In a process, 25 J is transferred into a system by heat and 45 J is transferred out. What is the net heat transfer, Q?

1. Net heat Q is the energy transferred in minus the energy transferred out. 2. Substitute the values: Q = 25 J minus 45 J. 3. Subtracting gives Q = -20 J. 4. -70 J and 70 J would come from adding the two values instead of subtracting them, and 20 J drops the correct sign. 5. So -20 J is correct, with the negative sign showing net heat leaves the system. _Source: OpenStax Physics (CC BY 4.0), Ch 12 "Thermodynamics", section 12.2 First law of Thermodynamics: Thermal Energy and Work_