Experiment Series and Parallel Circuits

INTRODUCTION
Ohm’s law describes the relationship between electric potential, current, and resistance. In this experiment, you will examine how combinations of batteries and resistors affect both the energy and the flow rate of charge in electrical circuits.

OBJECTIVES
In this experiment, you will ? ? ? ? ? ? Measure potential difference and current at various places in series and parallel circuits. Track the energy/unit charge and the current as charge flows through batteries and resistors in series and parallel circuits. Determine the relationship between potential difference and resistance in series circuits. Determine the relationship between current and resistance in the branches of a parallel circuit. Determine expressions for equivalent resistance for both series and parallel circuits. Account for differences in bulb lighting in series and parallel circuits.

MATERIALS
Vernier data-collection interface Logger Pro or LabQuest App Vernier Circuit Board or batteries, mini bulbs and holders, power resistors, and clip leads Vernier Differential Voltage Probe Vernier Current Probe Vernier Power Amplifier or two D-cell batteries

Advanced Physics with Vernier – Beyond Mechanics

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Experiment 7

PROCEDURE
Part 1 Series circuits

1. Connect the Differential Voltage Probe and the Current Probe to the interface and start the data-collection program. Two graphs, potential vs. time and current vs. time, will be displayed. You will need to view only one graph at a time for this experiment. In LabQuest App, choose to view one graph. Make sure the vertical axis label on the remaining graph is potential. 2. The default data-collection rate of 10 Hz is suitable but change the duration to 5 seconds. Zero each probe before the first run. Make sure that you touch the leads of the voltage probe together when you zero it. 3. Set up the series circuit shown in the schematic at right. R1 and R2 are the 10 ? and the 50 ? resistors. The labeled points indicate where you will make your connections with the voltage and current probes. Use the switch, SW1, just below the batteries to complete or open the circuit. Note: For all subsequent readings with the voltage probe, place the black lead of the voltage probe at the starting or reference point and probe with the red lead. In this way, increases in potential are indicated by positive readings; negative readings indicate that potential decreases. Figure 1

4. Determine the increase in potential provided by the battery. To do this, connect the black lead to A and the red lead to B. Start data collection, then close the VAB = switch to complete the circuit. When data collection is complete, open the switch. Store this run. To determine the average value of the change in potential, select a portion of the graph after the switch was closed and choose Statistics from the Analyze menu. Record the mean value of the potential difference for this region of the graph. Be sure to keep track of the signs of these values.

VCD = VEF =

5. In like manner, determine the potential drops at each of the resistors (from C to D and from E to F). Be sure to store each of these runs. Record the voltage changes on the fill-in schematic (Figure 2 on next page) 6. Speculate for a moment about how you might expect the flow rate of charge (current) to compare at the following places in the circuit: before the first resistor (from B to C), between the resistors (from D to E), and after the second resistor (from F to A). Record your predictions here. Should the three currents be the same? If so, why? If not, which will be largest and why?

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Advanced Physics with Vernier – Beyond Mechanics

Series and Parallel Circuits Note: For all subsequent readings with the current probe, it is important that you reroute the current through the probe from red to black (as indicated on the body of the probe itself) and then back to the circuit. Using red and black leads connected to the terminals of the probe may help you keep track of the direction of current.

7. For the next three runs, change the vertical axis label of your graph to Current. To measure the current at each of the three locations described in Step 6, connect the current probe in series, start data collection, then close the switch to complete the circuit. When data collection stops, open the switch, store the run, then move the probe to the next location. Save your file.

IBC = IDE = IFA =

Figure 2

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Experiment 7
Part 2 Parallel circuits

1. Choose New from the File menu and change the data-collection setup so that the parameters match those in Part 1. Zero the probes and choose to view one graph (potential vs. time), as you did before. 2. Set up the parallel circuit as shown in Figure 3. R1 and R2 are the 10 ? and the 50 ? resistors. The labeled points indicate where you will make your connections with the voltage and current probes. You will need to connect two leads at points C and F. 3. As you did in Part 1, determine the increase in potential provided by the battery. Start data collection, then close the switch to complete the circuit. When data collection stops, open the switch. Store this run.

Figure 3

To determine the average value of the change in potential, select a portion of the graph after the switch was closed and choose Statistics from the Analyze menu. Record the mean value of the potential difference for this region of the graph. Be sure to keep track of the signs of these values.

VAB = VCF =

VDE = 4. In like manner, determine the potential drops across each of the resistors. Store each of these runs and record the average potential rises/drops in the table and on the fill-in schematic (Figure 4 on next page)
5. Speculate about how you expect the flow rate of charge (current) to compare before the first resistor (from B to C) and in each of the branches. Should the currents be the same? If so, why? If not, which will be largest and why?

6. For the next three runs, change the vertical axis label of your graph to current. To measure the current at the three locations described in Part 2, Step 5, make the connections, begin data collection, then close the switch to complete the circuit. When data collection stops, store the run, record the average value of the current, and move on to the next location. Record your average current values in the table and on the fill-in schematic on the next page.

IBC = I1 = I2 = IFA =

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Advanced Physics with Vernier – Beyond Mechanics

Series and Parallel Circuits

Figure 4

Advanced Physics with Vernier – Beyond Mechanics

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Experiment 7

EVALUATION OF DATA
Part 1 Series circuits

1.

Considering the units of the volt, write a statement describing the change in the energy of the charge carriers as they move from point A through the battery, then through the two resistors, returning to point A. Keep in mind that the resolution of the voltage probe is ±3 mV. Write an equation that describes this relationship, using Vb, V1, and V2 as variables describing the potential differences.

2. Describe the flow rate of charge at these locations in the circuit. How do these values compare to your predictions?

Part 2 Parallel circuits

1. In what ways are the changes in potential similar to those in the series circuit? How are they different?

2. As you did in Part 1, determine the average value of the current for the final three runs. Write a statement describing the flow rate of charge as it moves from the battery to the junction of the two branches, then through the two resistors, returning to point A. Write an equation that describes this relationship, using IT, I1 and I2 as variables describing the current.