Electric Circuits. Consider a simplified water cycle which might be a water feature in a lawn or garden:

Transcription

1 Electric Circuits The word circuit comes from the same root as the words circle and cycle. An electric circuit is a device which moves electric charge around in a circle or at least a loop. It is related to other cycles such as the water cycle. Consider a simplified water cycle which might be a water feature in a lawn or garden: Water cascades down a short waterfall. In the process it turns a small water wheel. At the bottom of the waterfall the water flows through a drain and into a pipe. The water flows through the pipe to a water pump. The pump pushes the water up through another pipe to the top of the waterfall. The process repeats itself constantly. Now consider a simple electric circuit: One end of a battery (or cell) is connected to a wire. The wire is connected to a glowing light bulb. The other end of the light bulb is connected to another wire. This second wire leads back to the other end of the battery. This water feature and this electric circuit are closely related! In the space below, draw a sketch of the water feature on the left and a sketch (or diagram) of the electric circuit on the right. Below your diagrams try to identify the corresponding pieces of the two systems. (For example, what piece of the electric circuit corresponds to the water pump of the water feature? What part of the water feature corresponds to the wire from the battery to the light bulb?) 1

2 Charge: the stuff that moves. The stuff that moves through the water pipes is, um, water. The stuff that moves through the wires is called electric charge. A few experiments will illustrate the nature of electric charge. 1. The sticky tape experiment(s). The equipment for this experiment is extremely simple and cheap. Even so, correctly setting it up and doing the experiment can be tricky. It is best to get instructions from somebody else, usually called an instructor. If at all possible, watch your instructor do the experiment once before you try to do it yourself. Take a piece of sticky tape (such as magic tape ) about two or three inches long and fold one end over on itself so that you have a non-sticky tab (a quarter to a half an inch long) on one end. Smash the tape down on a hard smooth surface (such as a clean tabletop). Grasp the non-sticky tab of your tape and quickly peel it off the table. Let your tape hang down (you might need to shake it off of your hand) and try holding it near the tape of one of your classmates. What happens? Sticky Tape II (the sequel) That was so much fun we ll do it again with TWO sticky tapes placed on top of each other. In order to tell them apart we ll need to label them, so you will need a pencil or pen that can write on your sticky tape. Make one sticky tape and smash it down on the table as you did above. This will be the lower tape so label the non-sticky tab with a lower case l for lower. Now make another sticky tape (the same size) and smash it down directly on top of the lower one. The two non-sticky tabs should be right on top of each other. This is the top tape so label the non-sticky tab with a lower case t for top. Grasp BOTH STICKY TABS between the thumb and finger of one hand and quickly lift them off of the table together. Once they are hanging (together), grasp the two sticky tabs, one with each hand, and quickly pull them apart. Slowly bring your t tape near your l tape. What happens? Bring your t tape near the t tape of a classmate. Bring your l tape near the l tape of a classmate. What happens? 2

3 (Sticky tape, continued) Talk to your classmates and try to come up with a general rule for the behavior of t and l sticky tapes. Your rule might sound like another rule you have heard before. As a clue to what s going on here, try looking at your sticky tapes sideways. One of them will have a sideways t and one will have a sideways l. What do they look like? (This is where you say something like, Oh, now I get it! ) Benjamin Franklin figured out that electric charges come in two types. If we have equal amounts of the two types then they cancel each other out. He recognized that these are the properties of positive and negative numbers so he called one kind of charge positive and the other kind negative. Based on what you have done in class (or elsewhere) have you seen anything other than a label (such as l, t, red or black ) that clearly identifies which kind of charge is + and which kind of charge is -? (Do light bulbs care? Would the sticky tape experiment work differently if we reversed the labels?) Interesting historical fact: Mathematicians of the 1700s were undecided about whether or not negative numbers were real or whether they should be taught in school. Franklin s discovery ended that debate. The t sticky tapes have more positive charge than negative charge. The l tapes have more negative than positive. Do you think your hand has more positive or negative charge? Why do you think so? What evidence do you see? Everything we encounter in our daily lives contains both positive and negative charges. What must be true about the positive and negative charges in most of these things? Why? 3

4 2. Other charged stuff. Franklin s definitions: We can identify positive and negative charges using sticky tape but Franklin s definitions were based on what was available at the time. o If you rub a piece of amber with rabbit fur, the amber will become negatively charged. If you don t have amber and rabbit fur, you can use a balloon and your own hair. Rub a balloon against your hair and the balloon becomes negative. o If you rub a piece of glass with silk, the glass will become positively charged. You can substitute acrylic for glass if you want and you can substitute plastic food wrap (such as Saran Wrap) for the silk. What does it mean to say that a balloon is negatively charged? Does that mean it has no positive charges in it? WARNING: This use of the word charged is different from the use of the word charged or charging when referring to a battery! Referring to a battery as charged or uncharged is actually sloppy use of language but it is very common. Before it has been rubbed with fur or hair, we say it is uncharged. What does this mean? An uncharged balloon is rubbed against some uncharged fur. The balloon becomes negatively charged. What do you suppose happens to the fur? You should be able to find a collection of things you can use as substitutes for Franklin s amber, fur, glass, and silk. Predict what will happen if we charge these objects in various ways and bring them close to each other. Once you have made some predictions, test your predictions with some experiments. Record your observations below. 4

