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
Angular frequency for the vector quantity angular velocity. One revolution is equal to 2π radians, hence where ω is referred to as the natural frequency (which can sometimes be denoted as ω). As the object oscillates, its acceleration can be calculated by where "x" is displacement from an equilibrium position. Using "ordinary" revolutions-per-second frequency, this equation would be The resonant angular frequency in a series LC circuit equals the square root of the reciprocal of the product of the capacitance ("C" measured in farads) and the inductance of the circuit ("L", with SI unit henry): Adding series resistance (for example, due to