Physics E-1b Expt. 2: DC Circuits Spring 2006 Introduction Preparation: Before coming to lab, read the lab handout and Giancoli Chapter 19, sections 19-1 through 19-3 plus section 19-8. Then prepare your pre-lab. Preparing a thorough pre-lab is especially important for your success with this experiment so your TF will check your pre-lab at the beginning of the lab section. The pre-lab must include a sketch of each circuit and preliminary answer to the bold questions. Be sure to leave plenty of extra space between answers. In the lab you will use this space for your observations as you build and test each circuit, and for revisions or final corrections to the bold questions if needed. Note questions 21, 26, 32, 34, and 35 cannot be answered until you take data in the lab. When you finish the lab, hand in one document that contains your pre-lab, your observations and your answers to all the bold questions. All tasks to be performed in the lab appear in italics throughout this handout. Be sure to bring writing paper, graph paper, a ruler, a calculator and your copy of the Lab Companion to the lab. Objective: In part I, series and parallel circuits will be studied using light bulbs. In Part II, Kirchhoff’s Rules will be verified using carbon resistors. Finally, ohmic and nonohmic devices will be investigated with a variable voltage divider. Theory If two circuit elements are in series, the same current must flow through them. If two circuit elements are in parallel, the voltage across each element is the same. At any junction, the current that flows in will equal the current that flows out. For any loop in a circuit, the sum of the voltage changes around the loop is zero. The voltage change across a resistor is negative if the change is in the direction of the current flow (a voltage drop). The voltage change across a resistor is positive if the change is in a direction opposite to the current flow (a voltage rise). An Ohmic device is one which obeys Ohm’s Law, V = IR, where R is constant. A non-Ohmic device is one that does not have a constant resistance. A light bulb is a simple example; the filament undergoes huge changes in temperature when current passes through it. Therefore, the resistance of the filament is not constant, rather, it increases with increased current.2 Circuit 1C+Part I: Qualitative Measurements with Light Bulbs Circuit 1. The Single Bulb Circuit In the following qualitative experiments, we will assume that if more current flows through a light bulb, it glows brighter. In Part II, circuit 8 we will test this assumption. Examine any standard light bulb made with clear glass. Suitable light bulbs are available in the help room for prelab inspection if needed. 1→ How many conductors are on the outside of the bulb? How many insulators are in the bulb? 2→ How many wires connect to the filament? Do they go to the same place? 3→ Draw a sketch of your bulb clearly showing which parts of the bulb are conductors and which parts are insulators. 4→ Examine the light bulb socket: How does it work? Draw a sketch of the socket. Now build circuit 1. The bulb should light when the switch is closed. The brightness of this bulb will be used as a standard of comparison in subsequent circuits. → Caution: Be careful not to short-circuit the battery! Never, directly connect the battery terminals to each other through a wire. Also in order to avoid draining your battery; please open the switch as soon as measurements and observations have been made! 5→Will the same result be achieved if the direction of the current is reversed? Explain. Remember that “conventional” current flows from high to low potential (except in the battery!). Switch the wires at the battery. Can you observe any difference? Leave Circuit 1 hooked up. You will need it for comparison below.3 Circuit 2. The Two-Bulb Series Circuit Now let’s consider a more complicated circuit — the series circuit. Refer to the diagrams of circuits 1 and 2 for the following discussion. First, consider circuit 2. Bulbs A and B are in series. The bulbs and the battery form a single loop. In such a circuit, the current is the same through each circuit element. 6→Given that the bulbs and batteries in both circuits 1 and 2 are identical, rank what you would expect the order of brightness of the bulbs to be when the switch is closed. That is, relate the brightness of A, B, and C with >, <, or = signs. Explain. 7→What happens if you unscrew one of the bulbs in circuit 2? What would happen if you short out one of the bulbs? How does the brightness of the unshorted bulb compare to that of the single bulb C in circuit 1? 8→What would you predict for the brightness of the bulbs if a third bulb were added in series with A and B? Explain your reasoning. 9→For a given battery, does the same amount of current always flow or does it depend on the particular circuit connected to it? Explain your reasoning. Build circuit 2 and check your answers. If any answer differs from your pre-lab please discuss it. Circuit 3. The Two Bulb Parallel Circuit A second way two bulbs can be connected in a circuit is as shown in circuit 3. Bulbs D and E are connected in parallel. Again, let’s compare this new circuit to circuit 1. 10→Rank the brightness of bulbs C, D and E using >, <, or = signs when the switch is closed. Explain your reasoning. 11→Would removing one bulb change the brightness of the other bulb? Explain.4 12→Will the total current through the battery increase, decrease, or stay the same when you unscrew one bulb? Explain. Build circuit 3 and check your answers. If any answer differs from your pre-lab please discuss it. 13→Suppose we were to increase the number of bulbs to three. What would happen to the brightness of each bulb? Explain. Add a third bulb to Circuit 3 to check your answer. . If your answer differs from your pre-lab please discuss it. 14→Would you say your battery is a constant voltage source or a constant current source? Explain. Circuit 4. The Three Bulb Series Parallel Circuit Now you can combine the knowledge gained in the one and two bulbs circuits. Consider circuit 4. With the switch 2 open and switch 1 closed, this circuit is