I looked at this video :
http://cbse.meritnation.com/img/lp_videos/1/11/12/190/665/1309/929/video_1_11_12_190_665_1309_929-1.swf
It says that it expands isothermally first and the adiabatically, but why does it expand so....is it due to any external force or just due to the pressure and...