# Section 4 WHAT MAKES CHARGE MOVE IN A CIRCUIT?

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1 Section 4 WHAT MAKES CHARGE MOVE IN A CIRCUIT? INTRODUCTION Why does capacitor charging stop even though a battery is still trying to make charge move? What makes charge move during capacitor discharging even though there is no battery to cause movement? Clearly, the complete story of why charge moves in circuits has to involve more than just batteries. In this section you will investigate the non-battery causes of charge movement in circuits. INVESTIGATION ONE: WHAT HAPPENS WHIE A CAPACITOR CHARGES? 4.1 Activity: Experimenting with an already-charged capacitor Charge a 25,000 µf capacitor through two long bulbs, using a 3-cell battery as shown in Figure 4.1a. Use a compass under one of the wires to monitor direction of flow. Figure 4.1a Figure 4.1b Figure 4.1c CHARGING CHARGING COMPETED ADDED BATTERY 1. Draw arrows on Figure 4.1a to show charge flow in all parts of the circuit while the bulbs are lit. Don t attempt to use arrowtails to show flow rate just show directions. 2. Figure 4.1b shows the capacitor after it has been charged. Draw (+) signs by the plate that has gained charge, and ( ) signs by the plate that has lost charge. PASCO scientific Student Manual 39

2 Next, imagine that you have opened the circuit and placed a second battery pack in the loop as shown in Figure 4.1c. Don t actually do this right now. Just think about what might happen if the already-charged capacitor is suddenly connected to a stronger battery with 6 cells. 3. Predict: Will the bulbs light again if you add the second 3-cell battery pack and close the circuit? Why or why not? Now add the second battery pack as shown in Figure 4.1c, with the positive end of one battery pack connected to the negative end of the other one. Make sure the compass is under one of the wires. 4. Did the bulbs light? If they did, draw an arrow on Figure 4.1c to show the direction charge was moving everywhere during the second bulb lighting. 5. Did more charge go into the (+) capacitor plate and out of the ( ) plate? What is the evidence? Now, remove both batteries from the circuit and connect the free ends of the wires to each other to form a closed circuit with a compass still under one wire. 6. Regarding both the bulbs and the compass, what did you observe? Explain why this happened. Demonstration: The teacher will now charge a capacitor with one battery pack as in Figure 4.1a, then add a second pack as in Figure 4.1c, and then add a third battery pack to the circuit. 7. How many times do the bulbs light? 8. Why do you think bulb lighting stops each time? PASCO scientific Student Manual 40

3 4.2 Activity: Exploring air as an analogy In Section 3 an air capacitor provided insight into non-battery origins of charge in electric circuits. In this activity an air capacitor provides insight into non-battery causes of movement in circuits. In the previous activity we found that a battery can push additional charge into a capacitor plate that is already full. We can make a similar situation for air by (a) connecting two syringes that are already filled with air (b) pushing some of one syringe s air into the other syringe Set up the apparatus shown in Figure 4.2a by pulling the plunger of syringe A all the way out, pulling the plunger of syringe B half-way out, and connecting the 2 syringes with a short length of clear tubing. One person should hold plunger B steady to mimic the charge-holding region of a capacitor plate, while a partner pushes on plunger A to mimic stronger pushing by a battery. Syringe A Air Tubing Figure 4.2a SYRINGES CONTAINING AIR CONNECTED BY TUBING 1. Can you push air from syringe A into syringe B? Air Syringe B 2. Describe how hard you have to push on plunger A, as you drive more and more air into syringe B. 3. Describe how much force you must exert to keep plunger B from moving while plunger A is being pushed in. 4. How does the air pressure change as syringe A s plunger is pushed in? 5. et go of syringe A s plunger, and describe what happens. Then start over and let go of syringe B s plunger. Describe what occurs. 6. Using the connected syringes, air provides a model for explaining the observed electrical behavior in the circuit of Figure 4.1c. What are a) the advantages, and b) the limitations of this model? PASCO scientific Student Manual 41

