G azərbaycan respublikasi təHSİl naziRLİYİ azərbaycan texniKİ universiteti M. M. QƏNDİLova e. Q. İsmayilova

JOKES, LAUGHS, SMILES
“Where have you been for the last four years?”

“At college, - taking medicine.”

“And did you finally get well? ”
UNIT XV
Tear always springs from ignorance

Emerson, Ralph, Waldo
TERMINOLOGY AND SYMBOLS FOR

ELECTRIC CIRCUITS

In order to study even a simple electric circuit, a knowledge of the relationship between practical units such as volts, amperes, ohms, and watts is necessary.

Amperes. The flow of electrons through the circuit is called an electric current. To determine the amount of current, a unit of measure is needed with which to work. The term ampere, used to define unit current flow, is analogous to the rate of flow of water through a pipe in gallons per second, or the rate of flow of an oil well in barrels per day. When 6.28x10¹⁸ electrons flow past a fixed point in the circuit in one second, a current in amperes in an electric circuit, an instrument called ammeter is used. Current flows from the direct current generator through the heater and back to the positive terminal of the generator. The ammeter indicates that current is flowing. The symbol of current flow in electrical equations is I.

Volts. No matter how many free electrons are in a conductor, they will not move until some force causes them to move. This force is called an amf, voltage, or a difference in potential.

When a difference in potential exists between two charged bodies that are connected by a conductor, electrons flow along the conductor from the negatively charged body until the charges are equalized and the potential difference no longer exists.

In liquid systems, a potential difference is expressed as pressure drop. The term “pounds per square inch” is used to denote the flowing pressure of a well, or a drop in pressure. (In the metric system, pounds per square inch are expressed in Pascals or kilopascals. One pound per square inch is equal to 6.894)

When it is stated that a lease secondary system carries 480 volts, that is an expression of the emf, voltage drop, potential difference, or electrical pressure supplied to the system. Just as pressure drop in a pipeline is expressed in pounds per square inch or kilopascals, the voltage drop to a certain well on the lease is expressed in volts.

To measure pressure drop in a pipeline, the difference in pressure at two points in the line is measured by a gauge. To measure voltage drop in an electric circuit, an instrument called a voltmeter is connected across the load, or across the section of line where the voltage drop reading is sought. The symbol for emf or voltage used in electrical equations is F.

Watts. Power is the rate at which electric current performs work, and the watt is the unit that expresses that rate. The power, or wattage, of an electric circuit is volts (E) times amperes (I). In electrical equations, the symbol for watts is W.

OHMS. Resistance can be considered as opposition to the flow of electrons, or friction. The free electrons in materials such as copper and sliver require a low emf to move them from atom to atom, hence they offer little resistance to the flow of free electrons; these materials are good conductors. Other materials have free electrons that require a very high emf to move them from atom to atom, and these materials are good insulators; they are said to have resistance to the flow of electrons.

The unit used to measure electrical resistance- which is present in all parts of an electric circuit to some degree-is the ohm, named in honor of the German physicist, Georg Simon Ohm (1787-1854). Ohm’s observations concerning pressure and resistance in an electrical circuit established Ohm’s Law: The current flowing in a circuit, in amperes, is directly proportional to the emf, in volts, and inversely proportional to the resistance of the circuit, in ohms. Ohms of resistance are indicated in electrical equations by the letter R.

Expressed as an equation, Ohm’s Law is I=E/R where I represents the current in amperes, E represent the emf in volts, and R represents the resistance in ohms. Ohm’s Law may be applied to an entire circuit or to any portion of circuit. If any two of the quantities are known, the third may be found by applying the foregoing equation.