Single Bulb Circuits. Materials: Flashlight battery Light bulb Wire

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1 Single Bulb Circuits Materials: Flashlight battery Light bulb Wire We begin our study of electric circuits by connecting a battery and a bulb together and observing what happens. We investigate the conditions under which the bulb lights brightly, dimly or not at all. Obtain one battery, one light bulb, and one wire. Connect these in as many ways as you can. Sketch each arrangement on the next page. On one side of the page, list arrangements in which the bulb lights. On the other side of the page, list arrangements in which the bulb does not light. You should make sketches of at least four different arrangements that light the bulb. How are they similar? How are they different from arrangements in which the bulb fails to light? Use the space below to state what requirements must be met in order for a bulb to light. 1

2 ARRANGEMENTS THAT LIGHT THE BULB ARRANGEMENTS THAT DON T LIGHT THE BULB 2

3 A Model for the Electric Circuit Materials: 6V Battery Light bulbs Heavy hookup wires Thin wire Coffee Stirrers Straws Cup of Water An arrangement of a bulb, battery and wire that allows the bulb to light is said to be a closed electric circuit. The terms complete circuit, or just circuit are also used. The word circuit was originally used to mean a circular route or course. In working with electric circuits, you may have noticed some regularity in the way they behave. Perhaps you have begun to form a mental picture, or a model, that helps you think about what is happening in a circuit. In this experiment, we will begin the process of developing a scientific model for an electric circuit. A scientific model is a set of rules that applies to a particular system that makes it possible to explain and predict the behavior of that system. We would like to build such a model for electric circuits that will enable us to predict the behavior of any circuit of batteries and bulbs. If we connect several bulbs and batteries together in a circuit, we would like to be able to predict which bulbs will light, which will be brightest, dimmest and so forth. Activity 1: Briefly connect the terminals of a battery with the thin wire until the wire feels warm. Don t maintain this connection for more than 5 seconds at a time since the wire will get very hot. Does the wire seem to be the same temperature along its entire length or are some sections warmer than other? What might this observation suggest about what is happening in the wire at one place compared to another? 3

4 Now make a single bulb circuit using the 6V battery and hook-up cables you have been provided. Sketch your circuit below. Show the working circuit (with the bulb lit) to the instructor before moving on. Note the brightness of the bulb. When a wire or a light bulb is connected across a battery, we have evidence that something is happening in the circuit. The wire becomes warm to the touch; the bulb glows. In constructing a model to account for what we observe, it is helpful to think in terms of a flow around a circuit. We can envision the flow in a continuous loop from one terminal of the battery, through the rest of the circuit, back to the other terminal of the battery, through the battery, and back around the circuit. We have found that a light bulb included in this circuit will light. We shall assume that the brightness of the bulb is an indicator of the amount of flow through the bulb. Brighter means more flow (though twice as bright does not necessarily mean twice the flow). The assumptions that something is flowing through the entire circuit (including the bulb) and that a light bulb can be used as an indicator of the flow are both consistent with our observations. Follow-Up Question: 1. Can you tell from your observations thus far the direction of the flow through the circuit? Why or why not? Can you think of any way you might be able to figure this out? 4

5 There is a standard way to draw diagrams that represent electrical circuits. Shown below is a circuit diagram for a bulb attached to a battery: wire bulb battery wire Notice that the diagram indicates the way in which things are connected: A wire goes from one side of the battery to one terminal on the bulb. A second wire goes from the other side of the battery to the other terminal on the bulb. Wire up a circuit in this way and verify that the bulb lights up. Consider the following dispute between two students: Student 1: When the bulb is lit there is a flow of electric current from the battery to the bulb. There is also an equal flow of electric current from bulb back to the battery. Student 2: There is only flow from the battery to the bulb. We know this is so because a battery can light a bulb, but a bulb can t do anything without a battery. Do you agree with student 1 or student 2? Explain your reasoning. 5

6 Activity 2: Set up a circuit having two bulbs connected one after the other, as shown in the diagram to the right. When bulbs are connected one after the other in this way they are said to be in series. Compare the brightness of the two bulbs. Pay attention to large differences only small differences may be due to the fact that no two bulbs are exactly the same: Compare the brightness of each of the bulbs in the above circuit with the brightness of the bulb in the single bulb circuit of Activity 1. Switch the order of the bulbs in the above circuit and see if this makes any difference: If one of the bulbs is unscrewed, what happens to the brightness of the other bulb? How does the amount of electric current that flows from the battery in the series two bulb circuit compare with the amount of electric current that flows from the battery in the single bulb circuit? Explain your reasoning. 6