4 4.3 Commentary: Compression, concentration, and trying-to-expand When you pushed plunger A inward, the air in the syringes was compressed into a smaller volume. The air responded to this compression by trying to expand. The evidence for trying-to-expand was clear: When you released plunger A, you saw it being pushed back out by the compressed air. Increased concentration particles more tightly packed is the reason compressed air tries to expand. But making the volume smaller is not the only way to increase the concentration. When you pump air into a car tire, you increase the concentration by adding more air in a given volume. You are creating the same basis for trying-to-expand. The proof is that the extra air will expand out through any hole you make in the tire. You can perform a thought experiment that combines volume reduction with adding more: Visualize a tire that s full of normal air. Then visualize this air being compressed into part of the tire. Finally, visualize more air being pumped into the part that was left empty when the volume of the original air was reduced. The fact is that air tries to expand no matter how you make it more concentrated. The term compressed air is generally used for all trying-to-expand situations. 4.4 Commentary: The electric pressure idea Compare extra charge being pumped into a capacitor plate (by a battery) with extra air being pumped into a tire: As charge flows in, the concentration of charge in the plate increases. You can imagine the charge in the plate being compressed to make room for more like air in the tire being compressed to make room for more. Does compressed charge try to expand back out of the plate through a wire like compressed air expands back out of the tire through a hole? If compressed charge behaves the same way as compressed air, then the following events will happen: Increasingly strong reverse pushing by increasingly compressed charge in the (+) plate will make the battery less and less able to pump more charge into the plate. That will make the bulbs get progressively dimmer during capacitor charging. When the battery is removed, compression in the (+) plate will push charge in the reverse direction and discharge the capacitor. Decompression will weaken the reverse pushing and make the bulbs dimmer over time during discharging. These bulb dimming predictions were in fact observed. The observations provide evidence that compressed charge in circuits really does behave like compressed air. PASCO scientific Student Manual 42

5 AIR PRESSURE is the name given to the effort to expand by compressed air. The name EECTRIC PRESSURE is the same effort by compressed charge. Electric pressure is measured in terms of a unit called the VOT named after the Italian scientist Alessandro Volta, who introduced the concept in Commentary: Is the air analogy really right? Thinking about electric pressure in a container of charge as being like air pressure in a container of air helps you keep in mind that charge always tries to move from a place of higher electric pressure to a place of lower electric pressure. It reminds you that this movement continues until the pressures are equalized. The same idea that helps you understand when and where air moves will also help you predict when and where charge moves. Nevertheless, we shouldn t expect charge to behave like air in absolutely every respect. We will use the term pressure to emphasize that compressed charge behaves like compressed air in important respects. But we will add the qualifier electric as a reminder that differences of behavior may exist in circumstances that we have not yet encountered. PASCO scientific Student Manual 43

6 INVESTIGATION TWO: HOW IS EECTRIC PRESSURE INFUENCED BY A BATTERY? 4.6 Commentary: Proposed model of how a battery pushes on charge Suppose a battery moves charge internally as depicted in Figure 4.6a out of its bottom terminal and into its top terminal. The consequences of this movement are shown in Figure 4.6b charge depletion ( ) in the bottom battery terminal and charge compression (+) in the top terminal. Figure 4.6c shows the presence of below-normal OW pressure in the bottom terminal (produced by depletion) and above-normal HIGH pressure in the top terminal (produced by compression). + + HIGH - - OW Figure 4.6a Figure 4.6b Figure 4.6c CHARGE MOVED COMPRESSION (+) IN TOP RESUTING HIGH INTERNAY BY TERMINA AONG WITH PRESSURE IN TOP THE BATTERY DEPETION ( ) IN BOTTOM & OW IN BOTTOM Figure 4.6c describes a proposed model of how a battery pushes on charge in wires connected to it. This model needs to be tested to find out how well it works. But Figure 4.6c calls our attention to a role for below-normal electric pressure in circuits. Example: What does this OW pressure do during capacitor charging? We need to find out how below-normal air pressure behaves before we can test a battery model that involves below-normal electric pressure. 4.7 Activity: How below-normal air pressure behaves Figure 4.7 shows an air capacitor with both sides open to the atmosphere through a tube in each side. There is atmospheric pressure in each side, which we will call NORMA air pressure. Balloon Sport Water Bottles Tape Figure 4.7 AIR CAPACITOR OPEN BEFORE INVESTIGATION 1. Blow air in through the tube on one side of an air capacitor, and hold the extra air inside by closing the tube. Draw a sketch of the air capacitor, and label the pressure in each side as NORMA or HIGH or OW. PASCO scientific Student Manual 44

7 2. Explain why the membrane between the two sides changes shape. 3. Release both ends of the air capacitor to normal atmospheric pressure. Then place your mouth over the tube at the other side of the air capacitor and inhale; hold the depletion inside by closing the tube. Draw a new sketch of the air capacitor, and label the pressure in each side as NORMA or HIGH or OW. Explain why the membrane between the two sides changes shape. 4. What do your observations tell you about the comparative ability of: (a) Above-normal pressure to push toward NORMA pressure? and (b) NORMA pressure to push toward below-normal pressure? 4.8 Exercise: Testing the pressure-creating model of a battery Consider a battery described by the model proposed in Activity 4.6. Suppose this battery is connected in a circuit with an uncharged capacitor. Before the circuit is closed, both capacitor plates will have a normal amount of charge. So they will be at NORMA electric pressure when the circuit is closed as shown in Figure 4.8. Use the air analogy test the predictions of the proposed model of a battery. Be sure to include the role of OW (below normal) electric pressure the analog of OW air pressure which you investigated using an air capacitor in Activity Set up the circuit in Figure 4.8 and observe bulb lighting during capacitor charging. According to the proposed model, what makes the top bulb light? HIGH OW NORMA NORMA 2. According to the proposed model, what makes the bottom bulb light? Figure 4.8 THE MOMENT CHARGING BEGINS 3. According to the proposed model, why do the bulbs become dimmer over time? 4. According to the proposed model, why does charging eventually stop? PASCO scientific Student Manual 45