5 Wires: pipes for electric charge When looking at an electric circuit, the wires don t move or light up. When an electrical device quits working we usually don t change the wires. The wires might not seem to do anything. 1) Set up an electric circuit with a couple of cells and one round bulb. Find a compass (the kind that points north, not the kind that draws circles) and place it on the table far from metal objects. When the needle comes to rest, lay one of the wires of your circuit directly over the needle (it should run right over the needle in the same direction as the needle, not perpendicular to it). Without moving this wire or compass, connect and disconnect the circuit a couple of times. Do you see evidence of something happening at the wire? 2) Prepare to set up an electric circuit with a couple of light bulbs (both long or both round) in series but BEFORE YOU CONNECT THE CIRCUIT, predict which light bulb will light first when you do connect the circuit. a. Discuss this with your classmates. You will probably hear different hypotheses about which bulb will light first and why. Write down some of these hypotheses so you can test as many as possible. b. Now connect the circuit. Which bulb lights first? Do you all agree about which bulb lights first? Does this change if you switch the order of the bulbs? Does it change if you reverse the batteries? What do you think: which bulb lights first? 3. You can think of your wire as solid piece of copper. In copper wire the positive charges (nuclei of atoms) stay put along with most of the electrons, while only a small fraction of the negatively charged electrons move. Does this sound more like a hollow water pipe or a pipe full of sand? Would this kind of water pipe lend itself toward charge that moves fast or slow? How does this analogy compare with your observations in part 2b, above? 5

6 4. Thinking about water When you turn on a water tap, how long does it take before water starts to flow? 5. Jane is shopping for an apartment. She sees one that she really likes but she notices that it is a couple of miles away from the nearest water reservoir. She reasons that water will come out of the tap at a rate of a couple of feet per second. If the reservoir is 10,000 feet away, then she figures she would have to wait 5,000 seconds for water to reach her from the reservoir. She doesn t want to wait 5,000 seconds (that s well over an hour) every time she needs a glass of water so she decides to look for another apartment. What do you think of Jane s reasoning? What s wrong with it? 6. When we use a water tap at GRCC, the water starts to flow right away. Immediately before we started to use the water tap where was that water? 7. So here s a little paradox to figure out. The electric charge that moves through a wire is like a fluid moving through sand the size of atoms. It flows very slowly. One centimeter per minute is not unusual and when speeds get up to one millimeter per second you can expect the wire to get hot! When you flip a light switch, the lights seem to go on or off instantaneously. Even if lights are across the room they seem to light immediately when the switch is flipped. How can both of these statements be true? (Which they are.) Check your ideas with an instructor. 6

7 Batteries: the pumps of electric charge. First a question about our water feature (remember the waterfall, water wheel, and the pump). Does the pump create the water? Does the pump store the water? Does the water come from the pump? What does the pump do? A water pump can do a combination of a couple of things. It can raise water up, or it can increase the pressure in the water, or both. This is similar to what is done at hydroelectric dams. There is a name for this. A water pump increases what property of the water? (Ask your instructor if you just don t know.) Electric potential Scientists use the word Potential to mean potential energy per unit of stuff. Exactly what is the most useful unit of stuff in any given field of science is something which is defined by experience and tradition. Here are some examples: Fluid potential = Gravitational potential = Chemical potential = Potential energy Unit of volume Potential energy Unit of mass Potential energy Unit of chemicals (the unit of chemicals is usually the mole ) For electricity, the stuff is electric charge, so the unit of stuff is the unit of electric charge. The standard unit of electric charge is the coulomb but don t worry about that yet. Just take it for granted that we have a unit for a certain amount of electric charge. Electric potential = Potential energy Unit of charge Electricity is probably the area of science where the concept of potential is used the most. 7

8 So based on our analogy with the water feature, what do you think batteries do? Let s test your ideas. Set up a circuit where you light two light bulbs in series (and you might as well use long ones) with one cell. That s not very impressive. If one cell raised the potential by one unit (we ll get to the units of electric potential in a minute) what do you predict two cells in series would do to the potential? What do you predict this would do to your circuit? Test your prediction. Go ahead and test what happens with three cells in series and four cells in series. You might burn out a bulb but you can try it this once. Record your observations. Now what do you expect would happen if you set up the circuit below? This circuit should behave the same way as what circuit you ve seen before? Test your prediction. (Pay careful attention to the arrangement of the cells and you might need extra hands to hold all of the cells.) 8

9 Now what do you expect would happen if you set up the circuit below? Notice that it is not the same as the last circuit. Even so, this circuit should behave the same way as what circuit you ve seen before? Test your prediction. (Pay careful attention to the arrangement of the cells and you might need extra hands to hold all of the cells.) Electric potential is potential energy per unit of charge. If you know the electric potential difference between two points (which might be the ends or terminals of a battery), how could you figure out how much energy is required to move a certain amount of charge through the circuit? One more step. Electric potential is potential energy per unit of charge. Power is energy per unit of time. If you know the electric potential difference between two points (which might be the ends or terminals of a battery), how could you figure out how much power is required to operate an electric circuit? 9

