Key Terms
conductor
current
insulator
Ohm's Law
resistance
voltage
Atoms are built from three particles - proton, neutrons, and electrons. Protons have a positive electric charge (+) , neutrons are electrically neutral, and electrons have a negative electric charge (-). Protons and neutrons comprise most of the mass of the atom and reside in the center of the atom called the nucleus. The identity of an atom is determined by the number of protons in the nucleus. For example, the element iron has 26 protons and carbon has 6 protons. Electrons orbit the nucleus of the atom and have only a tiny fraction of the mass of a proton or neutron. If the atom is electrically neutral, it will contain the same number of electrons as protons.
Most of the elements on the periodic table are classified as metals or non-metals. Besides some similarities in appearance, metals tend to be good conductors of electricity and non-metals tend to resist the flow of electricity through them. This stems from the properties of the outer electrons (called valence electrons). Good conductors have loosely held valence electrons which can easily disassociate themselves from the atom and enter what scientists call the "conduction band" (link 3.1.a) where they drift freely through the substance. When these drifting electrons move in one direction through a conductor, we call that electricity.
How many times have you walked across a carpet (with hard soled shoes) and zapped yourself on a door knob or other metal object? Have you ever opened a package mailed to you, only to find those nasty packing peanuts relentlessly sticking to your arm? Why does dust seem to collect on your TV screen? Do you buy Static Guard® for your laundry? These are all examples where we see an object that has acquired an electrostatic charge. Technically, this is known as the "triboelectric effect", but the common name is just static electricity. In most cases, charge separation is initiated by friction or rubbing (tribo in Greek) between two surfaces of appropriate materials. For example, if you rub a rubber stick with fur, it will easily pick up a static charge. Loose electrons from one material are transferred to the other, in the process, leaving one surface with a net negative charge and the other with a net positive charge. A static charge rests at the surface of any insulated object (one that does not conduct electricity well) until it comes in contact with an electrical conductor (typically any metal)... and then ZAP ... a discharge occurs. In reality, the exact physics is still not completely understood, but it works!
Electrostatic charges result in forces that both attract and repel. Specifically, unlike charges attract and like charges repel. This can be demonstrated when you rub a balloon on your hair. The balloon will pick up a static charge (typically negative) and your hair picks up the opposite charge (positive). When you place the balloon close to your hair, there is an electrostatic attraction causing your hair to move towards the balloon. In fact, your hair will stand up all by itself because your positively charged hair will repel the like charges on each strand. Click link 3.1.b
Photocopiers make extensive use of static charges. First, a drum is given a positive charge (by a corona wire). Light is reflected off your original paper to the drum. Where the paper (master original) is white, the light will strike the drum which frees some electrons (from below the drum surface) and cancels the positive charge at the surface of the drum. If the paper (master original) is dark, no light reflects to the drum and it retains its positive charge. That is, the drum obtains an "electrostatic image" of your paper (master original).
Creating an electrostatic copy on the drum (animation)
Transferring the toner (animation)
Next, negatively charged toner is attracted to the positive areas of the drum. At the same time, a piece of paper (copy) gets a positive charge from the corona wire. You can probably guess what happens next ... the negatively charged toner is attracted to the positively charged paper. A heating element seals the marriage by fusing the toner to the paper. A laser printer works on the same principle with a few minor differences in procedures.
A person usually carries some small static charge (not electrically neutral). This is because we are constantly making contact with surfaces and distributing a few free electrons to/from the things we disturb. People who work with computers know this and must take extra precautions to ground themselves when handling circuit boards. Since we normally carry a small net electric charge, devices called triboelectric motion sensors use this property to detect your presence. Observe the animation below.
As the thief approaches observe what happens in the sensor (animation)
Suppose a thief is approaching your valuable treasure (don't ask me why he already has handcuffs on), and you wish to be alerted. If you place a metal sensor (greatly enlarged) in the vicinity of the item, you can see that the approaching man will initiate a charge separation in the metal detector (remember electrons can move easily in metals) which can be detected and triggers an alarm.
