Lab 5: Simple Electrical Circuits Introduction: In this laboratory you will explore simple DC (direct current) electrical circuits. The primary goal of the lab will be to develop a model for electricity. A model is a clear mental picture that helps to explain how a physical system behaves and allows you to correctly predict what will happen in new situations. Note that a model is not a complete theory. A theory is a complete explanation in terms of known physical laws and mathematical equations. A model is the conceptual basis on which the complete theory is constructed. The method we will use to help you develop your model is a three-step procedure: predict/test/evaluate. You probably already have some concept, some idea of what electricity is and how it behaves. It may be right or wrong. Whatever it is, it is your initial model. You will be asked to predict what you think will happen in a given electrical circuit, based on this model. You will then test your prediction by making measurements on a real circuit. This, essentially, is how all good science is done. We make predictions based on what we think is a correct explanation of a system, then experiment to verify or refute our prediction. If the prediction was correct, that says the model is at least partially right. If the prediction was wrong, that says the model is either wrong or incomplete, and must be revised. The goal, of course, is to develop a model that always results in correct predictions. When we have done that, we can then say with some justification that we understand the system. So don t worry if your predictions are wrong at first. You will not lose points for incorrect predictions. In fact, correcting an erroneous model often results in more effective learning than simply verifying a correct one. So use this opportunity to make some mistakes and to learn from them. 1
1. A Simple Circuit a. Making a Bulb Light Your first objective is to make a bulb glow using a power supply, wires, and a bulb in a socket. A power supply is much like a battery, it is a source of Electromotive Force (EMF). EMF is the term we use to refer to the potential difference (or voltage ) that a battery develops across its terminals. For example, an ordinary flashlight battery (D-cell) has an EMF of 1.5 Volts. Procedure: Turn on the power supply, move the STBY-ON switch to the ON position, make sure the meter range switch is in the down (0-25V) position, and set the power supply to 5 Volts. You can now use the STBY-ON switch to turn the supply on or off without having to reset the voltage each time. Check that the current range control is set to 2A and the small center knob is turned fully clockwise. The power supply is now a source of 5 Volts of EMF. You are now ready to try and light the lightbulb. Connect a red wire to the positive terminal of the power supply and a black wire to the negative terminal and turn on the power supply by moving the STBY-ON switch to the ON position. Now experiment and see what it takes to light the bulb. Don t worry if the current limiter happens to turn off the power supply. Just reset it by pushing the red button near the current limiter knob, and resume your experimenting. When you have gotten the bulb to light, draw a picture on the worksheet of the circuit you used. Be sure to clearly indicate what wires were connected where. As a precaution against possible damage to the apparatus, always move the STBY-ON switch to the STBY position after you are finished experimenting. Further Investigation: Hopefully you found that in order to light the bulb you needed to create a closed path for the current to travel around. But what about the details? In particular, how is the filament (the part of the bulb that actually glows) connected to the external wires from the power supply? Procedure: Use the magnifying glass to inspect the bulb. Identify the filament and see how it is connected to the base of the bulb. Now unscrew the bulb from the socket. Turn the power supply on and try to make the bulb light by touching the connecting wires directly to the bulb itself. See just where on the bulb you have to make the connections in order to get the bulb to light. Answer the worksheet question about how the bulb works. 2
b. Measuring Current To light the bulb, you needed a complete circuit in which current can flow. But how does the current behave as it travels around the circuit? Is the current the same in the wire leading from the positive terminal of the power supply to the bulb as it is in the wire leading from the bulb to the negative terminal, or does the bulb somehow use up current? Let s apply our predict/test/evaluate method and find out. On the worksheet, write your prediction of what happens to the current in the circuit you have just been working with. Screw the bulb back into the socket and construct the circuit shown in Figure 1 below. Connect the multimeter as shown in the figure in order to measure the current flowing in the wire which leads from the positive terminal of the power supply to the bulb. Turn on the power supply and record the reading on your multimeter. Now turn the power supply off, disconnect the multimeter, and construct the circuit shown in Figure 2. below. Turn on the power supply and again record the reading on your multimeter. 3
Now evaluate your results. Were the two readings the same, or did they differ significantly? Note here that some slight variation in the last one or two digits of the meter reading is to be expected. No two measurements, even of the same quantity and made with the same instrument, can be exactly the same. There is always some uncertainty in any measurement. What you are looking for is significant differences (say greater than five or ten percent) between the two readings. If no significant differences are noted, you can conclude that the two measurements in fact agree. Record your evaluation on the worksheet. Was your prediction correct? If not, how do you have to modify your model in order to account for this new information? Use the space provided on the worksheet to explain what modifications (if any) are necessary. 2. A Slightly More Complicated Circuit Now consider the circuit shown in Figure 3. below. Such a circuit is called a series circuit because the two bulbs are connected one after the other rather than side by side. How will this circuit behave? In particular (assuming the power supply is still set to 5 Volts) will the current flowing in this circuit be the same as in the one-bulb circuit, or not? Will the first bulb somehow use up some of the current so the second bulb gets less, or will the current through both bulbs be the same? Make your prediction on the worksheet. Now, making sure that the power supply is still set at 5 Volts, turn on the power supply and observe the behavior of this circuit. Record what you see. Now, using the same measurement technique you used in part 1, measure and record the current in all three wires of your series circuit. Now evaluate your results. Was your prediction correct? If not, what additional modifications in your model are necessary. Explain in the space provided on the worksheet. 4
