America is a very electric country. Every day, we use more electricity. It powers ours motors, our ovens, our televisions, our dishwashers, our computers, etc. Without electricity, this country would come to a standstill. In today's workshop, we will learn about electricity and the law that governs how it moves through simple circuits, Ohm's Law. Later, we will learn about electricity's sibling, magnetism.
What is electricity? This is a question that humans have been asking for a long time. Mankind has been fascinated with electricity, even when they did not know what it was. Lightning is a natural (and deadly) version of electricity. Static electricity (rubbing fur, shuffling your feet on carpet, etc.) is another example of electricity that has been around for a long time. However, it was not until the great American physicist Ben Franklin discovered electrical charge that we began to understand its properties and how to use it. (As an aside, it should be pointed out that the reason why Ben Franklin was a statesman was because he was so well respected as a scientist first. In fact, Franklin was the only foreign member of the French Academy of Science.)
Franklin discovered that there were two types of charges in the world, positive and negative. In most material, these two charges are present in equal numbers, thus making most substances electrically neutral. However, through certain actions such as rubbing rubber against fur, you can separate the charges. We now know that the negative charges are called electrons and the positive charges are called protons. It was not until later that we found that there was a third type of particle, called the neutron, which has no charge. The following activities demonstrate this idea of charge.
Balloon on a Wall and a Fluorescent Tube
One way to separate charges is by rubbing fur against rubber. Excellent examples of these two are balloons and hair (Hair is fur!). First, blow up a balloon and tie off the end. Rub the balloon against your head (If you are follicly-impaired, then rub it against your neighbor's head.). If a wood surface (door or paneling) is nearby, press the rubbed portion of the balloon against the wood. Release the balloon. What happens?
Another way to "see" charge is to rub the balloon against a fluorescent light bulb. Wash the outside of a long fluorescent tube and dry it thoroughly. In a darkened room, stand the tube upright with one end on the floor. Rub the balloon quickly up and down the tube and then hold the balloon near the tube. What happens?
When you rubbed the balloon against your hair or the glass, you were able to separate the charges, with the negative charges being left on the balloon. Bringing the balloon near the wood surface, causes the positive charges in the wood to be attracted to the balloon (unlike attract) while the negative charges in the wood were repelled (like repel). Thus, when the balloon was placed against the wall, the positive charges in the wood were close to the surface and their attractive force held the balloon in place. With the light bulb, the electrons on the balloon cause charges to separate with the tube. The vapor in the tube nearest the balloon becomes charged, just like it does when the bulb has current running through it during normal operations. The charged vapor bombards the fluorescent chemicals on the surface of the bulb as the vapor is attracted to the balloon. This causes the chemicals to light up.
Evaluation: Against the wall, the balloon held up quite well. However, due to the humidity, we got very little response out of the fluorescent light bulb. We did get it to work, but only by placing it next to a Van de Graf generator.
You can show electrical attraction with a plastic comb and a ping pong ball. Charge the comb up by rubbing it on fur, wool, or hair. Bring the comb near the ball. What happens and why? You can also attract other things with your charged comb. With it charged up, place the comb near a stream of water. The stream is suddenly bent towards the comb. Why?
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To see a Quicktime movie of this demonstration, click here. |
Evaluation: Even with the high humidity, these two demonstrations worked well. The comb near the stream of water was able to deflect the stream by about 3 inches.
Have you ever pulled an object out of a box filled with Styrofoam packaging pellets and never been able to get rid of them? They seem to stick to everything. The reason why is that they so easily pick up charge. You can make a great demonstration by putting small pieces of Styrofoam in a plastic container. After closing the container (a transparent plastic tube with rubber caps at both ends works well), rub the outside with a piece of cloth or fur. What happens? As you rub the plastic, electrons (negative charges) are deposited on the surface of the plastic. These charges repel negatively charged Styrofoam, driving them away from that area.
Evaluation: This demonstration worked o.k. We used 48 inch fluorescent light bulb covers and small bits of styrofoam packing for our tubes. Rubbing it with a cowhide glove provided enough static charge to make the styrofoam jump. The styrofoam pieces need to be fairly small (about a millimeter in diameter) to be seen jumping. A dessicant in the tube might enhance performance on humid days.
