Copper wires are used to conduct electricity because copper conducts electricity very efficiently. Efficient though it is, copper wire still carries some resistance to the free flow of electricity.
Current flows because the atoms of certain metals hold onto their electrons weakly. An electrical potential pushes some of the electrons off the atoms they are attached to and onto other atoms, which pass them further down the line. The more energy it takes a conductor to hop the electrons from one atom to another, the worse a conductor it is.
The more paths that exist between one atom and the next, the more opportunities each electron has to move. Therefore, the resistance of a wire grows less the wider it is. It grows greater the longer the wire grows, as each atom the electrons have to jump to slows them.
Stranded wire and whole wire have the same resistance for the same size. All that matters is the cross-sectional area of the wire, not whether all the parts of it touch each other.
Calculating the resistance of copper wire begins with dividing 1.68 by 100 million. The result is multiplied by the wire's length in meters and by the cross-sectional area in square meters. The final result is the wire's resistance in ohms.
An alternative is to look up your wire's gauge on a wire gauge resistance table. Then the ohms per meter can be multiplied by the wire's number of meters.
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