If to include different conductors and the ampermeter in an electrical circuit of one source of current, it is possible to observe that indications of the ampermeter at different conductors differ. It is explained by the electrical resistance of the site on which, as well as from tension, current depends.
Resistance as physical quantity
Electrical resistance of the conductor is the physical quantity designated by letter R. 1 Ohm – resistance of such conductor in which current is equal to 1 ampere at a voltage of 1 volt on the ends is taken for unit of resistance. Briefly it is written down a formula:
1 Ohm = 1B/1A.
Units of measure of resistance can be and multiple. So, milliy (my) is 1 0.001 Ohms, 1 kilohm (kOhm) – 1000 Ohms, 1 megaohm (megohm) – 1,000,000 Ohms.
In what the reason of electrical resistance in conductors
If ordered the electrons moving in the conductor did not test any obstacles in the way, they could move by inertia as much as long. But in fact it does not occur as electrons interact with the ions located in a crystal lattice of metal. Their movement from it slows down, and in 1 second through the cross section of the conductor there passes the smaller number of charged particles. Therefore also the charge transferred by electrons in 1 second decreases, i.e. current decreases. Thus, any conductor kind of renders counteraction to the current moving in it, resisting it.
Resistance reason – collision of moving electrons with ions of a crystal lattice.
In what Ohm's law for subcircuit is expressed
In any electrical circuit the physicist deals with three physical quantities – the current, tension and resistance. These sizes exist not separately in itself, and are connected among themselves by a certain ratio. Experiments show that current on the site of a chain is directly proportional to tension on the ends of this site and is inversely proportional to resistance of the conductor. Om's law opened by the German scientist George Om in 1827 also consists in it: I=U/R, where I – current on the site of a chain, U – tension on the ends of the site, R – site resistance. Om's law – one of fundamental laws of physics. Knowing resistance and current, it is possible to calculate tension on the site of a chain (U=IR), and knowing the current and tension, it is possible to calculate resistance of the site (R=U/I).
Resistance depends on length of the conductor, cross-sectional area and the nature of material. The smallest resistance is characteristic of silver and copper, and ebonite and porcelain almost do not carry electric current.
It is important to understand that resistance of the conductor expressed from the law of Ohm by a formula R=U/I – a constant. It does not depend at most current, on tension. If tension on this site increases several times, in as much time also current will increase, and their ratio will remain invariable.