Electroscopes are devices that allow us to measure the charges on an object. You can build one quite simply using a jar with a lid, a piece of wire or metal paper clip, thin aluminum foil, and clay or wax. Punch a small hole through the lid of the jar, big enough to fit the wire through it. Stick the wire through the hole, leaving a small loop of wire sticking out the top of the lid. Seal the hole with the clay/wax. Cut the wire off so that it extends about halfway into the jar. Bend the end of the wire and place two thin strips of foil at the end. Make sure that the foil is somewhat free to rotate. Screw the lid back onto the jar. You now have an electroscope.
Rub a piece of rubber against fur and bring the rubber close to the wire loop. What happens to the two pieces of foil? Touch the rubber to the loop and then draw it away. What are the pieces of foil doing now? When you touched the rubber to the loop, you deposited negative charges onto the wire, and thus, onto the pieces of foil. Since both pieces have the same charge on them, they repel each other. Now rub a piece of silk against glass. Bring the glass close to the wire loop. What happens? Touch the glass to the loop. Since the glass has a positive charge on it, touching it to the loop neutralizes the negative charge on the foil, reducing the repulsion of the foil. Try rubbing other objects against things and see if charges are separated.
Evaluation: Owing to the humidity, this experiment failed miserably. The foil in the tube did not move at all. A thinner foil (we were using heavy duty aluminum foil wrap) and a dessicant might make it work.
The fact that objects can become charged can be very useful. Since some objects charge faster than others, we can use electricity to separate certain mixtures. For this experiment, we will need some salt, some fine ground black pepper, a plastic comb, and fur/cloth. Mix the salt and pepper together and spread on a flat surface. Charge up the comb and pass it slowly over the mixture at a height of about one centimeter. Do not just hold the comb over the mixture. Examine the comb. Is there anything on it? Make several passes and see if you can totally separate out the pepper from the comb.
As you pass the comb over the mixture, both the salt and pepper are electrostatically attracted to the comb. However, since the pepper is lighter, it will more readily "jump" onto the comb (Note: As the substances are stuck onto the comb, they begin to assume a net negative charge, and are therefore eventually repelled. This is why you should not leave the comb over the mixture.). Even though some salt does stick to the comb, the majority of the particles on the comb the first pass are pepper. Eventually, though, the ratio of salt to pepper in the mixture becomes so great because of the pepper that has been pulled out that the majority of particles stuck on the comb are salt. By this time, the only pepper particles left in the mixture are quite large, and they can be removed by some other means. This method of separating a mixture by electrostatics is widely used in pollution control devices at coal burning plants. The ash particles can be removed by electrostatic fields just like the pepper.
Evaluation: This experiment worked for the smaller pieces of pepper, but it did not work that well for larger pieces. Using a finely ground pepper might work better.
Photocopying machines use electrostatics to put toner on a sheet of paper where it should be. Toner dust is attracted to regions of the paper that have been charged up electrostatically. The toner dust is melted onto the page by a hot wire and fixed into position, exactly where it was on the original. We can show this effect with a Petri dish, fine ground black pepper, paper, and a piece of silk. Place the pepper in the Petri dish and cover with the glass lid. Tape a piece of paper to the top of the lid that covers some fraction of it. Rub the top of the lid with the silk. What happens? To get a better understanding, rub a glass rod with the silk and insert it into the Petri dish. Is the pepper attracted or repelled by the rod?
Evaluation: Not done.
As useful as electrostatics is, we are more concerned with moving charges. At the power company, some type of fuel is being consumed, releasing large amounts of energy. This energy needs to be transported in some fashion from the plant to our homes and offices. A static charge does not do this; a moving charge does. Moving charges are called currents and are the method for transferring the energy from the power plant to us. Two things are necessary for a current. One is a potential difference. This means that you need an excess of one type of charge in one location relative to another (ex. more electrons on one pole of a battery than on the other pole). The other thing that you need is a conductive path. All matter has a certain ability to conduct electricity; some things better than others. To study conductivity, let us look at the following experiment. We will need a D-cell battery, a clothespin, a flashlight bulb, aluminum foil, masking tape, scissors, a ruler, and testing materials (rubber bands, paper, wood, coins, strips of plastic, plastic wrap, glass, etc).
Cut two pieces of foil that are 60 cm. x 15 cm and fold them in half lengthwise until they form two thin strips 60 centimeters long. Tape one end of each strip to the ends of the battery. Connect the other end of one of the strips to the base of the flashlight bulb using the clothespin. Test the materials by touching the bottom of the bulb to the material while the material is in contact with the other aluminum strip. Which ones cause the bulb to glow?
Evaluation: The participants liked this experiment. It allows students to test many materials in a safe environment, allowing them some creativity.