8 INVESTIGATION THREE: HOW CAN WE VISUAIZE PRESSURES IN A CIRCUIT? This investigation introduces the use of colors to represent electric pressure values in circuits. Color-coding a circuit enables you to visualize pressure differences as the causal agents that determine where and when charge moves. 4.9 Commentary: Color coding for electric pressures in a circuit Electric pressures can be indicated on circuit diagrams by using colors to represent pressures on a relative scale. The following coloring system will be used: RED HIGHEST Above Normal ORANGE Above Normal YEOW NORMA GREEN Below Normal BUE OWEST Below Normal Rules For Color Coding 1. A battery maintains highest electric pressure in the metal terminal labeled (+) and lowest electric pressure in the terminal labeled ( ). Therefore: Use RED for the (+) battery terminal and wires directly connected to it. Use BUE for the ( ) battery terminal and wires directly connected to it. 2. Use YEOW to represent normal electric pressure due to the normal amount of charge that exists in a connecting wires and uncharged capacitor plates before the wires are connected to a battery. 3. Battery terminal colors transfer to connecting wires as soon as the wires touch. Use only one color throughout each wire -- and throughout any group of wires that touch each other -- as well as throughout any capacitor plate connected to it. 4. Use different colors for the two wires connected to opposite sides of a lit bulb, because a pressure difference is needed to cause charge flow through a filament that resists flow. The colors may change over time during a transient process. 5. Do not color light bulbs -- because a lit bulb filament does not have the same pressure at all points. For the same reason, do not color the interior of a battery. PASCO scientific Student Manual 46

9 4.10 Commentary: Why wires are given uniform colors A battery terminal transfers its electric pressure to a wire everywhere in the wire as soon as the wire touches it. Why??? The wire does not resist charge flow to any significant degree. So charge flow into or out of the wire will equalize the pressure everywhere within it and with the battery terminal in a super-fast transient process. When a wire is connected to a capacitor plate, the plate has so much more metal than the wire that very little charge needs to leave or enter the plate in order to make the pressure in the wire equal to that in the plate. Therefore the pressure-equalizing process will not appreciably change the pressure in the plate. Whenever two or more wires touch each other, charge flow between them will equalize the pressure everywhere in the connected wires in an instantaneous transient process Activity: Why wires are given uniform colors Place one end of a soda straw against the skin of your arm or hand. Blow air into the straw at the other end then suck air out. 1. How much time elapsed between blowing air into the straw and feeling its pressure on your skin? How much time did it take to suck the air out? 2. For how long did the pressure keep changing after you first felt a change? 3. How much air do you feel you pushed into the straw, or sucked out of it, in order to change the pressure compared with the amount you blow in to supply air flow through an open straw? 4. If a wire is to charge flow as a straw is to air flow, what can you conclude about how much time it takes a wire to reach uniform pressure throughout the wire? PASCO scientific Student Manual 47

10 4.12 Exercise: Color coding the circuit for capacitor charging + _ Figure 4.12a Figure 4.12b NO BATTERY CHARGING BEGINS Color the battery terminals, the wires, and the capacitor plates in the diagrams in Figures 4.12a and 4.12b as you read the explanations that follow. Figure 4.12a shows a circuit containing a capacitor and two light bulbs. Since the circuit has no battery, the original NORMA pressure in the wires and capacitor plates has not been altered; therefore the wires and plates are all colored YEOW. In Figure 4.12b, a battery has just been inserted into the circuit. The (+) terminal of the battery is a place that the battery keeps at HIGH electric pressure, and so it is colored RED. The red-to-yellow pressure difference will instantly push extra charge into a non-resisting wire attached to the battery s (+) terminal --- enough charge to raise the pressure in that wire to the same RED value as the battery terminal. Because the light bulb resists movement of charge, hardly any charge will have moved through the upper bulb and into the top capacitor plate during the negligible amount of time it takes the wire to reach RED pressure. Because an enormous amount of extra charge is needed to raise the pressure in the very large top capacitor plate, it and the wire attached to it are still at essentially the original YEOW pressure. The ( ) terminal of the battery is a place that the battery keeps at OW electric pressure, and so it is colored BUE. The yellow-to-blue pressure difference will instantly push charge out of a non-resisting wire attached to the battery s ( ) terminal --- enough charge to lower the pressure in that wire to the same BUE value as the battery terminal. Because the light bulb resists movement of charge, hardly any charge will have moved out of the bottom capacitor plate and through the lower bulb during the negligible amount of time it takes the wire to reach BUE pressure. Because an enormous depletion of charge is needed to lower the pressure in the very large bottom capacitor plate, it and the wire attached to it are still at essentially their original YEOW pressure. The pressure difference in the two wires connected to a bulb is what drives charge through the bulb. A large enough pressure difference will drive a flow rate that is great enough so that friction between the moving charge and the material of the filament will make the filament hot enough to glow. The glow is what you see when a bulb lights up, but the pressure difference in the wires is what drives the flow that makes the bulb lighting happen. PASCO scientific Student Manual 48

