Physics 144 Chowdary How Things Work. Lab #5: Circuits

Transcription

1 Physics 144 Chowdary How Things Work Spring 2006 Name: Partners Name(s): Lab #5: Circuits Introduction In today s lab, we ll learn about simple electric circuits. All electrical and electronic appliances (for example: toasters, computers, radios, telephones, CD players, refrigerators) use electric circuits. In this lab, you will learn how to light a light bulb, build a circuit, see how to change the brightness of light bulbs in a circuit, read schematic circuit diagrams, and measure voltages and currents in a circuit. Much of this lab is introductory; next week in class and lab we ll learn concepts and techniques that will help us to understand much of what we see today. Equipment At your lab bench, you will find several batteries mounted in battery holders. There are several screw-type light bulbs screwed into sockets mounted on black or white blocks. Knife switches are mounted on the yellow blocks. You should also find lots of red and black banana plug wires. There is a small Christmas-tree type light bulb, along with a bulb with smooth sides. There should also be several pieces of plain wire and some tape. There should also be several magnifying glasses. General Warning: In today s lab, you will often make short circuits. These short circuits won t harm you, but they will drain your batteries very quickly and cause certain elements in your circuit to become quite warm. Upon completing a circuit and making any observation, quickly disconnect your batteries from your circuit. Note: During this lab exercise, you will often be asked to make a prediction BEFORE you connect up a circuit or make a measurement. Please make sure you make your prediction and RECORD your best guess BEFORE you make any circuits/measurements. Part I: Lighting a Light Bulb At your lab bench, you should find a little light bulb of the kind that go on the strands of light bulbs used for the winter holidays (usually on Christmas trees, etc.) If you examine this little light bulb closely, (use the magnifying glass at your bench if needed), you ll notice that there are two wires sticking out the bottom/side of the light bulb. The wires go into the bulb, and are connected by another wire, called the filament. The filament is the active element in a light bulb; when energy is delivered to the filament, it gets very hot and begins to emit light. In all incandescent light bulbs, light is emitted by a hot filament (fluorescent light bulbs work on a different principle). You might also notice a second wire parallel to the filament; that s there so that if the filament burns out, the rest of the strand can remain lit up (we ll see how that can be in a little while).

2 Obtain the smooth-sided light bulb (make sure you aren t using the screw-type light bulbs for this part). If you look inside the light bulb, you should see the filament and the two wires attached to the filament (use the magnifying glass if needed). For this light bulb, the wires don t directly protrude outside the bulb as they do for the little bulb, but if you look closely inside the bulb, you should be able to see that one of the wires runs straight through to the bottom, and the other is connected to the smooth metal side. Get a battery, removing it from its holder if necessary. The battery can be considered as the source of energy of electric potential energy; th e light bulb is the object through which the charge travels, resulting in the bulb heating up and glowing. This will occur if (1) enough charge passes through the bulb and (2) the charge has enough energy. Using two pieces of wire, set up the circuits pictured below in Figure 1, consisting of your battery, two wires, and the smooth-sided light bulb. Note that the pictures are not too scale; also, the smooth type bulb is drawn more spherical than it actually is. Use tape to secure the wires if needed. (**note that we encountered difficulties during lab lighting up the Christmas-tree bulbs; however since they are just analogs to the other bulb, it wasn t a problem**) Write underneath each sketch whether the associated circuit resulted in the bulb lighting up. Write DID if the light bulb did light up; write DIDN T if the light bulb did not light up. A B C D E F G H Figure 1: Connecting a battery and light bulb using two wires.

3 Figure 2 shows several arrangements of a light bulb connected to a battery using a single wire. Look at each arrangement, and PREDICT whether or not the light bulb will light up. Below each picture, write down WILL if you predict the bulb will light up, and write down WON T if you predict the bulb will not light up. Don t set up any of the arrangements until after you ve made your predictions. Discuss your predictions with your partners. - A B C D E F G H SHOW your predictions to your instructor. Make sure your instructor initials your predictions before moving on. initials Now, using a single battery, a single wire, and the smooth-sided light bulb, set up each arrangement of Figure 2. Secure the wire with tape where necessary. Next to your prediction, write down your result. Write DID if the bulb did light up, and write down DIDN T if the light bulb did not light up. What features of the circuit configuration are necessary for the light bulb to light up? Describe in a few brief sentences how you would tell someone who hasn t done this lab how they could get the smooth sided light bulb to light up with one battery and one wire.

