# Direct-Current Systems

Resistance of flow through a wire, measured in units called ohms (), depends on
the wire material. Metals like copper and aluminum have low resistance and are
classified as conductors.

Resistance for a given material varies inversely as the area of the cross section
and directly as the length of wire.
Ohms law states that the voltage E (volts) required to cause a flow of current
I (amperes) through a wire with resistance R (ohms) is given by Large amounts of power are measured in kilowatts (kW), a unit of 1000 W, or megawatts (MW), a unit of 1,000,000 W.
Electric Energy. The energy expended in a circuit equals the product of watts and time, expressed as watt-seconds or watt-hours (Wh). For large amounts of energy, a unit of 1000 watt-hours, or kilowatt-hours, kWh, is used.
Charges for electric use are usually based on two separate items. The first is total energy used per month, kWh, and the second is the peak demand, or maximum kW required over any short period during the month, usually 15 to 30 min.
Power Transmission. Power is usually transmitted at very high voltages to minimize the power loss over long distances. This power loss results from the energy consumed in heating the transmission cables and is equal to the square of the current flowing I, times a constant representing the resistance r of the wires, / ft, times the length L, ft, of the wires. Measured in watts,
Heat loss = I^2rL

Series Circuits. A series circuit is, by definition, one in which the same current I flows through all parts of the circuit (Fig. 15.1a). In such a circuit, the resistance R of each part is the resistance per foot times the length, ft. Also, by Ohms law, for each part of the circuit, the voltage drop is  Series circuits are most commonly used in street, airport runway, and subway lighting. Most building systems use parallel circuits for both motor and lighting distribution.
Network systems consisting of a combination of series and parallel circuits are used for municipal distribution.