Fundamentals of Circuits I: Current Models, Batteries & Bulbs

Name: Lab Partners: Date: Pre-Lab Assignment: Fundamentals of Circuits I: Current Models, Batteries & Bulbs (Due at the beginning of lab) 1. Explain why in Activity 1.1 the plates will be charged in several different ways. 2. Sketch below one arrangement of the battery, bulb and wire (other than the one in Fig. 3 which you will try in Activity 1.2. 3. At this time, which model for current in Fig. 6 do you favor? Why? PHYS-204: Physics II Laboratory i

Name: Date: Lab Partners: Fundamentals of Circuits I: Current Models, Batteries & Bulbs Objectives To understand the nature of electric currents and why the current is the same at all points in simple circuits. To understand that an electric potential difference (voltage) can cause a flow of electric charge in a conductor. To gain practice in designing and building simple circuits with batteries, light bulbs and switches. To learn to draw circuit diagrams using electrical symbols. To learn to make measurements of current ant voltage using microcomputer-based probes Overview In the labs during the first part of this semester you are going to do a series of activities designed to help you develop a strong conceptual understanding of the nature of electric current and electric potential difference (commonly referred to as Voltage), and the essential features of simple electric circuits. These labs have been adapted from the Real Time Physics Active Learning Laboratories [1]. The goals, guiding principles and procedures of these labs closely parallel the implementations found in the work of those authors [1, 2, 3]. The ideas that you will learn in these labs are the same ones that go into making all the electrical devices that play an important role in our modern world-everything from electric lights and motors to TV sets, computers and photocopy machines. In several of these labs the electrical components we will work with are batteries and light bulbs. A battery is a device that uses chemical energy to do work on the electric charges that flow in it. As it does work it increases the electrical potential energy of these charges, which generates an electric potential difference. The energy added to the electric charge by the battery can be used to operate an electrical device. In today s experiment that device will be a light bulb. But in order to deliver the energy to the light bulb the electric charge must flow through the battery and light bulb. This requires the light bulb and battery to be connected using conductors, materials that allow electric charges to flow in them. In this lab you will consider several models that might explain the flow of charge and predict the measurements that would result if the model is correct. You will then make the measurements to determine which of the models correctly describes the flow of charge, and which models do not. PHYS-204: Physics II Laboratory 1

Investigation 1: Models describing electric current What is electric current? Today, as you can read in any textbook, we believe that current is produced by electric charges in motion. The electric current in a copper wire, for example is produced by the motion of electrons through the wire. Do we know this? If so, how do we know? How can we experimentally distinguish this model of current from other models? These questions were studied and tested by Michael Faraday, a famous early nineteenth century scientist. Faraday examined the effects of electricity from many different sources, including animals such as electric eels, batteries, and electromagnetic induction in addition to the electric charge produced by rubbing objects, and concluded that electricity, whatever may be its source, is identical in its nature. Let s put Michael Faraday s conclusion to a small test. Is the electricity in static electricity identical in nature to the electricity in a battery or power supply? Objects which are rubbed in particular ways are found to attract or repel other objects which have been rubbed in particular ways. These forces between objects are attributed to a property of matter known as charge. Rubbing the objects produces and induced charge on the objects. This induced charge is an example of static electricity. The purpose of the first activity is to compare the current produced the static charges deposited by rubbing materials together. to the current produced by a battery or power supply. You will observe a demonstration using the following materials: Large variable capacitor with two metal plates Electroscope foil covered Styrofoam ball on a string 300 V battery pack or power supply PVC rod and wool cloth acrylic rod and polyester or silk cloth alligator clip leads Activity 1.1: Comparing charge from a power supply and charge produced by rubbing Step 1: The metal plates of a variable capacitor and the conducting ball will be set up as shown in Fig. 1. By charging the metal plates in two ways, we can test whether or not the different charging methods have different effects on the ball dangling between the plates. The two methods we will test are: a. Electrostatic Charging by Rubbing. Stroke one plate with a PVC (gray) rod that has been rubbed with the cat fur or wool. Repeat this several times. Stroke the other plate with the acrylic (clear) rod that has been rubbed with the silk or polyester cloth. PHYS-204: Physics II Laboratory 2

