ELECTRICAL UNITS

Before going further with our study of electricity, it may be helpful to look at some of the standard units, notations, and  symbols used in electrical work. In the fields of electricity and electronics, a variety of terms are used to describe quantities in electrical compo-nents and circuits. Some of these units are volts, amps, coulombs, ohms, and watts. All these terms are listed in Table 3.

You’ve already learned about volts and amps. Remember that the strength of EMF or electrical potential is measured in volts, and the voltage in an electrical system can be compared to the water pressure in a water system. The stronger the voltage, the stronger the flow of electrons in an   electrical circuit. Voltage is abbreviated V or E.

Amperage refers to the amount of electric current flowing  through a circuit and  is measured in amperes, or amps. The amount of water flowing through a pipe can be compared to amperage. Amperage is abbreviated A or I.

When measuring electricity, the quantity of electrical charge is measured in coulombs.

One coulomb is a charge that’s equal to 6,240,000,000,000,000,000 electrons (these electrons are standing still, not flowing). The  abbreviation for coulomb is C.

Electrical resistance is the opposition that a circuit creates against the flow of electrical current. The magnitude of electrical resistance in a circuit is measured in ohms. One ohm is equal to the resistance of a circuit in which one volt of EMF is applied to produce one ampere of current. The abbreviation used for ohm is the symbol Ω.

The use and measurement of resistance is very important in industry. You’ll often rely  on resistance measurements to judge if a circuit or component is working properly. For  example, as you just learned even copper wire offers a small amount of resistance to the flow of electrical current. Since    an electric motor depends on a long length of copper wire wound into a coil shape, you can tell if a motor is in good electrical condition by mea-suring the resistance of its wire coil. A failed or openwire within the coil is indicated by a  much higher resistance reading than a wire that isn’t failed.

Another electrical unit you should be familiar with is the watt. Watts are used to measure power, or the amount of useful work that can be done by a circuit. The abbreviation for watt is W. You’ll often encounter lighting and heating equipment rated in watts.

HOW ELECTRICAL UNITS ARE RELATED

Three of the electrical units we just discussed have a special relationship to one another. These three units are the ohm, the ampere, and the volt. The mathematical connection between these units was first defined in the nineteenth century by the physicist  Georg Ohm, and his statement of this relationship is called Ohm’s law.

Ohm’s law states that the voltage in an electrical circuit is equal to the current multipled by the resistance. If we use the letters E, I, and R to represent voltage, current, and resistance, we can state Ohm’s law with the following mathematical formula:

E = I ´ R

In this formula, the letter E stands for voltage (in volts), the letter I stands for current (in amperes), and the letter R stands for resistance (in ohms).

In your work with electricity, Ohm’s law will have many practical applications. Using the Ohm’s law formula above and a little basic math, you can determine the voltage, current,  or resistance in any circuit.

PREFIXES

When electrical units are shown in writing, it’s often necessary to indicate very large or very small numbers of watts, volts, ohms, and so on. For this reason, you’ll often see pre-fixes attached to these electrical units. Prefixes are short word parts that are attached to the beginnings of words. In  science, prefixes are often used to indicate a  number.

A list of standard prefixes is shown  in Table 4. Any of these prefixes may be used with electrical terms to indicate very large and very small numbers. Be sure to study these  prefixes carefully so that you’re familiar with    them.

The first prefix  in the table is giga, which means  “one billion.” If you place the prefix giga  in front of the term watt, you create a new term, gigawatt. One gigawatt is equal to one  billion watts. This word may be used when talking about the output of a large power plant.

The next prefix is mega, which means “one  million.” If you place  thep refix mega in front of the term watt, you create the term megawatt. One megawatt is equal to one million watts. The output of a large electrical generator at a utility company is commonly measured in megawatts. Or, you may hear that a large commercial building or factory consumes several megawatts of electrical power.

In another example, look at the prefix milli. The prefix milli can be placed in front of the term volt to create a new term, millivolt. The prefix milli means “one thousandth,” so this  tells you that one millivolt is equal to one thousandth of a volt.