CURRENT AND RESISTANCE
The electric current i is established in a conductor when a net charge q passes through it in time t. Thus, the current is
The units for the parameters are
● i: amperes (A)
● q: coulombs (C)
● t: seconds (s)
The electric field exerts a force on the electrons to move them through the conductor. A positive charge moving in one direction has the same effect as a negative charge moving in the opposite direction. Thus, for simplicity we assume that all charge carriers are positive. We draw the current arrows in the direction that positive charges flow (Fig. 1.3).
A conductor is characterized by its resistance (symbol ). It is defined as the voltage difference between two points divided by the current flowing through the con- ductor. Thus,
where V is in volts, i is in amperes, and the resistance R is in ohms (abbreviated D).
The current, which is the flow of charge through a conductor, is often compared to the flow of water through a pipe. The water flow occurs due to the pressure difference between the inlet and outlet of a pipe. Similarly, the charge flows through the conductor due to the voltage difference.
The resistivity p is a characteristic of the conductor material. It is a measure of the resistance that the material has to the current. For example, the resistivity of copper is
1.7 X 10-8 D·m; that of fused quartz is about 1016 D·m. Table 1.2 lists some electrical properties of common metals.
The temperature coefficient of resistivity ex is given by
It represents the rate of variation of resistivity with temperature. Its units are 1/°C (or 1/°F). Conductivity (C), is used more commonly than resistivity. It is the inverse of conductivity, given by