7 Activity 3: Set up a two-bulb circuit having the terminals of the bulbs attached together as shown in the diagram to the right. When bulbs are connected in this way they are said to be connected in parallel. Compare the brightness of the two bulbs: Compare the brightness of each of the bulbs in the above circuit with the brightness of the bulb in a single bulb circuit: Switch the order of the bulbs in the above circuit and see if this makes any difference: If one of the bulbs is unscrewed, what happens to the brightness of the other bulb? How does the amount of electric current that flows from the battery in the parallel two bulb circuit compare with the amount of electric current that flows from the battery in the single bulb circuit? Explain your reasoning 7

8 How does the amount of electric current that flows from the battery in the parallel two bulb circuit compare with the amount of electric current that flows from the battery in the series two bulb circuit? Explain your reasoning. It sometimes helps to think of the flow of electricity in wires and bulbs as being similar the flow of water through pipes. In this picture, a wire behaves like a big fat pipe through which water flows very easily, and a bulb is like a skinny pipe through which water flows more slowly. In the same way, electric current flows very easily through the wires used when hooking up a circuit, but faces more resistance when flowing through a bulb. You can think of a battery as a pump. A water pump pushes water through pipes by creating pressure. A battery pushes electric current through wires and bulbs by creating a voltage. Just like pressure is a measure of how hard a pump can push water, voltage is a measure of how hard a battery can push electric current. big pipe wire bulb battery narrow pipe pump wire big pipe Notice that you need the water pipes to make a complete circuit in order for water to keep flowing. You also can t have any leaks. These things are true in electric circuits also: You need a circuit for electricity to flow, and you can t leak electricity anywhere. To make water flow through a narrow pipe we need pressure. If we increase the pressure we increase the flow. If we make the narrow pipe longer, it s harder for the water to move, and the flow becomes smaller. In the same way, to make electric current flow through a lamp we need voltage. If we increase the voltage we increase the flow and the lamp becomes brighter. If we put two lamps one after the other it becomes harder for current to flow, and the lamps become dimmer. For electric circuits this is summarized by Ohm s Law: I = V/R which says that the current I through any part of a circuit is equal to the voltage V pushing across that part of the circuit 8

9 divided by the resistance R of that part of the circuit. Resistance is just a measure of how hard it is for current to flow (like the skinniness of the pipe). We can look at this more directly. Fill your cup with water. Try and drink some of the water through the regular drinking straw. Now try and drink some through one of the coffee stirrers. How much water can you get through the coffee stirrer as compared to the drinking straw? Now, connect two coffee stirrers end-to-end. (This may take a little work to get it so that it doesn t have any leaks.) Try and take a drink through the two coffee stirrers end-to-end. Is it easier or harder than with a single coffee stirrer? Now try the same thing with the two coffee stirrers side-by-side. Is this easier or harder than a single coffee stirrer? How would this change if you were to use a bunch of coffee stirrers together, all side-by-side. What do you observe? The drinking straw represents a low-resistance object, like a wire. The coffee stirrers represent a high-resistance object, like the light bulb. Your mouth is acting like the battery, and the water is acting like the flow of electricity. When you had the two coffee stirrers endto-end, that was like the two light bulbs in series. You notice that it was harder to pull the water through the two coffee stirrers, so as a result, there was less flow. Similarly, the two light bulbs in series were dimmer then the single light bulb. The two coffee stirrers side-byside were easier to drink through. This was like the two light bulbs in parallel. You were able to pull more water through, but each stirrer only got half of the water. Even though there was about twice as much water flow, each stirrer only got half of it. Similarly, with the two light bulbs in parallel, there was twice as much flow, but the two bulbs had to share it, so they were each as bright as the single bulb by itself had been. 9

10 Activity 4: You are about to set up the three-bulb circuit shown in the diagram to the right. A Before connecting the circuit, think carefully and answer the following questions: 1) Predict what you expect brightness of each of the three bulbs (A, B and C) will be compared to each other. A and B: B C A and C: B and C: Hook up the circuit, observe the brightness of the three bulbs, and compare them: A and B: A and C: B and C: Explain in words how you think the electric current flows through your circuit in order to explain your observations. It may help to think of the water analogy. How does the amount of electric current that flows from the battery in the three bulb circuit compare with the amount of electric current that flows from the battery in the single bulb circuit? Explain your reasoning. 10

