Physics 101 Laboratory 4: Series and Parallel Resistors

Transcription

1 Physics 101 Laboratory 4: Series and Parallel Resistors In this exercise, you will gain more experience with basic DC circuits, wiring both series and parallel circuits. Your equipment for this exercise: One circuit board and cables DC power supply (this time from the benchtop outlets) One digital multimeter Several color-coded resistors Background Information As described in your textbook, and in lecture, resistors can be wired one after another in a series configuration, side by side in a parallel configuration, or with three or more resistors, a combination of the two. Figure 1 shows the most basic parallel arrangement, and Figure 2 shows the most basic series arrangement. The Multimeter Measurements As with the last lab, be careful with the multimeter. If you ask it to measure one quantity while wiring it inappropriately, you can blow a fuse and waste a lot of lab time. To reiterate, make sure both you and your lab partner have closely looked over a given circuit before applying power. Feel free to ask your lab instructor for help at any time. Combining Resistors You will now combine the two resistors in two different configurations. In Figure 1, the two resistors are connected in a parallel configuration, while they are wired in series, in Figure 2. You will predict and then measure quantities in both of these circuits. In the figures below, the black dots show junction locations on your circuit pegboards. The V source in this laboratory will actually be power supplied from the terminals in your bench. Please note: this is not the standard 3-prong 120 V outlet, but rather a 10 Volt outlet, with red and black terminals. R 1 V source R 1 R 2 V source R 2 Figure 1: Resistors in parallel Figure 2: Resistors in series Exp. 4: Resistor Combinations Page 1 of 5

2 Part I: Parallel 1. Choose two resistors with different values. Predict their total resistance together in parallel combination. 2. Using your multimeter as a voltmeter, please measure the power from your bench terminals it should be about 10V. Look at Figure 1, and using the two resistance values, along with what you have learned about parallel resistors, make three predictions: I, the total current leaving the battery, I 1, the current through resistor #1, and I 2, the current through resistor #2. Write down your predictions. 3. Now set up the circuit according to Figure 1, using your two resistors. Don t connect the circuit to the DC voltage outlet until you and your partner have double-checked the circuit. 4. Use your multimeter as an ammeter (I measurement.) Recall from Lab #3 that a current measurement puts the ammeter in series with the branch you wish to measure. Also recall that you should start on the higher current setting, to make sure the current is below 200 ma before switching to the smaller current setting. 5. Now measure these three currents, one after another: I leaving the battery, I 1, the current through resistor #1, and I 2, the current through resistor #2. Note that you will have to move the ammeter in the circuit each time you make a new measurement (see Figure 3). Make sure to disconnect the DC voltage supply (from the outlet in the bench) between each measurement. In general, remove electric power from a circuit each time before you touch the circuit components or move the multimeter. 6. Compare your measured values from step 5 to your calculated values in step Now set your multimeter to Ohms, using the black and red terminals. It now measures resistance directly. 8. Disconnect the power supply from your circuit, but leave the resistors alone. Connect the two probes of the multimeter to the two junctions shown in Figure 1 as the black dots. Record your measurement of the net resistance. 9. Compare your measurement in step 8 to your prediction in step 1. Exp. 4: Resistor Combinations Page 2 of 5

3 Part II: Series 1. Keep the same two resistors. Predict their total resistance together in a series combination. 2. Look at Figure 2, and using the two resistance values, with what you have learned about parallel resistors, make three predictions: I, the total current leaving the battery, V 1, the voltage dropped across resistor #1, and V 2, the dropped across resistor #2. Write down your predictions. 3. Now set up the circuit according to Figure 2, using your two resistors. But do not connect it to the DC Voltage outlet on the bench yet. And again, throughout this exercise, make sure to disconnect the DC voltage outlet (in the bench) each time you want to touch the circuit components or move the multimeter. 4. Set up your multimeter as an ammeter (I measurement) that will measure the I leaving the DC voltage outlet. Again, connect the DC voltage supply last, after you and your lab partner have double-checked your set up. Connect the circuit to the DC voltage and measure the total current I entering your circuit (see Figure 3). Compare this measured value to your calculated value for current in step Disconnect the circuit from the DC Voltage. 6. Now set your multimeter to Volts. Recall from Lab #3 that in voltage mode you must use the multimeter in parallel with a resistor. Again, see Figure 3) 7. Using the same set up of Figure 2, carefully measure the voltage dropped across resistor #1, and then measure the voltage dropped across resistor #2. Compare these values to your predicted values from step Disconnect DC Voltage, and disconnect your multimeter from the circuit, but leave the resistors alone. 9. Now set your multimeter to Ohms, using the black and red terminals. 10. Disconnect the power supply from your circuit, but leave the resistors alone. Connect the two probes of the multimeter to the two junctions shown in Figure 1 as the black dots. Record your measurement of the net resistance. 11. Compare your measurement in step 10 to your prediction in step 1. Exp. 4: Resistor Combinations Page 3 of 5

4 I. I1 I2 I V2 V2 Figure 3. DMM positions for parallel and series resistor configurations Exp. 4: Resistor Combinations Page 4 of 5

5 University of San Francisco Physics 101 Laboratory #4 Answer Sheet(s) Lab section: (Day-Time) Date: (mm/dd/yyyy) Lab Partners: Each member of the lab group should hand in an original lab report. Photocopies will not be accepted. Circle your name in the list at left. Add extra sheets as needed. Please staple pages of report. Exp. 4: Resistor Combinations Page 5 of 5