If you complete an electrical circuit by placing at its ends a potential difference, then it will run an electric current, by virtue of which can be measured by the ammeter.

But this force will vary, if you replace one wire to another circuit.

This suggests that not only affects the voltage current, but also the material of the conductor is made.

This is the property of the conductor interfere with the passage of an electric current is called resistance.

Each body with respect to the electric currentIt is characterized by its resistance. If you recall the electron theory, according to her, all matter consists of atoms and molecules. These atoms and molecules have different structure in different materials. And they occur in the path of free electrons in a conductor when a current flows through an electrical circuit. That is, when a free electron collides with the crystal lattice of the material ion conductor, it inevitably loses some of its kinetic energy and feels as if resistance to its movement.
The greater the resistance of the conductor, the worse it passes an electric current. Represented by the electrical resistance of the Latin letter R, and the unit of measurement adopted by the 1 ohm.
The reverse characteristic substance resistanceis its conductivity. The higher the electrical conductivity of the material, the better it conducts current. Insulators are different from the conductors for conductivity in a great number of times measured unit with twenty two zeros!

## Resistivity. The definition and calculation

Thus, the electrical resistance depends onthe material from which made the conductor. But there are two important parameters - it is the conductor length and cross-sectional area. Obviously, the longer the wire, the longer its substance ions will interfere with the movement of free electrons.
But to better understand why the resistancedepends on the cross-sectional area, it is necessary to draw an analogy with the water. Imagine two identical vessels connected in one case, the thin tube and the other - colon. According to thin or thick tube water will spill from one vessel to another? It is clear that large.
Resistivity - a resistance conductor and a length of 1 meter and a cross section of 1 mm2.
The lowest resistivity have silver and copper.
Thus, to calculate the electrical resistance of the conductor, it is necessary to use the formula:
R = pl / S,
where p - resistivity, l - length of conductor, S - conductor cross-sectional area.

## Interesting Facts

When the temperature of the metalconductor, its resistance increases. To explain this phenomenon is the fact that the transfer of body heat increases the intensity of the motion of the atoms of its substance, and it is increasingly interfere with the free current of electrons.
With a decrease in the temperature of the metals arethe best conditions for carrying electric current. There is even such a thing as superconductivity, that is a state of a metallic conductor when its resistance is zero. The metal atoms almost freeze in place, absolutely without hindering the movement of free electrons. This occurs at temperatures -273oS.