Figure 1: Visualizing Current b. Charging with a Battery or power supply. Connect a wire from the negative terminal of the battery pack to one of the angle irons. At the same time connect a wire from the positive terminal of the battery pack to the other plate. Step 2: With the metal-covered ball removed from between the plates, charge the plates using the charged rubber and glass rods (method a). Measure the effects with an electroscope. Step 3: Adjust the metal plates so that the gap between them is just barely larger that the diameter of the metal covered ball. Again charge the plates using the charged rubber and glass rods. Then carefully lower the ball between the plates. You should see something pretty unusual. Question 1.1 What happened when the ball was placed between the plates? In terms of the attraction and repulsion of different types of charges, explain why this unusual phenomenon happened. Step 4: With the ball removed from between the plates, charge the plates by connecting the power supply (method b). Step 5: Then carefully lower the ball between the plates, and again observe its behavior. Question 1.2 Describe what happened when the power supply was used to charge the plates. What differences (if any) between method (a) and method (b) did you observe in the response of the ball to the charges on the plates? PHYS-204: Physics II Laboratory 3

Question 1.3 Do the charges generated by rubbing and by the power supply cause different effects? If so, describe them. Do the charges generated in these two ways appear to be different? The rate of flow of electric charge is more commonly called electric current. If charge Q flows through a conductor in time t, then the current can be expressed mathematically by the relationship I = Q t. The SI unit of current is called the ampere (A). One ampere represents the flow of one coulomb of charge through a conductor in a time interval of one second. Another common unit of current is the milli-ampere (ma). (1 ampere = 1000 milli-amperes). It is common to refer to current as amps or milliamps. (a) (b) v v Figure 2: Moving Charges Question 1.4 Consider Figs. 2(a) and 2(b). Do both figures show a current to the right? Explain. Activity 1.2: Arrangements that Cause a Bulb to Light In this activity, you can begin to explore electric current by lighting a bulb with a battery. You will need the following: flashlight bulb (#14) flashlight battery (1.5 V D cell) PHYS-204: Physics II Laboratory 4

one wire (6 inches or more in length) Step 1: Use the materials listed above to find some arrangements in which the bulb lights and some in which it does not light. For instance, try the arrangement shown below. Figure 3: A wiring configuration that might make a bulb light with a battery. Question 1.5 Sketch below two different arrangements in which the bulb lights. (Note: there are a total of four including the one pictured above). Question 1.6 Sketch below two arrangements in which the bulb doesn t light. Question 1.7 Describe as fully as possible what conditions are needed if the bulb is to light, and why the bulb fails to light in the arrangements drawn in answer to Question 1.6. Activity 1.3: Other Materials Between the Battery and Bulb In the previous activity you found that a wire, properly connected to a battery and a bulb, allows current to flow. By definition we refer to materials that allow current flow as conductors. By contrast materials which do not allow current flow are called nonconductors. PHYS-204: Physics II Laboratory 5

Lets examine some common materials to see which are conductors and which are nonconductors. We can do that by determining which materials allow the bulb to light. You will need common objects: paper clips, pencils, coins, rubber bands, fingers, paper, keys etc. Connect a single wire, the battery, and a bulb so that the bulb lights. Choose one of the arrangements drawn in your answer to Question 1.5. Then, with the help of your partner, stick in a variety of the common objects available between the battery and the bulb. Question 1.8 List the materials that allow the bulb to light. Question 1.9 List the materials that prevent the bulb from lighting. Question 1.10 What types of materials seem to be conductors? What types seem to be non-conductors? By now you will have discovered that it is difficult to hold your circuits together? From here on, you can make it easier by using a battery holder and a bulb socket. These things, with the addition of a switch are found on the circuit board provided. Activity 1.4: Using a Battery Holder, Bulb Socket and Switch For this activity, in addition to the materials you ve already used, you will need: battery holder (for a D cell) several wires (6 inches or more in length) flashlight bulb socket contact switch Step 1: Examine the bulb socket carefully. Examine what happens when you unscrew the bulb. Note which parts of the bulb make contact with which parts of the socket. Step 2: Examine the bulb closely. PHYS-204: Physics II Laboratory 6

