In the first law equation, ΔU = Q minus W, what sign convention applies to Q and W?

1. By convention, a positive Q means there is net heat transfer into the system. 2. A positive W means there is net work done by the system on its surroundings. 3. Negative Q instead means energy leaves the system by heat, and negative W means work is done on the system, both of which raise internal energy. 4. This is the opposite pairing from the second option, and the third option mismatches W's sign meaning. 5. Heat and work can each be negative depending on direction, so both being always positive is false. 6. So positive Q into the system and positive W done by the system is the correct convention. _Source: OpenStax Physics (CC BY 4.0), Ch 12 "Thermodynamics", section 12.2 First law of Thermodynamics: Thermal Energy and Work_