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Resistance

When voltage is applied to a piece of metal wire, as shown in figure 1.2 (a), the current I flowing through the wire is proportional to the voltage V across two points in the wire. This property is known as Ohm's law, which reads

tex2html_wrap2119 where R is called resistance, and G is called conductance. The resistance R and the conductance G of the same piece of wire is related by R = 1/G. Resistance is measured in ohms (tex2html_wrap_inline2115) and conductance in siemens (S or tex2html_wrap_inline2117).

 figure110
Figure 1.2: Ohm's law. (a) Metal wire; (b) circuit symbol 

Any apparatus/device that has this property is called a resistor.   Study of the physics of resistance shows that it is proportional to the length of the metal wire, l, and inversely proportional to the cross-sectional area, A, i.e., tex2html_wrap5532 where the proportionality constant tex2html_wrap_inline5073 is known as the resistivity of the metal.

We may calculate the power required to pass current I through a resistor of resistance R using the previously derived formula, i.e.,
displaymath5023
Using the Ohm's law equation, we get tex2html_wrap5534 The last inequality defines a property called passivity.



Michael Tse
Tue Mar 10 13:15:28 HKT 1998