Figure 4: Diagram of wiring inside a light bulb: The dashed lines show wires hidden from view. The black portion is a ceramic insulator, while the grey pieces are a conductors. Question 1.11 Why is the filament of the bulb connected in this way? Question 1.12 Explain which parts of the bulb socket come in contact with which parts of the bulb. Why doesn t the bulb light when it is unscrewed? Prediction 1.1 Consider the circuit shown in Fig. 5. If the switch is open, will the bulb light? Will the bulb light when the switch closed? Explain your predictions. Figure 5: A circuit with a battery, light bulb and switch. Step 3: Wire the circuit shown in Fig. 5 and test it. Use the battery holder, bulb socket, and switch on the circuit board provided. Step 4: Close the switch and leave it closed so that the bulb remains lit for 20 seconds. Release the switch and immediately feel the bulb. PHYS-204: Physics II Laboratory 7

Question 1.13 What did you feel? when there is a current through it? Besides giving off light, what happens to the bulb Question 1.14 What can you say about the path needed by the current to make the bulb glow? Your explanation should be based on the observations you have made so far. Model A Next we explore and contrast some competing models for current in a circuit. Consider a circuit equivalent to the one shown in Fig. 5 with the switch closed. Fig. 6 depicts four different models for current in this circuit. These models are based on models that students often propose. These models are described below. 1 Model B 1 2 2 Model C 1 Model D 1 2 2 Figure 6: Four possible models for current. Model A: Current flows from the positive terminal of the battery through wire #1 and into the bulb. The bulb uses up all of the current and no current flows back to the battery in wire #2. Model B: Currents flow from both terminals of the into the bulb. Model C: The current flows from the positive terminal of the battery into the bulb. The bulb uses up some of the current and a smaller current returns to the negative terminal of the battery. Model D: A Current flows from the positive terminal of the battery into the bulb, and the exact same current returns to the negative terminal of the battery. PHYS-204: Physics II Laboratory 8

Question 1.15 How can you determine which model, if any, correctly describes the current through the bulb? What would you measure, and where would you measure it? Discuss what measurements you might make to eliminate at least three of the four competing models for current. Talk things over with your partners, and explain your reasoning. In Activity 1.5 you will use the computer based laboratory system and two current probes in your circuit to measure current and test the four models above. In addition to the battery, bulb and wires you used above, you will need: Computer with Logger-Pro software Lab Pro interface two current probes The current probe is a device that measures current. Your computer will take measurements from the current probe and display them, as a function of time, on the computer screen. The current proble will allow us to measure the current at different locations and under different conditions in electric circuits. CP Figure 7: Measuring current in a circuit with a battery, bulb and current probe. In order to measure a current, the current must pass through the current probe. For example, to measure the current in the bottom wire of the circuit in Fig. 5, the current probe should be connected as shown in Fig. 7. To connect the current probe at this location we must disconnect the circuit at the point where you want to measure the current (the bottom wire), and insert the current probe. That is, disconnect the circuit, put in the current probe, and reconnect with the probe in place. Keep in mind that the current probe measures both the magnitude and direction of the current. Just like we did for velocities, forces, and other vector quantities in Physics I, we distinguish opposite directions for current flow by positive and negative. A current in through the terminal and out through the - terminal (in the direction of the arrow) will be displayed as a positive current. By contrast, when the current probe measures a negative current, the current is directed into the terminal and out of the terminal of the probe. Take a close look at Fig. 8. If the current in the loop is clockwise both ammeters will measure currents which are positive. PHYS-204: Physics II Laboratory 9

