Electrical resistance (R) is a measure of the resistance that a material or electric circuit presents to the flow of current, and is defined as the ratio of the voltage (V) applied to the electric current (I) which flows through it. The unit of resistance is the ohm. If the resistance is constant over a wide range of voltage, then Ohm's law (I = V/R) can be used to predict the behavior of any material through which a current flows. This definition holds for both DC and AC circuits. Thus, for a given voltage, materials or circuits with a high resistance will allow a small current relative to a material with a low resistance. At the atomic level, currents are defined as the flow of the valence electrons of atoms through a given material. Resistance in this context results from collisions of...