Ohm, Ohm on the Range – Parallel Circuits
Conceptual Physics
Name:
Hour:
Discussion
When electric current takes a variety of paths in going from one place to another we have a parallel circuit.
What is the total resistance in the circuit when they are connected in parallel? Also, what happens to the
currents and voltages across the resistance? This experiment explores these questions.
Purpose
To determine the relationships among the voltages, currents, and resistances in a parallel circuit.
Procedure

Step 1: With the power switch off, connect a resistor (R1 = 100 ohms) and an ammeter in series with a
voltage source. Connect a voltmeter across the resistor, as in the circuit diagram in the figure. You
may choose to use a breadboard circuit instead of how it is shown in the figure below.
Circuit Diagram
Voltmeter
[set to 20 DCV]
voltage
source
Ammeter
[set to 200m DCA]

Just like in the “Ohm’s Law Lab” turn on the power and measure the current (I) through the resistor and
the voltage (V) across the resistor. (Do not let the voltmeter read above 5 V and keep your current
below 50 mA!) Record your measurements and compute the resistance (R1), using the relation R1 =
V/I.
o Current I =
Amps
o Voltage V =
Volts
o Resistance R1 =

KEEP YOUR POWER SUPPLY
KEPT TO THE SAME LEVEL
THROUGHOUT THIS LAB! Just
turn the switch off when finished with
each part. Keep the rheostat the same.
ohms (compute this)
Step 2: Use the breadboard and place two equal resistors in parallel on the breadboard circuit. Refer to
the picture of a sample breadboard for the setup of the resistors.

Predict: Using your knowledge of Ohm’s law and parallel circuits, how does the current measured by
ammeter A3 compare to the current measured in Step 1? (greater than) (less than) (same)

Predict: Using your knowledge of Ohm’s law and parallel circuits, how does the voltage V1 across the
first resistor compare to the voltage measure in Step 1? (greater than) (less than) (same)

Turn the power on. Measure and record the actual currents and voltages. Turn power off when done.
Compute R1 and R2.
A1 =
A2 =
A3 =
V1 =
V2 =
V3=
R1 =
R2 =

What is the relationship amongst the current readings? (Hint: Do two of them add up to the other one?)

How do the three voltage readings compare?

Compute the total resistance Req of the circuit by dividing the voltage of the source by the current
through the source (A3).
Req =

ohms
How does Req compare with the two values you computed for the individual resistances? Does this
make sense? What formula supports your answer?

Step 3: With the power switch off, add a third resistor of equal resistance in parallel with the first and
second resistors.

Turn on the power, measure and record the currents and voltages.
A1 =
A2 =
A3 =
A4 =
V1 =
V2 =
V3 =
V4 =
R1 =
R2 =
R3 =

Compute R1, R2, and R3 in the table. Also, compute the total resistance Req of the circuit by dividing
the voltage of the source by the current through the source (A4).
Req =

ohms
How does Req compare with the three values you computed for the individual resistances? Does this
make sense? What formula supports your answer?

Step 4: Repeat Step 3 with three resistors of unequal resistance (use 100, 150 and 200 ohm resistors).
Measure and record readings for the ammeter and voltmeter. Compute the values of the resistances R1,
R2, and R3.
A1 =
A2 =
A3 =
A4 =
V1 =
V2 =
V3 =
V4 =
R1 =
R2 =
R3 =

Compute R1, R2, and R3 in the table above. Also, compute the total resistance Req of the circuit by
dividing the voltage of the source by the current through the source (A4).
Req =

ohms
How does Req compare with the three values you computed for the individual resistances? Does this
make sense? What formula supports your answer?
Analysis
1) Describe the relationship among the total current and the individual currents in a parallel circuit.
2) Describe the relationship among the voltages in parallel branches of a circuit.
3) Describe the relationship among the equivalent resistance and the individual resistances in a parallel
circuit. Give the formula, but also explain what happens to Req as you add more resistors in parallel.
4) Why does current increase as you add resistors and pathways in a parallel circuit?