Heat

The symbol is $Q$ and the unit is the joule, but the quantity names a process rather than a property. The AP Physics 2 CED is blunt about this in its appendix on physics language: using the physics definition of heat, an object cannot have heat any more than an object may have work.

What a system stores is internal energy. Heating and cooling are two of the ways that store gets changed, and the CED separates them at 9.3.A.1.i and 9.3.A.1.ii: heating is the transfer of energy into a system by thermal processes, cooling is the transfer of energy out.

The sign is forced by the sheet's first law, $\Delta U = Q + W$.

• $Q > 0$ means the system was heated and energy entered it.
• $Q < 0$ means the system was cooled and energy left it.
• $Q = 0$ means nothing crossed by a thermal process, which is what adiabatic names.

Three thermal processes can carry it, and 9.3.A.2 names exactly those three: conduction, convection and radiation.

Two confusions to kill early. Heat is not temperature: $Q = mc\Delta T$ says the same $Q$ produces different temperature changes in different objects. And heat is not the only route into internal energy, since work does the same job, which is why the first law needs two terms.