Resistance

Resistance is defined as the ratio of the voltage measured across a circuit element and the current passing through it.

R = V/I

Note:

It is important to read the definition carefully. When resistance is not constant (for example, when the temperature of a metal increases) the V:I plot is a curved line. The resistance is the ratio V/I .... NOT the slope of the line.  

When resistors are connected in series the total resistance is the sum.

R = R1 + R2 .... etc.

When resistors are in parallel the recipricals are added.

1/R = 1/R1 + 1/R2 ... etc.

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