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Ohm's Law
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Here is the "Electricity - Basic Navy Training Courses" (NAVPERS 10622) in its entirety. It should provide one of the Internet's best resources for people seeking a basic electricity course - complete with examples worked out. See copyright. See Table of Contents. • U.S. Government Printing Office; 1945 - 618779 Chapter 3: Electricity in Motion - Moving Electrons
Fresh water must be carried to many parts of a ship. It is fed to the boilers, to the laundry, to the galleys, to the scuttlebutts, and to the heads. The water system is complicated-it requires pumps and many pipe lines to do its job. The electrical system is very much like the water system. Electricity must be "piped" to the lighting circuits, to the interior communications circuits, to the battle circuits, and on some ships, to the propulsion circuits. The "pipes" of an electrical circuit are METAL WIRES and the "'pumps" are the GENERATORS. Current flowing through a wire is surprisingly like water flowing in a pipe. If you
were to put an electron into one end of a piece of copper wire, this added electron would
unbalance the charges of the molecules in the wire and would act as a repelling force
on the nearest electron in the wire. (Actually, the added electron gives one end of the
wire a higher POTENTIAL than the other end.) The push by the added electron breaks one
electron away from the nucleus of the FIRST molecule and forces it on to the SECOND molecule.
The electron you put in the end of the wire then fills the empty space left in the first
molecule. Now the electron expelled from the first molecule forces an electron out of
the second molecule and into the THIRD molecule - and so on through the whole length
of the Wire.
Figure 11. - Electron flow in a conductor.* When this shifting of electrons reaches the last molecule in the wire, you have, in effect, transported one electron the entire length of the wire. Not the same electron you started with-but, since electrons are all alike anyway, you can say that there has been a FLOW of one electron through the wire. Figure 11 enormously magnifies the mechanism of this moving electron. You know that ONE electron by itself is not enough electricity to be of any use. In an actual circuit, there would be billions upon billions of moving electrons. AMPERES Measuring the size of a waterfall seems easy-if you merely say it's large or small. Actually, however, there's more to it than meets the eye. First of all, you've got to have a UNIT OF MEASURE. Drops, ounces, pints, quarts, gallons, or barrels are all quantity measuring units for a liquid. You'd select the one unit that fits the problem best-neither too small nor too large. "Gallons" might do the trick. With the unit of measure-gallons-selected, IS it possible to determine the SIZE of a waterfall if you're told ONLY THE NUMBER OF GALLONS spilling? How about this: "Niagara has 5,000,000 gallons of water falling." Exactly how much do you learn from that statement? Not much! Sure, 5,000,000 gallons is a lot of water, but you must also know HOW LONG it takes to spill that much. One year, one month, one week, one day, or one hour! Not much of a falls, if it takes a year. But you know Niagara is a roaring giant, and the 5,000,000 gallons are spilled in ONE HOUR. The point is, you must know TWO things in order to measure the size or strength of a waterfall-the number of MEASURING UNITS moving in a UNIT OF TIME. This is called TIME RATE OF FLOW. Flow of water is commonly measured in GALLONS PER SECOND, but it could be measured in "quarts per second" or "barrels per day." That takes care of a water system-now, how is "flow" measured in an electrical system? First, select a unit of quantity measure. The electron won't do as a unit because it's much too small. A larger unit-made up of' 6.3 billion billions of , electrons-is the COULOMB. And the coulomb is the standard electrical UNIT OF QUANTITY MEASURE. Coulombs ALONE can no more measure the STRENGTH of electrical current than can the gallon ALONE measure the strength of a waterfall. In order to measure electrical strength, coulombs (quantity) must be hooked up with TIME. COULOMBS PER SECOND correspond to gallons per second. And "a coulomb per second" equals an AMPERE. The AMPERE IS THE UNIT OF MEASURE OF CURRENT STRENGTH. ONE COULOMB passing a point in a circuit in ONE SECOND is ONE AMPERE. One ampere of current means that one coulomb (or 6.3 billion billions of electrons) passes a point in the' circuit each and every second. Two coulombs each second would be two amperes; and 100 coulombs each second would be 100 amperes. Likewise, 100 coulombs in 2 seconds would be only 50 amperes (50 coulombs EACH second). AMPERAGE is the measure of the RATE OF FLOW of electrons. An ordinary light. bulb requires one-half an ampere. But a 36-inch naval searchlight
requires 150 amperes. This shows that the current to a searchlight is 300 times as large
as the current to an ordinary light bulb. The searchlight is about 300 times as strong
as the lamp. RESISTANCE Copper wire is used to carry' current because copper has many FREE ELECTRONS (easily dislodged electrons). Of course, every copper nucleus tries to hang on to its own electrons including the free ones. And the attraction for the free electrons must be OVERCOME before current can flow. The property of "hanging on" is called RESISTANCE. All matter, including a copper
wire, has a certain amount of resistance. When a current flows, the resistance of the
circuit must be overcome by the potential of the circuit. If the POTENTIAL is large,
or the RESISTANCE small-strong current flows. On the other hand, if the POTENTIAL is
small. or the RESISTANCE large-LITTLE current flows. CONDUCTORS AND INSULATORS Just what makes some materials carry current more easily than others is not thoroughly known. Most scientists believe that it is because molecules differ in the number of their free electrons-electrons which can be broken away from a molecule and forced along to the next molecule. It seems that the molecules of most METALS are loosely hung together-they have many free electrons. That is, the attraction between electrons and nucleus is weak, and it is easy to push out electrons. In other words, most metals have LOW resistances and are called GOOD CONDUCTORS. Most NON-METALS are just the opposite of this-they have tight molecules which have few tree electrons. In fact, for all practical purposes, some of the non-metals have no free electrons. It is almost impossible to force electrons through substances of this kind. Such non-metals have a HIGH resistance. They are extremely POOR CONDUCTORS and are called INSULATORS. It is wrong to say that all substances are either conductors or insulators. There
is no sharp dividing line. Electricians simply use the BEST conductors for wires to CARRY
current, and the POOREST conductors for insulators to PREVENT the passage of current.