CP1 CP2 Figure 8: Circuit with two current probes, one measuring current from the positive terminal of the battery, the other measuring current into the negative terminal of the battery. Note how the and terminals of the current probes are connected, Prediction 1.2 The table below is a prediction table for each of the four models. Before making any measurements fill in Table 1 to show how the readings of current probe 1 and current probe 2 in the circuit in Fig. 8 would compare with each other for each of the current models described in Fig. 6. Model A Model B Model C Model D Probe CP1 CP2 CP1 CP2 CP1 CP2 CP1 CP2 Current, or 0 CP1>CP2, CP1<CP2 or CP1=CP2 Table 1: Fill in your predictions Activity 1.5: Eliminating Models for Current in a Circuit Step 1: Be sure that current probes are connected plugged into the Lab Pro interface. Step 2: Open the experiment file L1A1-5 (Current Model). The two sets of axes that follow should appear on the screen. The top axes display the current through probe 1 and the bottom the current through probe 2. The amount of current is also displayed on the screen. PHYS-204: Physics II Laboratory 10

Current 2 (A) Current 1(A) Time (s) Step 3: Zero the probes with the switch open. Step 4: Set up the circuit in Fig. 8 with the terminals marked and as shown. Begin graphing, and try closing the switch for a couple of seconds and then opening it for a couple of seconds. Repeat this several times while you are graphing. Sketch your graphs on the axes above. Note: You should observe carefully whether the current through both probes is essentially the same, or if there is a significant difference (more than a few per cent), and whether it is positive or negative. Question 1.16 How could you determine if an observed difference in the currents read by the top and bottom current probes is real or if it is the result of small calibration differences in the current probes? Question 1.17 Did you observe a significant difference in the currents at these two locations in the circuit, or was the current the same? Question 1.18 Based on your observations, which models seems to correctly describe the behavior of the current in your circuit. Explain carefully based on your observations. PHYS-204: Physics II Laboratory 11

Investigation 2: Using electric circuit symbols Representing electric circuits by drawings, like Fig. 5, can be very tedious. It can also be confusing for circuits that have many components. To make it easier to design electric circuits standard symbols have been established to represent the various components that make up an electric circuit. A few of these are shown below. More will be introduced in future experiments. V = 1.5V S Battery Switch R I Bulb Figure 9: Circuit Elements Wire Activity 2.1: Drawing circuits Question 2.1 Draw the circuit represented by Fig. 5 using the symbols above. Question 2.2 On the battery symbol, which line represents the positive terminal-the long one or the short one? (Note: you should try to remember this convention. There are many situations in which it is important to distinguish the positive and negative terminals.) PHYS-204: Physics II Laboratory 12

Investigation 3: Current and Electric Potential Difference Since a battery is a device that has an electric potential difference across its terminals, it is capable of giving energy to charges, which can then flow as a current through a circuit. Exploring the relationship between the potential differences in a circuit and the currents that flow in that circuit is an essential part of developing an understanding of electrical circuits. Since potential differences are measured in volts, a potential difference is informally referred to as a voltage, and usually denoted by the symbol V. Voltage is an informal term for potential difference. If you want to talk to physicists, you should refer to potential difference. Communicating with a sales person at the local Radio Shack store or buying batteries at a hardware store is another story. There you should probably refer to voltage. We will use these two terms interchangeably. You will use voltage probes to measure electric potential difference. The voltage probe has two clips. When the clips at the ends of the wires are connected at two points in a circuit the computer can measure the potential difference. Fig. 10 shows our simple circuit from Fig. 5 with a voltage probe connected to measure the voltage across the battery. Note that the word across describes how the voltage probes are connected. VP1 Figure 10: Voltage probe connected to measure potential difference of the battery Let s measure the potential difference across different elements of this circuit. You have available the following equipment. Computer with Logger-Pro software Lab Pro interface D-cell battery and holder light bulb and socket SPST switch two voltage probes PHYS-204: Physics II Laboratory 13