11 Lighting a Light Bulb with a Balloon Materials: Latex Balloon Compact Fluorescent Light Bulb Piece of Wool The forces that actually move the charged electrons inside an electric circuit are carried by electric fields. We can see the effects from electric fields in many different ways. We will look at one of these in this next activity. Blow up your balloon to full size and tie it off. After it is fully inflated, rub the balloon with the wool cloth for a few minutes. Make sure to rub the same part of the balloon back and forth rather than all over the whole balloon. It also works if you have longer dry hair to rub the balloon on your hair. After rubbing the balloon, try sticking the balloon on the wall or the side of your head. What happens? Why does this happen? By rubbing the balloon, you gave it an electric charge. Electrons jumped from the wool onto the balloon, giving the balloon a negative charge (and the wool a positive charge). When an object has an electric charge, it generates an electric field. Sometimes this field can be very strong. (This is what allows you to shock people with a jolt of static electricity after rubbing your feet on the carpet during the winter.) This electric field pulls positive and negative charges together. Since all matter is made of positive and negative charges, this is why the balloon (negative charge) is pulled in toward your hair (positive charges). (This is also why your clothes can stick together in the dryer if you don t use a dryer sheet. The clothes rubbing together build up an electric charge and stick together. A dryer sheet uses some special chemicals that help keep the charges from jumping from one piece of clothing to another and can help neutralize the charges if they do jump over.) Electric fields can move charges for other interesting effects too. This is how a fluorescent light bulb works. Make sure that your balloon is fully charged. (You will need the classroom lights off to be able to see this next part.) Now, take the fluorescent light bulb and wave it near the charged part of the balloon. (If you don t see anything, try touching different parts of the bulb to the balloon.) What happens? Write down why you think happens. 11

12 Does it matter what part of the balloon you have near the bulb? Why or why not? What s going on: A fluorescent light bulb works because it has a special gas on the inside. Strong electric fields can accelerate the atoms, causing them to collide and excite the electrons in the atoms in the gas or even pull some of the electrons off of the atoms. When these electrons de-excite or recombine with the atoms, they emit light. (This is the same reason you can see lightning or static electricity sparks. When electrons accelerate or when they recombine with or de-excite in atoms, they emit light.) Normally, the voltage that we place across a fluorescent light bulb in a household circuit provides the electric field to make the light bulb glow. However, any charged object gives off an electric field. Even though the field is much weaker, even our charged balloon has an electric field, just like you d have in a household circuit. This electric field can also accelerate the charges in the gas in the light bulb. We see the result as light. (This also shows why this doesn t work with a traditional incandescent bulb They work on a different principle.) 12

13 Magnets Warning: The two neodymium magnets you will be given are very strong. You should keep them away from credit cards and mechanical watches or these may not work real well. The magnets are quite brittle and will break if dropped. Investigating Your Magnets Materials: Two neodymium magnets for each student (you keep these) Bar Magnet Compass Activity 1: Place a bar magnet on the table and make sure all other magnets are at least six feet away from it. You will use the cheap little compass provided to map out the magnetic field lines of the bar magnet, drawing your results on the diagram on page 2. Place the compass on the table, up against the bar magnet at one end. On the diagram, draw an arrow in the following way: - The position of the arrow should show the position of the compass relative to the bar magnet. - The direction of the arrow should be the direction of compass needle, with the head of the arrow representing the red side of the needle. Move the compass all around the bar magnet in ½ inch steps. The compass should be touching the magnet. For each step draw another arrow. Draw all of the arrows the same length - between ½ and 1 inch or so. Now move the compass about 1 inch away from the magnet. Move the compass around the magnet in 1 inch steps, keeping it about an inch away from the magnet at each step, and draw an arrow for each step. Repeat the procedure using 2 inch and 3 inch distances & steps. When your diagram is complete, share your findings with others and discuss as a class. 13

14 N S 14

15 Activity 2: Do the same thing as Activity 1 but map out the field of one of your neodymium magnets rather than the bar magnet. You should stand your magnet on end and keep it in place using a piece of tape. Move the compass in circles 1, 2, 3 inches away from the magnet disc using steps that are small enough that you can the change in the direction if the compass needle is not too big between adjacent locations. Draw your results on the view from above diagram below. Describe a way that you can figure out which side of each of your neodymium magnets is North. Color the north sides of your magnets with a marker for future reference. When your diagram is complete, share your findings with others and discuss as a class. 15

16 The Earth is a Big Beautiful Magnet Materials: Neodymium magnets Compass Thread Use the compass provided to find North (making sure no other magnets are nearby to mess this measurement up). Keep this direction in mind during the following exercises. Working individually, hang one of your neodymium magnets from a string a couple of feet long. Make sure you keep away from any other magnets. Move around a bit. What do you observe? Which side of your hanging magnet is attracted to the north pole of the earth? Is this consistent with the way magnets attract each other? 16

