Page 33 - Basic Electrical Engineering

Since these materials conduct electricity, they are called conductors. They
easily allow electric current to flow through them. The strength of current
will depend upon the flow of charge per unit time. This is expressed as

where charge Q is measured in Coulomb and time, t in seconds. The unit of
current, therefore, is Coulomb per second, when 1 C of charge flows in 1 s;

the magnitude of current is called ampere, named after André-Marie Ampere.
Thus, 1 ampere of current is equivalent to the flow of charge of 1 Coulomb

per second.
In earlier years, current was assumed to flow from positive to negative
terminals. This convention is used even now although it is known that current

is due to the movement of electrons from the negative to the positive
terminal.

1.5.2 Resistance

Electrical resistance is the hindrance or opposition to the flow of electrons in
a given material. It is measured in unit called ohm. Since current is the flow

of electrons, resistance is the opposition offered by a material, to the flow of
free electrons. Resistance, R, is directly proportional to the length of the

material, and inversely proportional to the area of the cross section of the
material, through which current flows. The resistance offered by conducting
materials like copper and aluminum is low whereas resistance offered by

some other conducting materials like nicrome, tungsten, etc. is very high. All
these materials are called conducting materials. However, the values of

resistivity of these materials are different. The resistance, R of a material is
expressed as

where ρ is the resistivity, ℓ is the length and A is the cross-sectional area of
the conducting material.