As you saw in the preceding experiment, metals are excellent conductors. This is the reason why electrical wires are made out of metals. However, other substances can conduct electricity. For instance, air is normally a poor conductor of electricity. But if a large enough potential exist, even it can become conductive (ex. lightning). Other materials can also conduct electricity. An example is salt water. To show this, we will need a D-cell battery, a beaker full of salt water, and two wires.
Attach the wires to the battery. Making sure to bare the wires on the other end, carefully insert the other ends of the wires onto opposite ends of the beaker. What happens? You might notice bubbles forming on both leads. Now, mix salt into the water. What happens? The bubbles are hydrogen and oxygen. As the electrons pass through the water, they disassociate the water molecules back into hydrogen and oxygen. When salt is added, the dissolved ions Na+ and Cl- allow more current to pass through the water, resulting in more gas being generated. In fact, this is one way that we produce hydrogen, which is almost non-existent in our atmosphere. You should be able to collect these gases with using two test tubes filled with water overturned near the wires. If you do, be careful, as both gases are highly flammable.
Evaluation: This experiment met with modest success. A better way to show the increased current might have been to connect a small light bulb in the circuit. When the current increases, the light bulb would get brighter.
As we stated before, a potential difference is also required for a current. One way to produce such a potential is with a battery. A battery is a device composed of two unlike metals or compounds in an electrolytic solution. How does it produce a current? The outer electrons in the atoms of one of the metals are at a greater energy level than the outer electrons in the atoms of the other metal. When these atoms are in solution (To get the atoms into solution, the metals are put into either an acid or base solution that will dissolve at least one of the metals), they can react, and the higher energy level electrons from one of the metals flow to the lower energy of the other metal. The difference in the energy between the electrons of the two different metals can be extracted as a current. The following experiments will demonstrate this.
Experiment 1: For this experiment, we will need a piece of citrus fruit (lemon, orange, grapefruit, etc.), a voltmeter, wire leads, and several strips of various metals. First, roll the fruit between your palm and a table top so as to break the internal membranes of the fruit. Insert two different metals into the fruit. Attach the wire leads to the voltmeter and then to the metal. Read the voltmeter. Repeat for all combinations of metal strips. Put your findings into a chart. Which two give the greatest potential? Why is this?
Experiment 2: Believe it or not, your skin is slightly acidic. Therefore, instead of using a citrus fruit, we could have just as well as used you. Place a copper strip onto one wire lead and an aluminum one on the other. With your hands completely dry, hold both pieces of metal. What is the potential? Now moisten your hands and repeat. Has the potential changed? Why or why not?
Experiment 3: One way to increase the voltage is to connect several electrolytic cells in series. To do this, gather three different pieces of fruit and three sets of two unlike metals (ex. three pieces of copper and three of aluminum). Place one set of metals into each fruit. Connect two of the fruits together by attaching a wire between two separate metals on opposite fruits. Connect the third fruit to the other two in the same manner. Connect the remaining two pieces of metal to the voltmeter. How does the voltage compare to that in your table for the same metals? To increase the current, you must connect the cells in parallel. To do this, connect all three fruits together by attaching each metal plate to its like composed partners in each fruit. Connect the voltmeter to any two unlike metal pairs. Turn the voltmeter to current and read the amperage. Disconnect one of the fruits. How does the current change? Remove another lemon. What does it read now?
Evaluation: This experiment worked well. The participants were able to determine which two metals provided the largest potential difference. There was sum difficulty with the lemon, owing to its internal membranes. A better experiment might be to squeeze the lemon juice into a small beaker or test tube to make the test.
Another way to produce electricity is with a thermocouple. This is a device that consists of two unlike metal pairs at different temperatures. This produces electricity due to the Seebeck effect. Just as in a battery, when two unlike metals are in contact, their outer electrons are at two different energy levels. This energy difference depends on the temperature at which these two metals reside. The thermocouple uses a second pair of the exact two metals at a different temperature to create a potential difference between the two. Interestingly enough, thermocouples are used on the Voyager spacecraft to supply power. As a hot source, they use the heat from the decay of radioactive elements (They are too far away from the sun to use it.).
We can build a thermocouple quite easily. We need several different pairs of metal (see previous experiment), a voltmeter, wire leads, an ice bath, and a hot source (flame, hot plate, etc.). Twist two sets of the metals from the previous experiment together. Attach two of the same type metals from each pair together. Attach the other two same type metals to the voltmeter. Place one pair into the ice bath while placing the other into the warm environment. Read the voltmeter and record the reading in the chart below. Repeat for all pairs. Which pair gave the highest voltage?