4 Part II: Complete Circuits, Switches, and Current. Now, we will consider some different circuits containing switches. A switch introduces a controllable break in the circuit. By holding the switch down (or closed), the switch temporarily joins the break and allows charges to move smoothly across the former break. When the switch is up (or open), the switch breaks the path of the moving charge and prevents it from flowing. The behavior of any electric circuit can be understood in terms of electric charges moving through the circuit. It is conventional, but not essential, to consider positive charges flowing from the positive to the negative terminal of the battery. The bulbs will only light up when charges flow through it with enough energy. This is analogous to the flow of water around a closed network of pipes with a pump taking the place of the battery; here each connecting wire corresponds to a very wide pipe that doesn t use up much energy and the bulb corresponds to a resistive paddle wheel that transforms electrical potential energy into heat and light. You will use the notion of circulating charge to predict what happens in the circuits of with circuit elements mounted on blocks. These can be connected with banana plug wires (red and black wires that plug into the mounted connectors). Inspect the underside of the blocks to see the connections. Place the batteries in the battery holders. Make sure each bulb is securely screwed into its socket. The knife switch (on the yellow block) is open when the arm is in the air, and disconnected, and it is closed when the arm is down. Disconnect the battery after you have observed each circuit. In this section, we introduce the idea of a circuit schematic drawing. Next to each figure of how to connect up the battery, light bulb and switch is a schematic drawing. These schematics are convenient ways to indicate how the various elements (here the elements are batteries, banana plug wires, switches, and light bulbs) are connected/arranged in a circuit. Pay attention to the representative figure as well as the schematic diagram and make sure you understand the connection. After this section, we ll only be using schematic diagrams. For each circuit shown, PREDICT and RECORD whether or not the light bulb will light for both positions (open or closed) of the switch. Your choices are OPEN: WILL; OPEN: WON T; CLOSED: WILL; CLOSED: WON T. Record your predictions right next to the figure. After making your predictions, build each circuit and RECORD whether or not the light bulb lit up for both positions of the switch. Your choices are OPEN: DID; OPEN: DIDN T; CLOSED: DID; CLOSED: DIDN T. Record your result right next to the figure, underneath your prediction. socket & bulb knife switch (yellow block) Schematic diagram

5 What features of the circuit configuration are necessary for the light bulb to light up? Describe in a few brief sentences how you would tell someone who hasn t done this lab what is required to get charge to flow through the light bulb to make it light up. Part III: Making the Bulb Light, Batteries, and Energy Previously, we saw how to make a complete circuit and get charge to flow through the bulb. But it s not enough to have charge flow through the bulb; the charge also needs to carry sufficient energy. In our circuit, the charges get energy by passing through the battery. We can think of the battery like a conveyor belt: when positive charge enters the negative end of the battery, it is lifted to the positive end, gaining electric potential energy. The charge then leaves the positive end of the battery and goes through the circuit, where the electric potential energy is transformed (in the case of the light bulb, the electric potential energy is transformed into heat and light). On the following page are various configurations of 2 batteries and 1 light bulb, shown as schematic diagrams. Before making any circuit, PREDICT and RECORD if the bulb WILL/WON T light up; right your prediction right next to the diagram. After making your predictions, build the circuit, and record whether the bulb DID/DIDN T light up for that configuration.

6 What general trends did you notice? What configurations of batteries let the bulb light up? Which configurations did not have the bulb light up? Of the configurations that did light up, which configurations resulted in the bulb lighting up the brightest? Write a few sentences describing your observations.

7 Part IV: More than one light bulb. Consider the circuits shown below. Construct the circuit shown in Figure A below. Which bulb was brigh ter (1 or 2), or where they the same? Now, disconnect bulb 2 (unscrew it from its socket or disconnect a lead). What happens to the brightness of bulb 1? Does this result surprise you? Construct the circuit shown in Figure B below. Make sure all bulbs are screwed in tight. Which bulb was brighter (1 or 2), or where they the same? Now, disconnect bulb 2 (unscrew it from its socket or disconnect a lead). What happens to the brightness of bulb 1? Does this result surprise you? Finally, construct the circuit shown in Figure C below; again make sure all the bulbs are screwed in tight. Rank the bulbs in brightness from brightest to dimmest (note any ties). Repeat, but this time add in a third battery into the circuit. Again, rank the bulbs in brightness from brightest to dimmest (note any ties). Now, disconnect bulb 3 (unscrew it from its socket or disconnect a lead). What happens to the brightness of bulbs 1 and 2? Does this result surprise you? Bulb 1 Bulb 1 Bulb 2 Bulb 1 Bulb 2 Bulb 2 Bulb 3 A B C