FUNDAMENTALS OF ELECTRIC SYSTEMS

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FUNDAMENTALS OF ELECTRIC SYSTEMS

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,

i = q/t

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.

 A conductor is characterized by its resistance. It is defined as the voltage difference between two points divided by the current flowing through the conductor. Thus,

R = V/I

where V is in volts, i is in amperes, and the resistance R is in ohms.

           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 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 * 10^-8 (ohm meter). that of fused quartz is about 10¹⁶ (ohm.m). Table 1 lists some electrical

properties of common metals.

The temperature coefficient of resistivity(alpha) is given by:

Figure: Electrons drift in a direction opposite to the electric field in a conductor.

It represents the rate of variation of resistivity with temperature. Its units are 1/°C (or 1/°F).

Conductivity, is used more commonly than resistivity. It is the inverse of conductivity,

given by:

The units for conductivity are (ohm.m)^-1

Across a resistor, the voltage and current have this relationship:

V = iR

The power dissipated across the resistor (conversion of electric energy to heat) is given by:

P = i²R

or

P = V²/R

where P is in watts, i in amperes, V in volts, and R in ohms.

    TABLE 1 Properties of Metals as Conductors

Figure: Lines of Bnear a long, circular cylindrical wire. A

currenti,suggested by the central dot, emerges from the page

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