Resistivity

You know that current is defined as the rate at which current flows per unit of time. You also know that resistance is the opposition to that flow of charge. So far, you’ve taken into account a resistor in a circuit but not taken into consideration the wires that connect the circuit together. A good conducting wire has very little resistance. The wire’s resistivity depends on the material that the wire is made of. Most circuits are made of copper wires because the resistivity value of copper is very low and it is cost-effective. Silver has the best resistivity value of all the metals but it is not nearly as cost-effective.

The resistance of a wire also depends on the length of the wire and the cross-sectional area of the wire. The longer the wire is, the more resistance it has; but the larger the cross-sectional area, the less resistance it has. This relationship can be defined by the mathematical model:

where ρ is the resistivity value of the material use, L is the length, and A is the cross-sectional area. Resistance also tends to increase as the temperature of the material increases (though this is not the case for some semiconductors).