Below is a table listing some of the best conductors and some of the best insulators
(poorest conductors).
Imagine that you are to run power from the dynamo room to the bridge searchlight. For your wire, you would choose a good conductor. Silver is too. costly, so you'd probably select copper. You would not be able to use a bare wire because in running through bulkheads, along overheads, and through decks, a good part of your se3.rchlight cur-rent would escape through the steel of the ship (a good conductor). To prevent this loss, you would use a wire coated by an insulator-probably rubber. The copper carries the current and the rubber prevents the current from escaping out of the wire. CONTROLLING CURRENT The amount of current that is wanted in any wire depends on the use of the circuit and the type of the wire. It would be foolish to send one-half an ampere to a searchlight needing 150 amperes. The amount of current can be controlled in two ways. First, by the amount of POTENTIAL DIFFERENCE and second, by the amount of RESISTANCE. Up to this point, potential has meant the charging of a body or the charging of one end of a wire. This charging results in a DIFFERENCE of potential between two bodies or between two ends of a wire. If you refer back to figure 8, you will see that this difference in potential is easy to calculate. From 0 to +4 is a difference of 4. From -2 to +3 is a difference of 5. (Note: -2 to 0 is 2, and 0 to +3 is 3. And 2 + 3 = 5.) To be ABSOLUTELY correct, you should call this POTENTIAL DIFFERENCE. Electricians often shorten this term to the one word POTENTIAL. Increasing the pressure in a water pipe increases the flow of water. Likewise, increasing potential difference on a circuit increases the flow of current. Compare the drawings in figure 12. If the resistance to flow remains the same; you can see that when you double the pressure
on the water in a tank, you force twice as much water through the pipe. :When the pressure
is tripled, the amount of water is tripled. Also, when you double the potential difference,
the current is doubled.
Figure 12. - Potential difference and current. For a difference in potential of 2, only one ampere flows, but for a potential difference (p.d.) of 4, two amperes flow. Calculate how many amperes will flow for a potential difference of 6. Then check figure 12 to see if you are correct. These ideas are stated in a fundamental law of electricity - CURRENT IS DIRECTLY PROPORTIONAL TO POTENTIAL DIFFERENCE. The second factor in controlling current is the amount of resistance. If the potential difference re-mains the same, an INCREASED resistance will DECREASE the current. Compare the drawings in figure 13. In the water system there are four factors, determining the resistance to the flow of water - (1)
Diameter of the pipe. The smaller the pipe, the longer the pipe, the dirtier the inside of the pipe-the more friction the pipe has. Friction is resistance, so the greater the friction, the smaller the flow of water through the pipe. Electrical wires are the "pipes" of an electrical circuit. The resistance of these wires, which is like the friction of the pipes, depends on four factors - (2)
Length of the wire. Notice in figure 13 that if the wire is longer or smaller, less current will flow. This is because the resistance has been increased. Likewise, if the wire is made of higher resistance material (iron) the current is less. These three factors are similar to the first three factors in the water - pipe system. Temperature - the fourth factor which affects the resistance of a wire - may be compared to the velocity of flow in a water-pipe. For some reason, not entirely Clear to scientists, the resistance of most conductors increases as the temperature increases. The effect of temperature change is so small that it may be neglected for all ordinary cases. If the RESISTANCE of a wire is INCREASED by any on of these four factors - size, length,
material, and temperature - the CURRENT is DECREASED. Thus, you have another fundamental
law of electricity- CURRENT IS INVERSELY PROPORTIONAL TO RESISTANCE.
Figure 13. - Resistance and current. Imagine again that you are running a power cable to the bridge searchlight. A wire long enough to reach from the dynamo room to the bridge will have considerable resistance because of its LENGTH. You don't want too much resistance, so you select a wire LARGE ENOUGH IN DIAMETER to carry the 150 amperes. Naturally, you use a wire of low resistance material-probably copper-and insulate it. In this chapter you have studied how a current flows and how its strength is measured. YOU MUST understand these fundamentals - how resistance and potential difference affect the strength of a current. Be sure you have these ideas straight. Chapter 3 Quiz |
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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ... All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged. My Hobby Website: AirplanesAndRockets.com
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