The 1st law of thermodynamics

If a gas expands adiabatically will the work done be positive or negative?

I think it will be positive as $\delta W=p dV$ and we have a positive sign due to the work done by the gas is positive as it pushes to expand. But this site has the negative of the answer I obtain.

http://physics.bu.edu/~duffy/py105/Firstlaw.html (See adiabatic processes section)

Answer

Generations of physics students, including me, have got mixed up about the sign of work done. That's because the phrase work done can mean work done on the gas or work done by the gas, and these are equal but with opposite signs. I don't think there is any perfect way to deal with this except by using your common sense. If an expanding gas does work then that work must come from the internal energy of the gas, and therefore the internal energy of the gas must decrease.

Having said all this, I think you've misinterpreted the article you quote. In the Adiabatic processes section it starts the equation with:

$$ W = -\Delta U $$

But in the expansion $\Delta U$ is a negative quantity because the internal energy decreases. That means $-\Delta U$ is a positive number and therefore that the work is positive.