In this module, the learning objectives are for the student to be able to:
OUR FRIEND THE ATOM
Figure 1. Illustration of the parts of an atom
Your level of understanding of the atom does not have to be great in order to have abasic understanding of electricity! We simply need to review the basic parts of a molecule.
The smallest particle of an element (e.g. copper or nickel, hydrogen or oxygen) is the atom. Atoms are comprised of a nucleus, which largely dictates the weight of the atom. The nucleus is composed of neutrons - neutrally charged particles (they will not be discussed further) and protons - positively charged particles.
The electrons orbit around the nucleus. They are held in place by the magnetic attraction - the protons attract the electrons. In most situations, there are an equal number of protons and electrons. The reason the electrons don't get "sucked into" the center is that they are whirling around at such a speed that the forces are balanced. The high electron speed would otherwise whirl the electrodes into outer space, but they are held in orbit by the protons' charge.
The analogy between atoms and the solar system is probably obvious. But none the less, I'll add a gratuitous graphic. And a poor one at that- the sun is of course much much larger than the planets!
The sun is analogous to the nucleus. The sun's gravitational pull is analogous to the protons' magnetic pull. The electrons orbit around the nucleus like planets do around a sun. Sort of.
Figure 2. The orbit of electrons around a nucleus is something like the orbit of planets around a sun.
Electrons of atoms orbit in "shells" - they are located a certain distance from the nucleus. (It would be similar to having a few planets at the same distance out from the sun.) How readily the electrons can be stripped and move to the next molecule determines whether the substance is a conductor. Conductors allow easy electron movement, while the electrons of insulators are firmly held.
WHAT IS ELECTICAL CURRENT?
When electrons "hop" from one molecule to the next, we have electrical current. Current is the flow of electrons through a conductor. One way of measuring it is to actually calculate the number of electrons (or protons) moving through a point in a given period of time.
An energy supply is needed to cause the movement of the electrons. You also need a ready supply of electrons. The energy supply and the electron sources differs with AC and DC electricity.
AC stands for alternating current. The power plant creates an alternating push/pull of electrons. One end of the two prong plug connects to the "hot" wire, which is doing the push/pull. The other end of the two prong plug is connected to "ground," that is truly the earth, which provides a limitless supply of electrons.
In the US, line current alternates from push/pull 60 times per second. The alteration is in the form of a sine wave. The change in polarity changes smoothly from positive to negative. (Other countries may have 50 Hz line current.)
DC stands for direct current. Rather than the voltage alternating from positive to negative as it does in an AC circuit, in a DC circuit the electron flow is always in the same direction. The battery has a different charge at the two ends - a source of electrons and an electron attractor, in essence. "Short circuiting" the two ends with, for example, a wire connecting the two ends, would cause a rapid rush of electrons, and would quickly use up the battery's energy supply.
In a circuit diagram, there are two popular ways of showing that a battery is in a circuit. Figure 3a illustrates how a battery is shown in a simple circuit, while Figure 3b shows how it may be shown when the DC current (usually a DC generator) powers a component such as an amplifier.
Figure 3. Manners in which DC inputs may be shown on a schematic diagram. A. shows a 9 volt battery. B. shows how voltage inputs to a component (in this case an operational amplifier). The form shown in B is more common when the voltage comes from a DC generator and is going to "pins" of a component.
RETURNING TO CURRENT, EXPLORING VOLTAGE AND RESISTANCE
As stated above, current is the flow of electrons through a circuit. In an AC circuit, the electron flow is back and forth, while it is in one direction only in a DC circuit.
The power supply is measured in volts. A standard electrical line provides 110 AC volts, which is essentially the amplitude of the push/pull strength. A clothes drier or electric stove uses more voltage, 220 volts, and has a different electrical connector going into the wall.
DC batteries can have various voltages. Hearing aid batteries, AAA to D cell batteries are 1.4 volts. OK, actually, the proper word is "cells". Each single what-we-call-a-battery is a "cell". A group of cells forms a battery. For example, your car battery is actually composed of individual cells. When you hook up the positive end of one cell to the negative end of the next cell, the total "battery" has the voltage of the sum of the cells.
Figure 4. When cells are placed "in series", the voltage resulting (the "battery voltage") is the combined voltage of the individual cells.
Resistance and Capacitance
Current flows freely through a conductor. When a part of an electrical circuit "uses up" some of the free movement of electrons - when it does not allow the current to flow freely - that part has "resistance". An electrical component called a resistor has as its purpose the opposing of energy. When you have a light on a dimmer, you are adding resistance to the circuit in order to change the electron flow to the light bulb.
Another electronic component is a capacitor. Its purpose is to block DC current. When I was working on the development of the Nicolet's digital hearing aid (about 1987), the engineers where hearing a nasty hum in the hearing aid. They kept talking about the fix - "the magic cap". I had no idea they were talking about adding a capacitor (that shouldn't in theory have been needed) until I saw the inside of the circuit. Capacitors look something like engorged ticks, as shown below (color varies). They can also look like miniature soda cans, usually colored black.
