Thermal energy

Three words get used as if they were one, and the exam separates them.

• Thermal energy is stored. A system has it.
• [Heat](/glossary/heat) is a transfer driven by a temperature difference. A system cannot have it, only receive or give it.
• [Internal energy](/glossary/internal-energy), symbol $U$, is the AP Physics 2 accounting name for a system's whole store: particle kinetic energy plus the potential energy of their configuration. For an ideal gas the configuration term vanishes, so $U$ and the thermal energy are the same number.

What it does in AP Physics 1. It is the destination for mechanical energy that goes missing. The Topic 3.4 boundary statement says AP Physics 1 expects students to know that mechanical energy can be dissipated as thermal energy or sound by nonconservative forces, and the Topic 3.2 boundary statement limits the course to analyzing the transfer of mechanical energy, although students should be aware that mechanical energy may be dissipated in the form of thermal energy or sound, adding that AP Physics 2 will study how thermal energy can be transferred between systems through heating or cooling. So AP Physics 1 names the destination without quantifying the transfer.

One number does get computed. EK 3.2.A.4.iii: the energy dissipated by friction is typically equated to the force of friction times the length of the path over which the force is exerted. Path length, not displacement, which is why a round trip dissipates on the way back too.

The phrase that costs the mark. "Friction generates heat" is the wrong sentence. The AP Physics 2 CED's appendix on physics language is direct: an object cannot have heat any more than an object may have work. Friction generates thermal energy. Heating is what moves it somewhere else afterward.