2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/83837The 1st law of thermodynamics for heat exchange is dQ=dU+PdV. According to K. Martinas etc., J. Non-Equil. Thermod. 23 (4), 351-375 (1988), for substances with negative thermal expansion coefficient, P in this law is negative. In the present paper it has been shown that P for such substances is positive but the sign before P must be minus not plus: dQ=dU-PdV.There was a misprint in Eq. (11), e-mail me istepanov@email.com, 7 pagesGeneral PhysicsClassical Physicstext