What
is Resistivity?
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in the box below:
• Resistance is determined by
the resistivity ( )
of a material and the material
geometry.
• Resistivity is a measure of
the ease with which electrons can travel
through
a certain material.
• The inverse of resistivity is
called conductivity ( ).
• Every material has a different
resistivity, which depends on temperature.
The resistance of
a particular object is obtained
by multiplying the
resistivity by the length of the resistor,
and dividing by the
cross-sectional area
(S).
These parameters are illustrated in
Figure 2.1.
Types of
Resistors
1. A common type
of resistor is the molded
composition (carbon resistor).
- These resistors are manufactured
in a variety of sizes and shapes.
- They are easy to manufacturer,
inexpensive, and have a tolerance
that suits most electrical and
electronic applications.
- The chemical composition of
the resistor determines its ohmic
value and is controlled by the
manufacturer in the development
process.
- They are made in ohmic values
that range from one ohm to millions
of ohms.
- Disadvantage of carbon resistors
is their accuracy and limited
power handling capacity.
2. Another type of
resistor is the wirewound
resistor.
- These resistors have very accurate
values and possess a higher current
handling capability than carbon
resistors.
- The material that is often
used to manufacture wirewound
resistors is German silver, which
is composed of copper, nickel,
and zinc.
- Disadvantages of the wirewound
resistors are that they are bulky
and costly.
Categories
of Resistors
There are two categories
of resistors, fixed and
variable:
- The fixed resistor that have
one value and will never change.
- The variable resistor has an
adjustable collar that can be
moved to tap off any resistance
within the ohmic value range of
the resistor.
There are two types
of variable resistors, one called
a potentiometer and
the other a rheostat:
- An example of the potentiometer
is the volume control of a radio
receiver.
- An example of the rheostat
is the dimmer control for the
dash lights in an automobile.
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