The SI unit of current is the ampere (1 A = 1 C/s), one of the base units of the SI. By convention, the direction of current is the direction of flow of positive charge. If the carriers are negative, the direction of the current is opposite the direction of motion of the carriers.
A complete circuit is required for a continuous flow of charge.
The current in a metal is proportional to the drift speed (vD) of the conduction electrons, the number of electrons per unit volume (n), and the cross-sectional area of the metal (A):
Electrical resistance is the ratio of the potential difference across a conducting material to the current through the material. It is measured in ohms: 1 Ω = 1 V/A.
The resistivity ρ is an intrinsic characteristic of a particular material at a particular temperature and is measured in Ω·m. For many materials resistivity varies linearly with temperature:
ρ = ρ0(1 + αΔT)
(18-9)
A device that pumps charge is called a source of emf. The emf ξ is work done per unit charge [W = ξq, (Eq. 18-2)]. The terminal voltage may differ from the emf due to the internal resistance r of the source:
V = ξ - Ir
(18-10)
Kirchhoff's Junction rule: ΣI = 0 at any junction [(Eq. 18-11)]. Current into the junction is positive; current out of the junction is negative. Kirchhoff's Loop rule: ΣΔV = 0 for any path in a circuit that starts and ends at the same point [(Eq. 18-12)]. Potential rises are positive; potential drops are negative.
Circuit elements wired in series have the same current through them. Circuit elements wired in parallel have the same potential difference across them.
The power—the rate of conversion between electrical energy and another form of energy—for any circuit element is
P = IΔV
(18-19)
The SI unit for power is the watt (W). Electrical energy is dissipated (transformed into internal energy) in a resistor.
The quantity τ = RC is called the time constant for an RC circuit. The currents and voltages are