11 Figure 4.12c shows the situation after enough charge has moved through each bulb to significantly change the amounts of charge in the capacitor plates. The increase of charge in the top plate has raised the pressure there to ORANGE, and depletion in the bottom plate has lowered the pressure there to GREEN. The wires attached to these plates will have the same colors (pressures) as the plates. + _ + _ Figure 4.12c Figure 4.12d CHARGING CONTINUES CHARGING COMPETED The red-to-orange and green-to-blue pressure differences are smaller than the earlier differences from red-to-yellow and yellow-to-blue. So the pressure differences driving charge through the bulbs are now smaller than they were earlier. These smaller pressure differences now drive charge through the bulbs at a lower flow rate. That reduces heat from friction in the filament, and makes the bulbs appear dimmer. In Figure 4.12d, enough charge has been driven through the top bulb so that the pressure in the (+) capacitor plate has become equal to the HIGH pressure in the (+) terminal of the battery. So that plate and the wire connected to it are now colored RED. Also, enough charge has moved through the bottom bulb so that the pressure in the ( ) capacitor plate has become equal to the OW pressure in the ( ) terminal of the battery. So this plate and the wire connected to it are now colored BUE. Now, notice that there is no longer any pressure difference in the pair of wires connected to either bulb. Since pressure differences are needed to drive charge through filaments that resist flow, there is no further charge flow through the bulbs. The bulbs are not lit, and the process of capacitor charging has stopped. 1. What is happening in the upper and in the lower capacitor plates during charging? UPPER: OWER: 2. Observations of the bulbs and compass indicate that the capacitor charging eventually stops. Why doesn t charging continue? PASCO scientific Student Manual 49

12 4.13 Activity: Color coding the circuit for capacitor discharging Figures 4.13a, b, c, d show the situation at selected times during discharging of the capacitor. The number of (+) and ( ) symbols show the degree of compression or depletion of charge in the capacitor plates. 1. Draw starbursts on Figures 4.13a, b, c, d to show bulb brightnesses, and arrows to show the flow rates that cause bulb lighting. Show the distribution of pressures that make charge move by coloring capacitor plates and wires BATTERY REMOVED DISCHARGING BEGINS Figure 4.13a Figure 4.13b DISCHARGING CONTINUES DISCHARGING COMPETED Figure 4.13c Figure 4.13d 2. Which figure has the greatest pressure difference across the bulbs? 3. Which figure shows charge driven through the bulbs at the greatest flow rate? 4. In which figure do the bulbs become dim? PASCO scientific Student Manual 50

13 4.14 Activity: Color coding in circuits that don t have capacitors Color each of the following circuit diagrams (Figures 4.14a through 4.14e). On the basis of your color coding, predict the direction of flow and the magnitude of the flow rate through each bulb by drawing arrowtails. Predict the relative brightness of each bulb in a given circuit by drawing starbursts. Be sure not to draw arrowtails and starbursts for bulbs that will not light at all. In making predictions, keep in mind that the flow rate and brightness for each bulb is determined by the pressure difference across it. Equal pressure differences cause equal flow rates and brightness for identical bulbs, and a greater or lesser pressure difference causes greater or lesser flow rate and bulb brightness. Figure 4.14a Figure 4.14b Figure 4.14c Figure 4.14d Figure 4.14e After color-coding each diagram, construct the circuits to confirm your predictions. Use a compass to check your predictions about the directions of flow and the relative magnitudes of flow rate. Making good predictions probably means that you have a good grasp of color coding and its relationship to charge flow. Be sure to resolve any differences between your predictions and your observations before you move on. PASCO scientific Student Manual 51

14 SUMMARY EXERCISE 1. Cite two examples of evidence that mobile charge in a circuit can be compressed. 2. What is meant by the term electric pressure? 3. How does a battery establish its pressure difference between the (+) and ( ) terminals? 4. When color-coding, a wire is always a uniform color, and any wires it touches are the same color as well. What is the reasoning for this rule? 5. Using the term pressure difference explain why bulbs light. 6. In a circuit with identical bulbs, how can you use color-coding to predict the brightness of each bulb? PASCO scientific Student Manual 52

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