10 Light bulbs and other workers: Incandescent light bulbs are essentially made out of wire with some fancy packaging (glass and a metal case). The filament of a light bulb is just wire, and yet there are other wires in the circuit. Why do you suppose the filament glows? Aside from being bright, what other word might describe the filament while it is glowing? The filament is a lot thinner than the other wires in the circuit. What difference would that make to the electric charge flowing through it? (You can think of water flowing through a water pipe.) The flow rate of electric charge is called current. In the circuit below, how would the flow rate (or current) through bulb A compare to the flow rate (or current) through bulb B? A B In the circuit above, how would the brightness of bulb A compare to the brightness of bulb B? (You can set up the circuit if you like.) What have you noticed about the relationship between the brightness of bulbs and potential difference? If two (identical) bulbs have the same brightness, what would you say about the potential difference of the two bulbs? 10

11 A B x For historical reasons, we draw arrows for electric current as though it comes out of the positive end of a battery and flows into the negative end (and all the way through the battery as well). To remind yourself of this, draw a clockwise arrow around the circuit above to indicate the direction we say the current is flowing. Now imagine we follow the current around this loop, starting at the x in the corner. The first thing that happens to the charge is that it hits that battery. When that happens, does the electric potential go up or down? What do you think? (You might want to check your answer with an instructor.) Then the charge travels across the wire at the top of the diagram. It turns out that very little happens to the potential in a large wire. No measureable work is done there. Work is done when the charge gets to the bulbs. If the charge has to do work as it moves through bulb A, what happens to its potential as it moves through A? Does the potential increase or decrease? (Think about what happens if you have to do work or a spring has to do work.) What happens to the potential as the charge moves through bulb B? If A and B are identical bulbs, how would the change in potential in A compare to the change in potential in B? The charge then moves back to the battery along a thick wire and not much happens to the potential along that wire. Put it all together. What happens to the potential as the charge moves around the loop? 11

12 C D x That was fun, let s do it again. The difference in this circuit is that there are two loops that pass through the battery which the charge can follow. The first loop goes from the x through bulb C and back to the x. The second goes from the x, through bulb D and back to the x. You may want to use colored pencils, but draw both loops in the diagram above. To start out, let s assume that bulb C and bulb D might be very different. One might be a huge spotlight and the other one is a small flashlight bulb. Is it necessarily true that the current (or flow rate) through bulb C will be the same as the current (or flow rate) through bulb D? Now the honors question: there is also current flowing through the battery. You can think of this as the current at the x if you like. Looking at your arrows for your two loops, you should be able to come up with a relationship between the current through the battery, the current through bulb C, and the current through bulb D. What is that relationship? Now think about potential differences. Look at the loop through bulb C. The electric potential rises as the charge moves through the battery and it drops as it moves through bulb C. If it all comes back to the same spot at the x, what must be true about the jump in potential at the battery and the drop in potential at the bulb? (Think about the water feature if you like.) Look at the other loop. What can you say about the potential difference across bulb D? What can you say about the potential difference across the battery the potential difference across bulb C, and the potential difference across bulb D? 12

13 A B C D x x Putting it all together: We would say that light bulbs A and B are connected in We would say light bulbs C and D are connected in When two light bulbs are connected in series, what must be the same in those two bulbs? When two light bulbs are connected in parallel, what must be the same across those two bulbs? (Note on language: Notice the use of the words in and across in the previous two questions. That isn t an accident. Current runs through things. Potential difference is measured between the two ends of things. Charge moves. Current flows. Potential difference doesn t move.) While we are on the subject of language: Electric current is measured in units called amperes or amps (abbreviated A). Electric potential is measured in units called volts (abbreviated V). Another name for electric current is amperage. The difference in electric potential between two points goes by the special name voltage. Voltage is also measured in volts. 13

14 Resistance: What makes a filament light up. When we rub our hands together, we say our hands get warm because of friction. When we run current through a wire, we say the wire gets warm because of resistance. Electric current through an object (a light bulb) increases when the potential difference (or voltage) across the light bulb increases. If the potential difference (or voltage) is zero, then the flow rate is zero. What sort of a relationship does that sound like? Electric current through an object (a light bulb) decreases when the resistance of the light bulb increases. If the resistance becomes infinitely big, then the flow rate is zero. What sort of a relationship does that sound like? The previous statements can be summarized in one equation. The equation is called Ohm s law and it can be written in a couple of ways. current = voltage resistance or voltage = (current)(resistance) Think about water pipes full of sand or gravel. If you want a water pipe with very low resistance, what properties would you give it? Think about what happens when you add more light bulbs in series (to a constant number of cells). What appears to be happening to the current? What must be happening to the resistance? Think about what happens when you add more light bulbs in parallel (to a constant number of cells). What appears to be happening to the current through the battery? What must be happening to the resistance? 14