Resistance is caused when the free current carrying electrons in a material are scattered due to collisions in the ion lattice. These collisions are caused by defects and imperfections in the lattice, and by the lattice vibrations. The resistance of an object is therefore given by
where is the length of the object, and is its cross sectional area. is the resistivity of the material the object, a measure of how much the free electrons in a material are scattered by the ion lattice.