where the first term on the right is the heat transferred into the system, expressed in terms of temperature "T" and entropy "S" (in which entropy increases and the change d"S" is positive when the system is heated), and the last term on the right hand side is identified as work done on the system, where pressure is "P" and volume "V" (the negative sign results since compression of the system requires work to be done on it and so the volume change, d"V", is negative when work is done on the system).