta Q - delta W , where W is work done by gas.

so delta U = 0 - delta W             imply delta U = - delta W

i did notice the applet is wrong cos
when Adiabatic,
T = constant, which should not be.
i did a simple calculation, PV = uRT,

i notice the u (number of moles of gas) is different/changing if i vary the slider at the bottom "volume".

but if i vary the slider on the right, it is correct!!

[b]maybe to solve the control error, disable the slider "VOLUME" when adiabatic is "TRUE" :)[/b]