Main data on electrical resistance
Speaking about conductors, we mean the materials and substances which are rather well carrying current. However not all substances called by conductors equally well carry electric current, i.e. they as speak, have unequal conductivity of current. It is explained by the fact that at the movement free electrons face atoms and molecules of substance, and in one substances atoms and molecules hinder the movement of electrons stronger, and in others - it is less. Speaking in other words, one substances show to electric current bigger resistance, and others - smaller.
From all materials, widely applicable in the electrician and the radio technician, the smallest resistance to electric current is shown by copper. Therefore electric wires also do most often of copper. The smaller resistance has silver, but it is quite expensive metal. Iron, aluminum and different metal alloys have the big resistance, i.e. the worst electric conductance.
Resistance of the conductor depends not only on properties of its material, but also on the size of the conductor.
The thick conductor has smaller resistance, than thin of the same material; the short conductor has smaller resistance, long - bigger, just as the wide and short pipe renders the smaller obstacle to the movement of water, than thin and long. Besides, resistance of the metal conductor depends on its temperature: the temperature of the conductor, the less its resistance is lower.
The ohm is taken for unit of electrical resistance (by name the German physicist G. Om). Resistance of 1 Ohm - rather small electrical quantity. Such resistance to current is shown, for example, by the piece of the copper wire with a diameter of 0,15 mm and 1 m long. Resistance of filament of the bulb of the pocket electric lamp about 10 Om, the heating element of the rangette - several tens of ohms. In radio engineering it is necessary to deal with big more often, than ohm or several tens of ohms, resistance. Resistance of high-resistance phone, for example, is more than 2000 Om's; resistance of the point-contact diode included in the direction which is not passing current, several hundred thousands of ohms.
Do you know what resistance to electric current is shown by your body? From 1000 to 20000 Ohms. And resistance of resistors can be to several million ohms and more. These parts as you already know, on schemes designate in the form of rectangles. In mathematical formulas resistance is designated the Latin letter (R). The same letter is put also near graphic symbols of resistors on schemes. For expression of big resistance of resistors use larger units: kilohm (in abbreviated form write kOhm), equal 1000 Ohms, and megaohm (in abbreviated form write megohm), equal 1000000 Ohms, or 1000 kOhm.
Resistance of conductors, electric circuits, resistors or other parts measure by the special devices called ohmmeters. On schemes the ohmmeter is designated the circle with the Greek letter the omega inside.Top