Name: ______________________________________ Period: ____
Teacher: ____________________________________ Date: ______
Measuring Voltage and Current in a Parallel Circuit
Purpose
In this laboratory exercise, we will construct a parallel circuit containing three light bulbs. We will also connect an ammeter and a voltmeter at various positions in the circuit in order to determine the voltage drop and current passing through each light bulb. Finally, we will use “Ohm’s Law” to determine the resistance values of all three light bulbs and the power output of each.
Introduction
A parallel circuit is a closed circuit in which the current divides into two or more paths before
recombining to complete the circuit. Since a parallel circuit consists of resistors (light bulbs, motors, etc.) connected individually across a common voltage source (battery), the voltage across any element (resistor, light bulb, etc.) is the same. Normal household wiring is connected in a parallel arrangement. Break the circuit in any branch and electricity will continue to flow in all the other branches of the circuit. The voltage in a parallel circuit can be expressed by the relationship:
VT = V1 = V2 = V3 = … Vn (Equation #1)
Where:
VT = the total voltage in the circuit
V1 = the voltage drop across resistor R1
V2 = the voltage drop across resistor R2
V3 = the voltage drop across resistor R3
The total current in the circuit is the sum of all the currents flowing through each branch of the circuit and is given by the relationship:
IT = I1 + I2 + I3 + … In (Equation #2)
Where:
IT = the total current flowing in the circuit.
I1 = the current passing through resistor R1
I2 = the current passing through resistor R2
I3 = the current passing through resistor R3
Finally, the total resistance of the circuit is the sum of all the individual resistances of each resistor and is given by the relationship:
1 𝑅𝑇
=
1 𝑅1
+
1 𝑅2
+
1
𝑅3
+ …
1
𝑅𝑛 (Equation #3)
Where:
RT = the total (equivalent) resistance of the circuit
R1 = the resistance of resistor R1
R2 = the resistance of resistor R2
R3 = the resistance of resistor R3
Equipment
(a) Bulb circuit board with three light bulbs (d) Ammeter
(b) Color-coded connecting cables with alligator clips (e) 12 volt power supply (c) Voltmeter
Procedure
Part A - Three Bulb Circui)
1. Make sure your Lab’s DC power supply is plugged in and the DC power supply dial is pointed at setting 3. Do not change this setting.
2. The leads (wires) you will be using today each contain specific ends for connecting with other electrical equipment. They may contain banana plugs (Figure 3) or alligator clips (Figure 4).
Note: Steps 3-5 may already be set-up for you at your station.
3. Make sure the tabs on your bulb board are oriented in the same positions as in Figure 5. Attach the black lead’s alligator clip from the DC power supply to Tab F
Connect black
alligator clip to Tab F here
Figure 2
Figure 3
Banana plugs
Figure 4
Alligator clips
4. Locate the red lead from the DC power supply. Connect three alligator clip wires to the banana plug on the other end. Connect each of the other ends of the three alligator clip wires to Tabs A, C, and E on the left side of the light bulb circuit board (See Figure 6).
5. All three light bulbs should now be connected in parallel circuit with the DC power supply and be lit. If all three bulbs are not lit, inform your teacher. 6. Measuring total voltage with the Vernier voltage
probe
a) Plug the voltage probe into the LabQuest 2. Join the alligator clips at the ends of the voltage probe together and press Sensors→Zero→Voltage to zero the probe.
b) Touch the ends of the leads from the
voltage probe to the corresponding colored ends of the leads from the DC power supply (black to black and red to red). This is now the total voltage
supplied to the circuit. Round off the voltage reading to 2 significant figures and record as VT in the Data Table 1.
7. Measuring individual voltages across each light bulb
a) Touch the red and black alligator clips of the voltage probe to the two metal strips on either side of Light Bulb 1. If you get a negative voltage, reverse the voltage probe alligator clips to get a positive voltage. Round off the voltage reading on the LabQuest 2 to 2 significant figures. This is the voltage across Light Bulb 1 (V1).
Record this measurement in Data Table 1.
b) Repeat the above procedure for Light Bulb #2 and Light Bulb #3 and record each voltage reading in Data Table 1.
8. Measuring current across each light bulb
In order to properly measure current, the ammeter must be connected in series.
a) Plug a red lead with two banana plug ends into the positive input hole in the ammeter (right most hole). This may already be in place.
b) Plug a black lead with two banana plug ends into the center hole of the ammeter. This may already be in place. The ammeter is now set up to measure up to 1 amp (the bottom scale of the ammeter).
c) Unclip the alligator clip from Tab A and attach it to the end of the red lead from the ammeter. Touch the black lead from ammeter to Tab A. (See Figure 7). The ammeter is now measuring the current of Light Bulb #1. Record the reading as I1 in
Data Table 1.
d) Reconnect the alligator clip from Light Bulb #1 to Tab A. Repeat procedure 8c for the clips attached to Tab C and Tab E. Record the current as I2 and I3 in Data Table 1.
9. Measuring total current of the circuit In order to measure total current, all three light bulbs must be connected in series with the ammeter.
a) Unplug the red lead from the DC power supply and “stack” it into the back of the black lead from the ammeter (See Figure 8).
b) Plug the red lead from the ammeter into the (+) input of the DC power supply. The ammeter is now connected in series with all three bulbs and is reading the total current of the parallel circuit. Record this measurement as IT in Data Table #1.
Part B – Two Bulb Circuit
10. Disconnect the first light bulb from the circuit by removing the alligator clip from Tab A. What remains is two bulbs connected in a parallel circuit. What happens to the other light bulbs? Record your observation in the Data Table 1.
11. The ammeter is already measuring the total current. Record this measurement as IT in
Data Table 2 – Two Bulb Parallel Circuit.
12. Return the wiring to measure the current across each light bulb by disconnecting the red probe from the DC power supply and “unstacking” the red and black probes.
13. Repeat step 8c to measure the current across each of the two remaining light bulbs. Record the currents as I4 and I5 on Data Table 2.
13. Measure the total voltage of the two bulb parallel circuit and the voltage across each bulb with the voltage probe and LabQuest2. Record the measurements on Data Table 2.
Figure 7
Data Table 1 – Three Bulb Parallel Circuit
Voltage (Volts)
from voltmeter
VT V1 V2 V3
Current (Amperes)
from ammeter
IT I1 I2 I3
Observation when one bulb is disconnected
Data Table 2 – Two Bulb Parallel Circuit
Voltage (Volts)
from voltmeter
VT V1 V2
Current (Amperes)
from ammeter
IT I1 I2
Post-lab Questions
1. Is there any change in brightness as the parallel circuit was changed from three bulbs to two bulbs? If so, how did the brightness change?
________________________________________________________________________ 2. Compare the voltage across each bulb in Data Tables 1 and 2. Did the voltage across
each bulb change as the number of bulbs decreased?
________________________________________________________________________ 3. How are the voltages across each bulb (V1, V2, etc.) related to the total applied voltage
(VT) in each circuit (within an acceptable amount of error)?
________________________________________________________________________ 4. How are the currents across each bulb (I1, I2, etc.) related to the total applied current (IT)
in each circuit (within an acceptable amount of error)?
________________________________________________________________________ 5. How does the total current (amps) supplied by the power supply change when one bulb
was removed? In other words, how does IT in the two bulb circuit compare to IT in the