Activity 3.1: Electric Potential (Voltage) Step 1: Unplug the current probes from the interface, and connect the voltage probes to channels 1 and 2. Step 2: Open the experiment file L1A3-1(Two Voltages) to display the axes that will allow you to compare two voltages. Step 3: Connect the circuit shown in Fig. 9, but do not connect the voltage probes yet. Step 4: Zero the voltage probes, being sure that the clips of the voltage probes are not connected to anything. Step 5: Connect both clips to the negative end of the battery holder in the circuit. Click on the collect button to start graphing. Collect data with the switch open, and with the switch closed. Record the potential difference with the switch open, and with the switch closed. V open = volts V closed = volts Step 6: Move the red clip and connect it at the other end of the wire connecting the negative terminal of the battery to the light bulb. Connected this way the voltage probe will measure the potential difference between the ends of the wire. Step 7: Collect data to determine the potential difference with the switch open, and with the switch closed. V open = volts V closed = volts Question 3.1 What do you conclude about the potential difference when the two leads from the voltage probe are connected to the same point? What do you conclude about the potential difference between the two ends of the same wire? Prediction 3.1 In the circuit in Fig. 11, how would you expect the voltage difference across the battery to compare to the voltage across the bulb with the switch open and with the switch closed? Explain. Step 8: Now test your prediction. Connect voltage probe 1 across the battery, and voltage probe 2 across the light bulb, as in Fig. 11. Step 9: Collect data and make graphs of the two potential differences. Make sure the switch is closed during part of the time that the computer is collecting data, and open during part of the time. PHYS-204: Physics II Laboratory 14

VP1 VP2 Figure 11: Circuit with two voltage probes Step 10: Sketch the graphs on the axes below. On the graph show the time intervals during which the switch was closed and when the switch was open. Voltage 1 (V) Voltage 2 (V) 2 0-2 2 0-2 Time (s) Question 3.2 What do you conclude about the voltage across the battery and the voltage across the bulb when the switch is open and when it is closed? Are the graphs the same? Why or why not? Is this in agreement with your prediction 3.1? What is the effect of closing the switch? Activity 3.2: Electric potential difference and current Now we want to find out about the potential difference across the battery, and the current that flows around the circuit. Step 1: Unplug the voltage probe from channel 2, and plug a current sensor into channel 2. PHYS-204: Physics II Laboratory 15

VP1 CP Figure 12: Measuring both current and potential difference. Step 2: Connect a circuit with the battery, switch and light bulb. Include the current sensor and voltage probe in the circuit connected as in Fig. 12. Step 3: Open experiment file L1A3-2 (Current and Voltage) to display the axes that follow. Voltage 1 (V) 2.0 0-2.0 Current 2 (A) Time (s) Step 4: Collect data to make graphs of the potential difference across the battery and the current through the circuit while you open and close the switch several times. Use the Store Latest Run command in the Data menu to keep your results. Sketch the graphs. Be as accurate as you can to show the magnitudes of the potential difference and the current. Potential difference V Current amps. Question 3.3 What happens to the current through the circuit as the switch is closed and opened? What happens to the potential difference across the battery? PHYS-204: Physics II Laboratory 16

Prediction 3.2 Now suppose you connect a second bulb in the circuit, as in Fig. 12. How do you think the potential difference across the battery will compare to the potential difference with only one light bulb? Will it change significantly? VP1 CP Figure 13: Circuit with two light bulbs Step 5: Connect the circuit as in Fig. 13. Step 6: Again collect data to make graphs of Potential difference and current. Sketch your graphs, using dotted lines to show your results with two bulbs. Represent the magnitudes of the current and potential as accurately as you can. Question 3.4 Was the current through the circuit significantly different with two bulbs than it was with one bulb (more than a few per cent)? Question 3.5 Was the potential difference significantly different with two bulbs than it was with one bulb (more than a few per cent)? Question 3.6 Does the battery appear to be a source of constant current, constant potential difference, or neither when additional elements are placed in the circuit? PHYS-204: Physics II Laboratory 17

These laboratory exercise has been adapted from the references below. References [1] David R. Sokoloff, Priscilla W. Laws, Ronald K. Thornton, and et.al. Real Time Physics, Active Learning Laboratories, Module 3: Electric Circuits. John Wiley & Sons, Inc., New York, NY, 1st edition, 2004. [2] Priscilla W. Laws. Workshop Physics Activity Guide, Module 4: Electricity and Magnetism. John Wiley & Sons, Inc., New York, NY, 1st edition, 2004. [3] Lilian C. McDermott, et.al. Physics by Inquiry, Volumes I & II. John Wiley & Sons, Inc., New York, NY, 1st edition, 1996. PHYS-204: Physics II Laboratory 18