21A
YOUR ANSWER:
Both of the above.
(From P1-3B)
Excellent.
You now know what resistance is, the standard symbol for resistance (R), the unit of
measurement for resistance (ohm), the abbreviation for that unit of measurement (), and finally the four factors
that determine resistance.
There is one more item to be discussed while we are on the subject of resistance, and that is specific
resistance. Don't let the big words scare you. It's really very simple. All materials are classified according to the
amount of opposition they offer to current flow. This opposition for any particular material is called its specific
resistance. The standard of measure for all metallic conductors is the resistance in ohms of a conductor that is one
foot long and one mil (.001 inch) in diameter. Since resistance varies with temperature, 20 degrees centigrade has
been set as the temperature at which specific resistance is measured. The standard of measure of nonmetallic
conductors is the centimeter cube. Stated in plainer words, the specific resistance of a nonmetallic conductor is
the amount of resistance, face to face, of a cube of that material, one centimeter long and one centimeter square.
Again, a temperature of 20 degrees centigrade has been set as the standard temperature for measuring specific
resistance. But don't forget that the most important factor in determining the specific resistance of a material is
the atomic structure of that material. Pick the correct statement.
The specific resistance of a material is the
resistance of any amount of that material at
20 degrees centigrade.
Page 1-10A
The specific resistance of a material is the
resistance of a unit volume of that material
at a temperature of 20 degrees centigrade.
Page 1-16A
The specific resistance of a material is the
resistance of that material at 30 degrees
centigrade.
Page 1-18B
1-21
IT0340