Figure 6. Picture of a capacitor, and the symbol used to illustrate capacitance in a schematic drawing.
The unit of measurement of capacitance is a farad or microfarad.
Figure 7. Illustrations of the different appearances of resistors. The color coded model on the left requires the user to remember what the different colors mean. Other times the resistance value is printed on the resistor.
A resistoris shown above. The amount of resistance provided is now often written on the component, but there are also color codes that can be used as well.
Figure 8. Ways in which a resistor is shown in a schematic diagram. In a simple circuit the resistance value can be shown, as in the left. In a more complex circuit, the value of R will be given in an attachment to the diagram. This is convenient when there are options in a circuit. For instance, changing resistor values can change the gain in a circuit, so the schematic can be drawn once, and the instructions would tell the person building the circuit what value resistors to use for what amount of gain.
A resistor (or an amplifier, lightbulb, speaker ) has a given amount of resistance - opposition to flow of energy. The circuit's "load" is anything that provides resistance.
Resistance is measured in "ohms". The symbol for ohms is the omega, shown in Figure 8. By the way, the opposite of resistance is conductance, how easily energy flows, and is measured in mhos. Get it? Ohm spelled backwards! And you thought electrical engineers were dull folk.
Resistance, voltage and current are interrelated. A circuit has a given voltage (e.g. 9 volts DC or your 220 volt electric clothes dryer.) It also has resistance to it. You can compute the current running through the circuit with the formula
which is called Ohm's Law. Using their keen sense of humor, the engineers have played a joke on us all by abbreviating current as I. And voltage is often E, and you may see Z for resistance, but there's a limit to my tolerance for these things, so I'll stick to V and R. Therefore, you can say I = V / R. And basic algebra says that V= I x R and R = V / I.
|Formula||Current =||Voltage /||Resistance|
There is another electrical principle - that of wattage, a measure of power consumed by the load. Watts are calculated by multiplying voltage by amperage. A 60 watt lightbulb will use less power than a 100 watt lightbulb.
|Formula||Power =||Voltage X||Amperage|
Current gives shock
Current is what gives you an electrical shock. When you are the path of least resistance, and the current flows through you, if you can sense it, the sensation can range from a mild tingle to electrocution.
When you hold the two ends of a battery in one hand, you have created a path for electron flow. Your skin isn't a terrific conductor, it has around 10,000 ohms of resistance. Once inside the body, electricity flows very well, but it must exit again through the other finger, providing another 10,000 ohms of resistance (20k ohms total). To calculate the current flowing through your body, apply the formula I = V/R. In this case 1.4 volts / 20,000 ohms is 0.00007 amperes. Since there are 1000 milliamps in an ampere, that is .07 milliamperes, a very minute amount of current, which is why you don't perceive a shock.
Consider instead what would happen if you stuck a fork in the positive socket of the wall outlet. The fork has no significant resistance, so the full 110 volts could now flow through your body. Assuming the same total 20 kohms of resistance, you have 5.5 milliamperes. Not so good. But please don't wet your skin and reduce the impedance, ala sitting in the bathtub. If the resistance drops to 1000 ohms, there would be 110 milliamperes of current.
Assignment - Let's test the technology and your understanding of the material. In a paragraph or so, answer the question:
Why is it that wearing rubber-soled shoes offers protection against electrical shock in case you come into accidental contact with a "hot" (positive) wire?
Use the email button
at the bottom of this document. Why not cut and paste the question
into your email?
You have probably noticed the plastic insolating coating around electrical wires. They serve the purpose of making sure that another wire nearby doesn't have conductor-to-conductor contact.
Redundant Grounds for Safety
If a positive (hot) wire inside an appliance breaks, and a connection is no longer present, the appliance won't work. If the hot wire comes in contact with a metal case of the appliance, some bad things can happen! Namely, if a person (or wet nosed dog) touches it, the electrons can flow through the person, who, being in contact with the ground, provides a path for electrons to flow. Zap!
A safety measure is to attach a "redundant" (duplicate) grounding wire to the appliance housing (metal case). This is connected to the third prong on electric plugs. The house should be wired so that these extra connectors in each outlet are connected to a grounding rod. A ground rod is pretty much what it sounds like. A large metal rod driven into the ground outside your house.
Now, if a hot wire comes in contact with the metal appliance case, the electron supply comes freely from the earth. A person touching the appliance would not be an easier path for electron flow, so the person (or pooch) is not a part of the circuit, and won't be shocked.
