neg pressure
Positive pressure pushes against the piston v. Negative pressure pulls in the piston
DV > 0 ? DU > 0 … Energy conservation OK? 1st law: DU= –pDV (DV > 0, DU > 0) ? (p < 0) just the required negative pressure
L ? constant energy per unit volume System can lower its energy by volume contraction (… pulls in the piston)