Metrication in the United States There are no U.S. customary units for electric current, potential difference, or charge since these concepts were developed after the international adoption of metric in science. The metric units ampere, volt, ohm and coulomb are the only units used. The SI term hertz has replaced the term "cycles per second" as a unit of frequency. Energy is often measured in watt hours, BTUs, therms or calories rather than the SI joule. Residential and commercial electrical energy use is metered and billed in kilowatt hours, kW·h, as in most metric countries. Force measurements regularly use the pound instead of the SI
Collision frequency Collision frequency Collision frequency is defined in chemical kinetics and collision theory, in the background of "theoretical kinetics". If a system contains "n" type-a molecules per unit volume and "n" type-b molecules in that same unit volume, then the number of a-b collisions per unit time in that volume will be proportional to the product "nn". or, in terms of molar volumes ("[x]=n /N" where N is the Avogadro constant) By kinetic theory it can be shown that: where The collision frequency ("Z") is usually expressed in units of "moles of collisions" per unit time per unit volume: or equivalently: