Thermodynamics, isothermal expansion for ideal gas?

consider the free, isothermal (constant T) expansion of an ideal gas. "Free" means that the external force is zero, perhaps because a stop has been opened and the gas is allowed to expand into a vacuum. Calculate ∆U for this irrevesible process. show that q=0, so that the expansion is also adiabatic (q=0) for an ideal gas. this is ogous to a clic experiment first performed by Joule.