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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