  Friction-driven static electrification is familiar and fundamental in daily
life, industry, and technology, but its basics have long been unknown and have
continually perplexed scientists from ancient Greece to the modern high-tech
era. Despite its simple manifestation, triboelectric charging is believed to be
very complex because of the unresolvable interfacial interaction between two
rubbing materials. Here, we for the first time reveal a simple physics of
triboelectric charging and triboelectric series based on friction-originated
thermoelectric charging effects at the interface, characterized by the material
density (${\rho}$), specific heat (c), thermal conductivity (k), and Seebeck
coefficient (S) of each material. We demonstrate that energy dissipational heat
at the interface induces temperature variations in the materials and thus
develops electrostatic potentials that will initiate thermoelectric charging
across the interface. We find that the trends and quantities of triboelectric
charging for various polymers, metals, semiconductors, and even lightning
clouds are simply governed by the triboelectric factor
${\xi}=S/\sqrt{{\rho}ck}$. The triboelectric figure-of-merit is expressed with
the triboelectric power K=${\xi}\sqrt{t/{\pi}}$, of which the difference can be
maximized up to 1.2 V/W cm$^{-2}$ at the friction time t = 1 s. Our findings
will bring significant opportunities for microscopic understanding and
management of triboelectricity or static electrification.