A similar effect could happen to you if you find yourself caught in a lightning storm. If a charged cloud (say positive) moves above you, it could attract electrons to your head (you play the part of the triboelectric sensor). You will feel your hair stand on end as the cloud approaches. This is a very bad sign because the charge buildup could become so strong that you are in danger of being struck by lightning. Quickly find a low area and kneel with your head facing down. Note: Scientists aren't really sure what causes the charge separation in the atmosphere in the first place. However, once the electric field becomes strong enough (about 70,000 volts per inch), the air breaks down and a conduction path is established where oppositely charged particles rush to meet each other. This super heats the conduction path and produces the flash you see. Thunder results from the sudden expansion (heating) and collapse (cooling) of the air column around the conduction path.
One type of touch screen relies on the same principles introduced above. A projected capacitive touch screen has thin conducting fibers embedded in the screen between two layers of glass. The fibers are running in both the horizontal and vertical directions.
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(animation) |
Since human bodies carry a small net electrical charge, as a hand approaches the screen it is able to alter the electrical charges in the conducting fibers embedded in the monitor (as seen in the animation above). A microcontroller monitors the signals in each conducting strand and is able to point the exact (x , y) location the person touched the screen.
TENG - triboelectric nanogenerators
Static charges are NOT electricity. It is only when there is a discharge do we get an actual flow of charged particles (that ZAP you get when touching a doorknob) ... and that IS electricity. OK, there isn't a lot of electricity in that ZAP but several modern devices really don't need all too much electricity to make them work. Electrical engineers are working on devices that make use of static charges as an energy source. Think of all the motion involved in just walking around. That motion can be used to build up static charges and then tapped to run a calculator, or charge a cell phone battery, or even power a heart pacemaker. These devices are known as TENG and they are still in the testing phase (as of 2019). It may not be too long when you can buy a pair of shoes that have TENG devices built in, or maybe woven into the fabric of your clothes, or even placed in water and let the waves do the static charging.
Electricity is nothing more than the flow of electrons through a substance. Electrons will not simply move through a wire without some outside force (remember Newton's Law?). In order to make the loose negatively charged electrons move through a metal wire, you have to provide an "electric force" or voltage across the wire. Basically this is an external electric field that charged particles (free electrons) interact with and cause them to move. One common device can supply this external electric field- a battery. The strength of the field provided by the battery is indicated by its voltage. When connecting a wire between the battery terminals (+ and -), free electrons "feel" the electric forces of attraction and repulsion provided by the battery terminals and electrons will migrate toward the positive terminal of the battery. In the process, they move through the "load" and make something happen. The "load" is the object you are trying to use such as a cell phone, flashlight, or GPS unit. In addition, as electrons move through the wire, they encounter resistance which is "electrical friction" as they encounter other atoms and interact with other electrons. This "friction" converts some of the kinetic energy of the flowing electrons to heat (random motion of molecules). In most cases, this represents an energy loss. In other situations your "load" is designed to make heat. For example, the filament in an incandescent light bulb provides enough resistance that the heat is converted to light.
A simple electric circuit (animation)
Some books provide a useful analogy to help understand the basic principles of electricity. You can compare the flow of water through your home's plumbing system with electric current in a wire. The water in your house pipes can be compared with electrons in a metal wire ... when the water flows (gallons per minute) through the pipes it is similar to an electric current (amperage) flowing in a wire. The water will not move unless there is a pressure difference within the pipes from its source (which is a pump or water tower) and an open faucet. The water pressure difference provided by the pump (and ending at your faucet) is similar to the voltage provided by the battery. When you open the faucet, water feels this pressure difference and flows toward the faucet. Likewise, free electrons will not flow unless there is an applied voltage. But the faucet opening is nothing more than a plunger valve or gate valve which restricts the flow of water. This is the resistance to the flow of water just like a filament provides resistance to electric flow. There is even internal resistance (due to bends and restrictions in the pipe) as the water moves through the pipes throughout the house (even if the faucet is wide open) just as there is internal resistance within the wire.
