# RESISTANCE

Cuthbert Nyack

When a voltage source V is applied to a resistor, then a current I flows through it.
The resistance R = V/I is a measure of the current which flows through the
resistor. Ohm's "law" states that V/I is constant. This is only correct for
some materials eg metals and alloys.
At the microscopic level, the relation between voltage and current becomes
a relation between Electric field and Current density. Their ratio being
the resistivity.
Metallic resistors have resistances which vary with temperature according to
the following relation( at least over a limited temperature range). Platinum
resistors are often used to measure temperature based on this relation.
When a current is flowing through a resistor, then power P is dissipated
in it. P can be written in one of the following ways. When an alternating
current is flowing through a resistor, then the voltage is in phase with
the current.
Resistors are one of the most important components in electronic circuits.
They are used for biasing, gain adjustment, impedance matching, temperature
measurement, current limiters etc.

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COPYRIGHT © 1996 Cuthbert A. Nyack.