3. Potential Differences in a Series Circuit Remove the multimeter and reconstruct your original series circuit, as shown in Figure 3. You should now have an accurate model of how the current in a series circuit behaves, but what about potential difference? The power supply develops a potential difference of 5.0 Volts across its terminals. In a circuit consisting of only a single bulb, this potential difference is obviously also developed across the terminals of the bulb. But what about in a circuit consisting of two or more bulbs in series? On the worksheet, predict, based on your model, how you expect the potential differences across the two bulbs to behave. Will the potential difference across each bulb be 5 Volts (since that s what the power supply is set at), or will something else happen? Now use your multimeter to measure and record the potential differences across the two bulbs. Also record, for comparison, the potential difference across the terminals of the power supply. Note that you may wish to measure this with the multimeter rather than simply relying on the power supply s voltmeter (which may not be as accurate). Now evaluate your results. Was your prediction correct or not. Explain in the space provided on the worksheet. 4. Resistance Resistance is defined as the ratio of the potential difference across the terminals of an electrical device to the current flowing through it. In equation form: R' V i Since a certain current flows through a bulb when a certain potential difference exists across its terminals, the resistance of the bulb can be easily determined using the above equation. For example, you probably found, for a single bulb, that a potential difference of 5 Volts resulted in a current of about 0.14 Amperes. So the resistance of the bulb, under these conditions, is about 36 Ω (ohms). But is the resistance of the bulb always this value? To answer that question, more data is needed. If current through a device is measured for several different values of potential difference, and the results graphed, we can learn more about the behavior of bulbs and other kinds of resistive devices. Figure 4, on the next page, shows several possible graphs of potential difference versus current through an electrical device. 5
If the graph is a straight line, then the resistance of the device is the same regardless of how much current is flowing through it. Such a device is said to be ohmic. If the graph is not a straight line, then the resistance of the device depends on operating conditions and the device is referred to as non-ohmic. Lets investigate the behavior of two kinds of resistive devices, the bulb, and a resistor designed for use in electronic circuits. We ll use the computer to make simultaneous measurements of current and voltage and create graphs of our data for each of these devices. The computer can measure current and voltage by means of two devices called, respectively, a current probe and a voltage probe. These connect to a dual channel amplifier which interfaces them to the computer. Note: The current probe and the dual channel amplifier are easily damaged by either excessive current or excessive voltage, so great care must be taken in their use. Please be sure to observe all the operating precautions described below. 6
Set the power supply to 0 Volts (but do not turn it off completely), make sure the meter range switch is in the 0-25V position, place the STBY-ON switch to DC ON, reset the current range control to 0.5 A, and make sure the small inner knob is fully clockwise. Now assemble the circuit shown in figure 5. Make sure the current probe is connected with the correct polarity. The voltage probe is just a pair of clip-leads connected to the dual channel amplifier. Connect them across the terminals of the bulb just as you would any voltmeter. You are now ready to investigate the resistive characteristics of the bulb. Procedure: Start the program Logger Pro. Click on File and open the folder Current and Voltage Probes. In it, find and open the file Simplecircuits1. You should see a graph of Voltage versus Current displayed. If this does not happen, consult your instructor. After making sure the power supply is at 0 Volts, but turned on, press the Collect button on the computer display. Then press the Keep button. This will put the first data point on your graph. Now increase power supply voltage to 1 Volt. Again press Keep. Increase to 2 Volts. Press Keep. Continue this process, putting data points on your graph, up to and including a supply voltage of 10 Volts. Then press Stop. Immediately return the power supply to 0 Volts and move the STBY-ON switch to STBY. Look at your graph. Which of the graphs in Figure 4 does it most resemble? Use the tangent button on the computer display to find the slope of your graph for the first, fifth and last data points on your graph, and record these values on your worksheet. Print a copy of your graph. Now connect the voltage probe leads across the terminals of the resistor, and repeat the above procedure. Remember to go no higher than 10 Volts on the power supply, and turn it off as soon as you are finished. 7
Now disassemble your circuit and use your multimeter to directly measure the resistances of the bulb and the resistor. Record your values on the worksheet. What do you conclude about the behavior of the bulb and the resistor? Is either of them ohmic? If so, which one? Or are both ohmic? If either one is not, how does its resistance depend on the amount of current flowing through it? Conclusion: You have now had some direct experience with electrical circuits. Perhaps you have found that what you already knew about electricity was correct, or perhaps you now know something you didn t know before. In either case, use the space provided on the worksheet to summarize your model of the behavior of electric circuits. If your initial model was correct, describe how the experiments you conducted verified it. If you had to revise your model, describe what your misconceptions were and what new understanding you have replaced them with. 8
Simple Electric Circuits Worksheet Name Name 1a. A simple circuit Draw the circuit you used to light the bulb. Draw an accurate sketch of the bulb showing clearly where on the bulb you had to touch the power supply wires in order to make the bulb glow. Clearly indicate how these points are connected to the filament. 9
1b. Measuring Current Prediction: Measurements: Current in wire from positive terminal to bulb: Current in wire from bulb to negative terminal: Evaluation: 10
2. A Slightly More Complicated Circuit Prediction: Observations: Measurements: Current in first wire: Current in second wire: Current in third wire: Evaluation: 11
3. Potential Differences in a Series Circuit Prediction: Measurements: Bulb One Potential Difference: Bulb Two Potential Difference: Power Supply EMF: Evaluation: 12
4. Resistance Resistance values: Bulb: First data point: Fifth data point: Last data point: Resistor: First data point: Fifth data point: Last data point: Direct resistance measurements (with multimeter) Bulb: Resistor: What are your conclusions about how the bulb and resistor behave? Record them here. 13
Conclusion: Summarize your model here: 14