In physics, resistance is the degree to which a material fails to conduct electricity. It is the opposite of conductivity, which is the degree to which a material conducts electricity. Most materials have a certain degree of conductivity and a certain degree of resistance, but metals tend to be strong conductors. No metal is a perfect conductor, though, and the level of resistivity will vary under certain conditions.
Electricty and the movement of electrons
Electricity is basically the conduct of electrons from one point to another. An electron is drawn from one atom, so that atom exerts a charge on the next atom, drawing an electron from it, and so on. Unfortunately, from a conductivity standpoint, electrons have a tendency to go off in a separate direction every once in a while, because of collisions or interactions with other subatomic particles. Metals are usually good conductors because their atomic structure allows the outer electrons to pass very easily from one atom to the next, but there will always be a few electrons that are bounced out of their intended pathways. This results in a loss of current, and the degree to which it happens is referred to as a material's resistance. There are several factors that may affect the level of resistance.
Length as a Factor in Resistivity
Length is one of several conditions that affect a material's conductivity and resistivity. The further an electric current has to travel through a substance, the greater the resistance of that substance. Simply put, the greater the length an electron has to go, the greater the chance that the electron will be bounced from the intended pathway, thereby decreasing some of the electric charge.
Width as a Factor in Resistivity
The width of a cross-section of the material through which the current passes also affects the resistivity and conductivity of a conductive material. The wider the cross-section, the lower the resistance of the conductor. A good analogy for the length and width factors would be a bowling lane. The longer the lane, the more chances a ball has to bounce into the gutter, but the wider the lane is, the better chance the ball has to reach the end without hitting the gutter.
Strain on a metal wire or other conducting material also affects the resistivity by altering slightly the length and width of the material. Pulling on a wire acts to increase its length and crimp its width somewhat, thereby increasing the resistivity.
Raising the temperature of a metal increases its resistance. The higher temperature causes an increase in vibration of subatomic particles, which increases the likelihood that an electron will collide with one of them.
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