Page 34 - Basic Electrical Engineering

The resistivity, ρ is also called the specific resistance of the material. The
most conducting material, silver has the lowest value of resistivity, i.e., 0.016
−6
× 10 ohm-m. After silver, copper is most conducting. The resistivity or
specific resistance of copper is somewhat more than that of silver, i.e., 0.018
−6
× 10 ohm-m. That is to say, copper is less conducting than silver. We will
see a little later why and how the value of resistance changes with
temperature.

1.5.3 Potential and Potential Difference

EMF produces a force or pressure that causes the free electrons in a body to

move in a particular direction. The unit of EMF is volt. EMF is also called
electric potential. When a body is charged (i.e., either defficiency of electrons
or excess of electrons is created), an amount of work is done. This work done

is stored in the body in the form of potential energy. Such a charged body is
capable of doing work by attracting or repelling other charges. The ability of

a charged body to do work in attracting or repelling charges is called its
potential or electrical potential. Work done to charge a body to 1 C is the
measure of its potential expressed in volts:

When work done is 1 joule and charge moved is 1 C, the potential is called 1

volt. If we say that a point has a potential of 6 volts, it means that 6 Joules of
work has been done in moving 1 C of charge to that point. In other words, we

can say that every Coulomb of charge at that point has an energy of 6 Joules.
The potential difference of two points indicates the difference of charged
condition of these points. Suppose point A has a potential of 6 volts, and

point B has a potential of 3 volts. When the points A and B are joined
together by a conducting wire, electrons will flow from point B to point A.

We say that current flows from point A towards point B. The direction of
current flow is taken from higher potential to lower potential while the flow

of electrons are actually in the opposite direction. The flow of current from