You can increase the flow of water by doing one of two things: increase the pressure at the pump or open up the faucet wider (reduce resistance). Likewise, you can increase electric current (amperage) by increasing the voltage or providing less resistance. This is known as Ohm's Law which is written:
Ohm's Law
Where I = current or amperage (flow of electrons measured in amps)
V = applied voltage
(electric force measured in volts)
R = resistance
(measured on ohms)
Click link 3.1.c to see a Java applet (which may need to be installed or updated to run), or read link 3.1.d if you are still confused.
AC vs. DC current (animation)
Direct current (DC) means that the flow of electricity is in only one direction. This is what you get with battery powered devices. Household current is a bit different. The electricity generated by power plants in the US is 60 Hz AC ... meaning the current goes through 60 "cycles" of flip-flops each second. The "mean" voltage is 120 volts. Thomas Edison was sure that DC was the way to deliver electric energy to customers. However, Tesla favored AC currents and this eventually became the standard. We will see the advantages of using AC very soon.
Thomas Edison was on a quest to perfect the light bulb. He realized that the high electrical resistance in his filaments was the reason they gave off light. He tried thousands of different materials until he found ones which could hold up to these high temperatures for a reasonable amount of time.
Some devices rely on the heat produced via electrical resistance. For example, an ordinary space heater converts electrical energy to heat energy because the filament ribbon is designed to offer a great deal of electrical resistance. From a cost standpoint, this is probably not the best way to heat an area, but it may be necessary if conventional heating devices are not available. Can you guess how the safety switch works (in case it gets too hot)? Other examples where heat is the goal can be found in an iron, soldering gun, electric charcoal lighter, and hair dryer. The flashing turn signal you use in your car may be initiated by a tiny device which quickly opens and closes a circuit. If interested, click link 3.1.e to read about how a thermal flasher works. The animation does a very fine job showing you how it all works.
Most of the time, the heat produced by electrical resistance is an unwanted waste of energy. The CPU (central processing unit) inside your computer generates heat that must be removed by a cooling fan and/or heat sink (a way of moving heat away from the chip). The idea is to provide a good heat conducting path away from the CPU. Heat sinks tend to have fins to increase the surface area. The larger the surface area, the more efficiently heat is dissipated.
(animation)
You have all seen touch screens before. Although there are several different ways to accomplish this, the cheapest way is relying on changes in electrical resistance. If you examine the animation above, you can see that there are two conductive layers (separated by tiny spacers). These layers form the front of the monitor. Normally a small current is fed through the layer with a certain electrical resistance (vertical line to the left). When you touch the screen, you offer an alternate electric path with a different resistance (indicated as a red path in the animation). This small change in resistance is detected as a slight change in current and this information is fed to the computer. The exact location is detected because the entire screen is crisscrossed (horizontally and vertically) with a grid of resistive wires ... so an exact X and Y position is pinpointed.
The same idea is being used in the field of robotics as tactile sensors. Nanowires embedded in polymer sheets give robots another way of interacting with objects by giving them the sensation of touch. You can bet this technology is being watched closely in the field of prosthetics.
Most sensitive electronics (like computer systems) are guarded by surge protectors (link 3.1.f). These devices are designed to protect your equipment from voltage spikes and surges which can occur from an electrical disruption such as lightning. If you have ever looked behind the wall plate of an electrical outlet, you will see that the incoming wires are color coated. You will see a white wire, a red (or black) wire and (possibly) a green (or the metal box acts as the green wire).
The neutral (white) and ground (green) both lead to the neutral earth. The main difference is that you expect the current to follow the neutral white wire ... the green wire is there as a safety net in case there is an electric short in the system. The surge occurs in the "hot" wire (red or black) which is normally 120 volts above the neutral (white) wire. If the voltage spikes above that, a surge protector can pass excess current to the ground (green) wire. This is accomplished by using a metal oxide varistor or MOV. The MOV consists of a metal oxide sandwiched between two semiconductors. When the voltage is normal, these semiconductors act like insulators, so very little current passes to the ground wire. However, when the voltage jumps significantly above normal, these same semiconductors become excellent electrical conductors ... passing dangerous excess current safely to the ground (and not your computer).