Evaluation: This experiment only worked due to the sensitivity of the voltmeters used. The effect would not have shown up too well on the standard voltmeters found in middle grade and high schools.
Georg Ohm discovered in the early 1800's that, for many conducting materials, the current passing through material was proportional to the voltage across it. This law (Unlike Newton's Laws, Ohm's Law is not a universal truth; it is an empirical law that holds for many conducting materials.) can be written as V = I R where V is the voltage, I is the current, and R is the resistance. The resistance of an object depends on several factors. The longer an object is, the more resistance it has. The larger the cross sectional area that the object presents to the flow of electrons, the smaller the resistance is. Finally, the resistance of an object is determined by the composition of the object; copper has a smaller resistivity than water. This is all expressed in the equation R = L/A where is the resistivity of the object, L is its length, and A is the area.
We can test this formula in the following way. We will need a D cell battery, several different diameter pencil leads, wires, and an amp meter (multimeter set on amperage). Connect two wires to the poles of the battery. Attach one end of one wire to the end of a pencil lead. Attach the other wire to the amp meter. With a third wire, attach the other end of the amp meter to a location on the pencil lead that is 1 centimeter away from the first. Measure the amperage. Move the wire to a location 2 cm away and measure the current. Repeat until you are 5 cm away. Repeat this with all of the pencil leads. Make a chart of your results. What is the value of the resistivity for the pencil leads?
Evaluation: This experiment failed, due to the fact that we used carbon pencil leads that had been strengthened with some type of polymer. This dopant lowered the resistance of the leads to about .5-1 ohm. When the batteries were connected, so much current was passing through the leads that they began to smoke. Make sure that the pencil leads are pure carbon (try the cheaper priced ones).
The problem with resistance is that it causes the wire to get hot. And if the wire is getting hot, this means that heat is being generated. And if heat is being generated, then energy is being lost. Considering that electricity has to travel over long distances to get to your house and that resistance increases with distance, a lot of energy is being lost in electric lines. In fact, only about 90% of the electrical energy that enters the line gets through to your house. To show that resistance causes heating, perform the following experiment. Create a 15-cm strip of aluminum by folding a larger piece of aluminum several times. Attach the end of the aluminum to both ends of a D cell battery. Do not do this for more than 20 seconds, as the aluminum will get very hot, and the battery will drain.
Evaluation: Did not do this one since the previous experiment had shown this effect.
When electricity is being delivered to only one appliance, wiring is quite simple. The circuit will form a loop, with one input to the appliance being hooked to one potential and the other input to another. However, when two or more appliances need to be hooked up, you have two options as to how to wire them. One of these options is a series circuit. In this type, the output current from one appliance is the input for the next appliance. The current, therefore, must go through each appliance. If there is a break in the wire (an appliance is turned off or is not working), then the current stops flowing in the circuit and every appliance shuts off. An example of this type of circuit is the old style Christmas tree lights. The other way to wire multiple appliances is in parallel. For this type, each appliance is wired directly to the voltage source. Each appliance has its own current. Thus, if any one appliance is turned off, it does not affect the other appliances.
In either scheme, the voltage supply simply "sees" that it has to supply current to a resistor. These two different wiring schemes present two different resistances to the voltage supply. In a series circuit, the total resistance of the circuit is calculated by summing up the individual resistances of the appliances (RTotal = R1 + R2 +...). Adding more appliances means that the total resistance will increase. According to Ohm's Law (V=IR), this corresponds to a reduction in the total current that is being delivered. In a parallel circuit, the total resistance is the inverse of the sum of the inverse of the individual resistance (1/RTotal = 1/R1 + 1/R2 +...). This means that the more appliances that you connect, the lower the total resistance becomes. The voltage supply must therefore produce more current, according to Ohm's Law. As we saw in a previous experiment, if the current becomes very high, the temperature of the wire will increase to a point where it might melt. For this reason, parallel circuits are often equipped with circuit breakers as a safety device.
To show these two types of circuits, we will build one of each in class. We will need a piece of plywood, screws, switches, light sockets, light bulbs, wire, and a power supply.
Evaluation: This demonstration was deemed very good by the participants. It showed Ohm's Law quite well. It also showed the differences between parallel and series circuits in a striking fashion (used 15 watt light bulbs).