17 Making a Magnet Materials: 1 meter length of wire Battery with leads & clips Neodymium magnets Thread Compass Razor blade Tape Working as a group, take a length of wire and wrap it repeatedly around a ¾ cylindrical object until only an inch or two are left at each end (see sketch to the right). Using a razor blade scrape about ¼ of the insulation off each end of the wire. Tie the loops together with string in two or three places so that they stay together as you handle it. Tie with string Scrape insulation off ends Activity 1: Stand the loop on its side and hold it in place with some tape. Hook the positive side of the battery up to one of the leads and the negative side to the other lead. The battery is now driving an electric current through the loop of wire. Move the compass around near the loop of wire. What do you see? Investigate the magnetic field of this loop in exactly the same way that you did for the neodymium magnet in activity 2 of experiment 1. How does the field of this loop compare to that of the neodymium magnet? 17

18 Activity 2: Suspend a neodymium magnet from a string and dangle it near the loop. Investigate the forces between the magnet and the loop when the battery is driving a current through the loop. Which side of the loop is North? Table Switch the way the leads from the battery are hooked up to the loop of wire. Now which side of the loop is North? Disconnect the battery from the loop. Describe the magnetic field produced by the loop now. 18

19 Making a Speaker Materials: Paper Cup 2 meter length of wire Audio plug connected to wires Neodymium magnets AM/FM Radio / CD Player / Laptop / or some other audio source with a standard headphone jack Just like a magnet can be used to move a coil of wire which is carrying an alternating current, a coil of wire can be used to move a magnet. Take the 2 meter length of wire and wrap it around the base of the paper cup to form a tight coil about 1 cm or so from the bottom of the cup. This should be similar to the coil that you used to make an electromagnet earlier. Once you have a good coil of wire, connect the two ends of the coiled wire to the two wires that are connected to the audio plug. Place the neodymium magnet in the bottom of the paper cup. Congratulations, you have just constructed a basic speaker. Now, plug the audio plug into your radio / CD player / whatever and play some music at a fairly high volume. (Note that even though you should now have a working speaker, it won t be a great speaker, so the more input volume your player can put out, the better your chances of being able to hear it.) Music or talking with a large dynamic range (changes in volume) works best. Now, place your ear up to the opening of the paper cup. Can you hear the music? Why does this happen. Write down your ideas and then discuss as a class. 19

20 What s going on: This speaker works about like any other speaker. When the music is played, an electric current runs through the wire. The direction of this current switches back and forth. As the volume changes, the flow gets stronger and weaker. As the flow of electricity gets stronger and weaker, the magnetic field that the wire is generating gets stronger and weaker. This will pull off and on on the neodymium magnet, causing it to vibrate. The magnet vibrating causes the drum of the cup to vibrate. This vibration causes waves in the air that you can hear as sound. This is the same way a tin can telephone works. As the speaker speaks into the first tin can, the sound waves cause the base of the can to vibrate. This vibration pulls off and on on the string. This string pulls, in turn, on the base of the other can. (This also shows why the string has to be pulled taut for the phone to work.) The pulling of the string on the base of the second can causes that to vibrate, producing sound. Source: /07/28/can-you-hear-me-now-part-2/ 20

21 Optional activity [you probably won t have time to do this in class but you have all of the needed materials]. Making a Motor Materials: Styrofoam base to hold motor. 1 meter length of wire Battery with leads & clips Neodymium magnets Thread Compass Razor blade Tape Paper clips Working individually, take a length of wire and wrap it repeatedly around a ¾ cylindrical object until only an inch or so is left at each end (see sketch below). This time the two leads should stick straight out at opposite ends of the loop. Tie the loops together with string in two or three places so that they stay together as you handle it. You may also want to use some tape to provide extra strength at the places where the leads stick out. Tie with string You will again scrape away part of the insulation from the leads that are sticking out, but this time you will do it a bit differently. Lay the completed loop on the table. Looking down on it from above it should look like the sketch above. Using the razor blade, remove the insulation from the top of the leads only. In other words, when you are done the top half of the leads should be shiny metal, but if you turn the loop over you should still see insulation on the bottom half, as shown in the zoomed in sketch to the right. If you have any questions or doubts about doing the right thing, ask the instructor before starting to remove the insulation. 21

22 Using paperclips and the base, make the two vertical supports that will be used to hold your wire loop as shown in the figure below. (The base doesn t have to be made out of Styrofoam any insulating material that can support the paperclips will do cardboard works fine). Place your magnet below the suspended loop as shown. You want the loop to turn just above the magnet, not much higher. Carefully balance the loop so that it turns easily as it is held by the supports. Hook the two wires from the battery up to the two supports and give the loop a little spin to get it started. If you can t get it to work after trying for a while, ask for help. See if you can figure out how the motor works. Explain why stripping the insulation from just the top half of the leads was the key to getting this motor to spin. 22