# Resistor Theory

Resistors provide an obstruction to
the flow of electricity around a circuit. A voltage is dropped across a resistor,
dependant on the current flowing through the resistor. Here are some useful formulae:

#### Voltage Dropped across a Resistor:

V = IR, where I is in Amps, and R in Ohms

This can be re-arranged to give Current or
Resistance if Voltage is already known (see ohms law).

#### Power Dissipated in a Resistor:

P = I^{2}R or V^{2}/R where P is in Watts

Always make sure the resistor's power rating is not exceeded!

#### Resistors in Series:

Putting resistors in series increases the total resistance:

R = R_{1} + R_{2} = R_{3} .....

Note that the total resistance should be greater
than that of any of the individual resistors.

#### Resistors in Parallel:

Putting resistors in parallel reduces the total resistance:

(1/R) = (1/R_{1}) + (1/R_{2}) + (1/R_{3}) .....

If you have only 2 resistors in parallel you can use:

R = (R_{1} R_{2}) / (R_{1} + R_{2})

Note that the total resistance will be less than
that of any of the individual resistors.

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