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
Electric current with formula_20 being the elementary charge and formula_21 the electron density. The carriers move in the direction of decreasing concentration, so for electrons a positive current results for a positive density gradient. If the carriers are holes, replace electron density formula_21 by the negative of the hole density formula_23. In linear anisotropic materials, "σ", "ρ" and "D" are tensors. In linear materials such as metals, and under low frequencies, the current density across the conductor surface is uniform. In such conditions, Ohm's law states that the current is directly proportional to the potential difference between two ends (across) of that