Sometimes equipment has a three-prong plug but the house / building only has two prong outlets. You've probably seen the adapter that assists in this situation. It often has a place where you can screw the adapter to the screw in the plate covering the wall outlet. The theory is that the electrician may have attached the ground wire to that screw, so connecting the redundant ground plug to the screw is equivalent. Nice theory, but I haven't seen an outlet wired that way. Unfortunately, that isn't enough to keep me from using those adapters, since most of my house is wired with 2-plug outlets (and some of the three plug outlets don't have a redundant ground attached.)
If you want to test your outlet, hardware stores and home supply warehouses (Builders Square, Home Depot, etc.) sell testers. If you are using medical instrumentation, and are in any doubt about the quality of your wiring, it would be a very good idea to check that the outlet is properly set up. It would be most unfortunate if, while doing laryngoscopy or electrophysiology, your hot wire becomes connected to anything in contact with the patient. If your electric circuit is not properly grounded, this can happen.
Audiologists may have noticed that electrode plug-in styles for evoked potentials testing have changed recently. There use to be a male looking part; now the metal conductor is inside in a female-style connector. My friends at Intelligent Hearing Systems tell me that was a Food and Drug Administration requirement. They wanted to make sure that no one could take an electrode lead and plug it into a wall socket. (Do you have the feeling that physicians are hiring technicians with marginal qualifications?)
Fuses / Circuit Breakers / Ground Fault Circuit Interruptors
Have you seen the square outlet boxes (wall sockets where you plug in appliances) with the red and black buttons on them? Those are ground-fault circuit interruptors. When there is too much current flowing between the positive and the negative, one potential reason is that a mammal is contributing to the circuit in a less than helpful manner - i.e. by completing the path of the hot wire to ground, "short circuiting" the circuit.
I recently remodeled my kitchen, putting in new tile, and moving the outlets a bit. It was truly scary to see what a poor quality job was previously done on the electrical wiring. There were no plastic outlet boxes behind the outlet (so if a wire comes in contact with, say, the wire in the plaster lathe, we could create a circuit in any number of ways!). We put in GFCI outlets. When cleaning the grout with a wet sponge, I wasn't being too attentive and wiped right over the outlet, permitting a connection between the hot and ground ends of the outlet with me and my wet sponge as the conductor. Fortunately, the GFCI simply clicked, shutting off the circuit. And so, I am still alive to write about exciting things like instrumentation.
Electron flow creates heat. If a conductor heats up enough, it will break. Fuses use this principle. If you look at a car fuse, you'll see a thin wire in the middle. If too much current is flowing, the wire will heat up and break, stopping the situation of excessive current flow, protecting against excessive heat and possible electrical fire. (If in the heating process the metal wires in the appliance or wall heat up sufficiently, a fire could start. Obviously not a desirable circumstance.)
Fuses come in fast-blow and slow-blow. Fast-blow fuses will break with a relatively brief high current spike, while slow-blows won't break until the condition has occurred for some time.
The electrical engineer designs the circuit and fuse for the circuit's load. It is not safe to put in a fuse of a different rating. If you put in too low a value fuse, it will blow all the time. If it is too high a value, you lose the protection the fuse was meant to provide.
If your medical equipment stops working suddenly, before you call for an electrician or service call, check to see if there is a user-serviceable fuse you can replace. Most medical equipment comes with a spare fuse.
If the equipment blows a fuse, there are a few possible reasons. The fuse may be old and worn, or was bumped, etc., and it is a fluke. Or, there may have been excessive current flow from an electrical fault. If a fuse repeatedly blows, it's time to call in the electrician.
Wall outlets are connected to circuit breakers more often than fuses these days. The principle is the same - excessive current flow will cause a break in the circuit. While a fuse must be replaced, a circuit breaker simply flips a switch.
So, now, when you have the refrigerator, microwave and coffee pot on all at once, and then put the toaster on - when the circuit breaker flips you know why. Yes, it's irritating to reset the circuit breaker, but it was telling you that so much current was flowing through the electric wires, the wires were in danger of overheating. The activation of the circuit breaker is preferable to an electrical fire!
The following is a brief quiz to see how well you have mastered the material. Sometimes these are used for self-analysis only, sometimes the results are sent to the instructor. Although this will not factor into your grade, this quiz is set up so that it serves both purposes. You will immediately see your results, but I will also see your performance.
Assignment - Let's test our powers of deductive reasoning - I'll tell you an anecdote, which eventually has a point.
When I was living in West Texas, my husband and I rented a house in an economically depressed town 20 miles east of Lubbock. Lorenzo, Texas. The sign said population 610. I think they counted the pigs. The house had a great big fenced in yard for the dogs, and rented for $250 per month (1988-1993). So the advantages did off-set some of the disadvantages, like having to fill the hot water in the tub with a hose running from the sink. One of the house's "features" was non-professionally renovated wiring. Of course, given the age of the house, all of the plugs were the two conductor type. One day while vacuuming, the vacuum bumped into the electric fan (no central air in the house, either). Sparks flew. What do you think caused this, and why wasn't I shocked? (I was wearing sneakers - would that matter?) .
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