Some materials change their electrical resistance when light shines on them. These photo-resistive materials can be found in common nightlights where a cadmium sulfide photo-resistor cell conducts electricity when light shines on it and becomes an insulator when it is dark. In the next unit you will see how a simple relay can be used to automatically turn your lights on at night.
Optoelectric motion detectors
One way to protect your valuables is to install an optoelectric motion detector. This system uses a lens to project an image of a room onto a grid of wires. These wires are actually photo-resistors and have a small electric current running through them. When light of different intensities strike the wires, the electric resistance of the wires also change. If no one is in the room, the image will not change (dramatically) so there is no change in the current. However, if an intruder enters the room, the amount of light projected on the photo-resistors dramatically changes ... changing the resistance in the wires ... changing the amount of current through the wires ... alerting a computer chip to notify the police. The system can be adjusted so that gradual changes in light intensity (day/ dusk/ night/clouds) are ignored.
Certain silicon based materials change their electric resistance when subject to applied forces. These piezoresistive materials are used as pressure sensors. As a diaphragm is flexed by external pressure changes, it exerts forces on a piezoresistive material ... the changes in electrical resistance through the material are fed into a computer chip ... which reads out the pressure. Examples are found in cheap digital tire pressure gauges and other electronic stress gauges.
Some substances (platinum, metal oxides and silicon) change their electrical resistance as the temperature changes. As the temperature increases, so does the resistance. This offers an alternate method of measuring temperature. Most digital thermometers use thermoresistors to determine your temperature. Of course, this information is fed into a microcontroller and the output to an LCD (liquid crystal display) panel. Your car has a thermistor in the cooling system to warn you if temperatures are getting too high.
Thermistors are also used in some infrared motion sensors. When a person enters a room, their body heat will emit slightly more infrared radiation (heat) than the surrounding environment. If this can be focused on a grid of thermistors (similar to the optoelectric motion detectors shown above), the slight increase in temperature can be converted to a change in an electric current. This alerts the police, turns on a light, or signals an alarm.
A rheostat is a variable resistor (often called a potentiometer) which can alter the amount of flow through a circuit. It could look something like this:
a rheostat (animation)
You can see that as the "wiper" moves to the right, resistance increases, and the current flowing through the circuit will drop. For this reason, a rheostat has a practical application as a position detector. By monitoring the current in the wire, the exact location of anything connected to the wiper can be determined.
Rheostats are found all over your house. They are found behind most volume controls (on radios & TVs), any appliance with variable speed controls, dimmer switches and joy sticks (link 3.1.g). Dashboard lights and windshield wiper speeds can be adjusted with a rheostat.
(animation)
From this animation you can see that when the volume is turned down low, the rheostat is set up so it offers the most electrical resistance. Thus the current flowing will be the least. As you turn up the volume control, the rheostat will offer less and less electrical resistance ... so the current goes up ... and pretty soon ... you hear ... "Turn down that *%&*%#@ head banging music!"
Need to measure the pressure of a gas? One application of a rheostat uses a tube of mercury as shown in the animation below. As the gas pressure (fed to the left side of the tube) increases, it pushes mercury up the right column of the tube, which shortens the resistor (shown inside the tube at the right). The change in resistance is inversely related to the pressure of the gas. That is, as the pressure on the left side increases, resistance through the wire decreases.
A simple way to measure gas pressure (animation)
If you read link 3.1.h , an excellent article about how a fuel gauge works, you begin to see how the many things we have already discussed in this class can work together to make a useful tool.
The image below shows a device known as an electrolytic tilt sensor. It uses a conductive liquid in a glass housing similar to one found in a common carpenter's level. Both ends of the tube are given a small positive electric charge and a ground wire is located in the middle. If the housing is perfectly level, the electrical resistance between A-C and B-C will be equal. However, if the system is off level, the air bubble alters the electrical resistance of both conducting paths so that the resistance of path A-C ≠ the resistance of path B-C. This is easily monitored by a microcontroller, letting you know things are tilted.
Electrolytic tilt sensor (animation)
©2001, 2004, 2007, 2009, 2016 by Jim Mihal - All rights reserved
No portion may be distributed without the expressed written permission of the author
