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Parallel Circuits Lab
When resistors are connected in parallel, each resistor provides a path for electrons to follow and,
therefore, reduces the equivalent resistance to the current. In Figure 1 (c), three resistors are connected
in parallel across a voltage source. There are three paths by which the current may pass from junction A
to junction B. it follows that more current will flow between these junctions than would be the case if
one or two resistors connected them. Each time a resistance is connected in parallel with other resistors,
the effective resistance decreases. The effective resistance of parallel resistors is found mathematically
by applying the equation: 1
1 1
1
    ...
REquiv R1 R2 R3
It will be necessary for you to follow the circuit diagrams of Figure 1 very closely. Although the
diagrams show several meters in use at once, you have been provided with only one ammeter and one
voltmeter. You must move the meters from position to position until all readings are obtained.
(a)
(b)
(c)
Above, Circuit diagrams for resistors connected in parallel. Be sure to note the positive and negative
terminals of the voltmeter and ammeter in relation to the positive and negative terminals of the voltage
source.
Objective: During this investigation you will apply Ohm’s Law to a parallel circuit.
Don’t forget to actually READ the instructions on the other side!
Materials:
Power supply
Three resistors
6 banana leads
6 Alligator-banana leads
Switch
Voltmeter
Ammeter
Procedure:
A. One Resistor
1. Set up the circuit as indicated in Figure 1 (a).
2. Close the switch and read the meters.
3. Record your observations in Table 1.
B. Two Resistors
1. Set up the circuit as indicated in Figure 1 (b).
2. Close the switch and read the meters.
3. Record your observations in Table 2.
4. Move meters as necessary to obtain all readings.
C. Three Resistors
1. Set up circuit as indicated in Figure 1 (c).
2. Close the switch and read the meters.
3. Record your observations in Table 3.
4. Move meters as necessary to obtain all readings.
If and only if you finish the entire lab with time to spare you may complete the Extension for
Extra Credit on a separate piece of paper.
Extension:
1. A technician needs a 5 Ω resistor and a 10 Ω resistor. The supply cupboard is out of 5 Ω and 10
Ω resistors, but there is an abundance of 15 Ω and 20 Ω resistors. How can the technician use the
higher value resistors to obtain a 5 Ω and 10 Ω resistor?
2. Draw the circuit diagram.
Name:
Date:
Parallel Circuits
Period:
Data and Calculations:
A. One Resistors
1. Printed value of R1
Table 1
Ammeter Reading
Voltmeter Reading
B. Two Resistors
1. Printed value of R1
2. Measured value of
V
R 1   ___________
I
and R2
2. Calculated equivalent resistance:
1

REquiv
1 1
→ REquiv  ______________

R1 R2
Table 2
It
Ammeter Reading
I1
I2
Vt
3. Measured equivalent resistance REquiv 
Voltmeter Reading
V1
V2
Vt
 __________
It
4. Calculated It  I1  I 2  ____________
V
5. Measured R1  1  ________
I1
V
6. Measured R2  2  ________
I2
C. Three Resistors
1. Printed value of R1
2. Calculated equivalent resistance
, R2
1
REquiv
and R3

1 1 1
  → REquiv  ______________
R1 R2 R3
Table 3
It
Ammeter Reading
I1
I2
I3
3. Measured equivalent resistance REquiv 
Vt
Vt
 __________
It
Voltmeter Reading
V1
V2
V3
V1
 ________
I1
V
5. Measured R2  2  ________
I2
V
6. Measured R3  3  ________
I3
4. Measured R1 
7. Calculate the equivalent resistance using the values of R1, R2, and R3 you found above in #4,5
and 6.
Conclusion Questions:
1. How does the current in the branches of a parallel circuit relate to the total current in the circuit?
2. How does the voltage drop across each branch of a parallel circuit relate to the voltage drop
across the entire circuit?
3.
a. What happens to the equivalent resistance of a circuit when additional resistors are added in
parallel with resistors already present?
b. What happens to the equivalent resistance when parallel resistors are removed?
4. Three 30Ω resistors are connected in parallel. Calculate their equivalent resistance showing all
steps.
5. The three 30Ω resistors are connected in parallel are then placed across a 60 V source.
a. Find the total current drawn from the source showing all steps.
b. Find the current through each branch of the circuit showing all steps.
.
c. Find the voltage drop across each branch of the circuit showing all steps.
.