SERIES AND PARALLEL Circuits – I
UM Physics Demo Lab 07/2013
Pre-Lab Question
What is the difference between voltage and current? Use the flow of water in a pipe as an
analogy.
EXPLORATION
Materials
1 green multimeter (with leads)
1 battery board
1 alligator lead card
1. Build the circuit shown in the schematic below. The numbers indicate contact points
where you will use the multimeter probes to measure voltage. Build the 6V battery from
1.5V cells.
Figure 1: A Simple Circuit
Measure the voltage across the battery. Although the cells are labeled 1.5V, the actual cell
voltage may vary. The actual voltage can be greater than 1.5V for brand new cells, and less
than 1.5V for cells that have been used. It is alright if the voltage is not exactly 6V for the
assembled battery. To measure the total battery voltage, be sure to measure from bottomto-top for the combined cells.
Actual Battery Voltage:
Number of Cells: _______________
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2.

First, predict the voltages (V) you will measure between the labeled points on the
circuit. Close the switch, and measure the voltages. Place the probes at contact
points shown on the circuit diagram above and record below.

Second, measure the current at each of the three points. To measure, you interrupt
the circuit with the multimeter which means you disconnect the circuit at a point and
use the multimeter to complete the circuit. Measure the current at the test points
with the meter set to milliamperes (mA) and record below.
Contact
Points
Predicted
Measured
Explanation of Prediction and Measurement
V1-2
V2-3
V1-3
I1
I2
I3
X
X
X
Explain your observations. How does the voltage across the light bulb compare to the
battery voltage? Does current behave differently than voltage in a circuit and how?
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Series Circuits
3. Build the circuit drawn in the schematic below.

Predict the voltage for each pair of test
points. Measure the voltages and record

Predict whether the measured currents will
greater, smaller or the same as you
measured for a single bulb in the simple
of Figure 1. Measure the current at each
and record.
Figure 2: A Series Circuit
Contact
Points
Predicted
Measured
below.
be
circuit
point
Explanation of Prediction
V1-2
V2-3
V1-3
I1
I2
I3
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Explain your observations. How do light bulbs in series share voltage? How does this
compare to the voltage of the battery? Why are they the same or different? How does the
current compare to that of the single-bulb circuit and why is it the same or different?
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Parallel Circuits
4. Build the circuit shown below.
Figure 3: A Parallel Circuit
5.

Predict what the voltage will be across each light bulb when you close the switch.
How does this compare with the total potential of the battery? Discuss with your
group. After recording your prediction, close the switch and measure the voltages
across the two light bulbs. Explain what you observe.

Based on the current you measured for a single light bulb in the simple circuit of
Figure 1, predict the current for each bulb and for the whole circuit (test points a,
b, and c). Discuss with your group and explain your prediction. Will current behave
the same as with two bulbs in series? Measure the current at points a, b and c and
compare with your prediction.
Contact
Points
Predicted
Measured
Explanation of Prediction
V1-4
V2-3
Ia
Ib
Ic
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Explain your observations. How do voltage and current behave in a parallel circuit
compared to a series circuit?
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Power
6. Fill in the table below with the values for voltage and current you measured for a series
circuit of two bulbs at 6V (Figure 2) and a parallel circuit of two bulbs at 6V (Figure 3)
Calculate and compare the power consumed by each of the bulbs.
Recall from the circuits lab:
Bulb
P  I V
Current
(Power = Current x Voltage)
Voltage
Power
1 of 2 in series
2 of 2 in series
1 of 2 in parallel
2 of 2 in parallel
Challenge Work:
1. Describe a fast way to check if a light bulb circuit is series or parallel, without tracing the
circuit.
2. (Series) Using the series circuit, remove one battery from the circuit by re-wiring. How
did this change the brightness of the bulbs? How does the voltage of the batteries affect the
brightness of the bulbs?
3. (Parallel) Why do you think most Christmas tree lights are wired in parallel?
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4. Which circuit, Figure 2 or Figure 3, consumes more power from the battery? Justify your
claim by calculating the power consumed by the circuit in each case.
Summary:
1. The total voltage available to a circuit is determined by the number of cells in series
for the source battery.
2. The electric force in a DC circuit is conservative, like gravity.
3. The sum of the component voltages in a series circuit is equal to the voltage of the
source (Kirchhoff’s Loop Rule). The Loop Rule results from the conservative
nature of the electric force and is equivalent to the statement that the electric force
does no net work over a closed loop path, exactly the same as for gravity.
Equivalently, the sum of the voltages around any closed loop in a circuit must be
zero.
4. The current is the same throughout each element of a series circuit.
5. The current is inversely proportional to the number of resistors (bulbs) in the circuit
(a consequence of Ohm’ Law).
6. The current divides in a parallel circuit, but the total current must be conserved
(continuity, charge conservation and the Junction Rule for electric current).
7. The voltage across each component in a parallel circuit is the same and equal to
the source voltage. This implies that the voltage across each element in a series
circuit is less than that for a parallel circuit for the same source potential (battery
voltage).
8. Power/brightness is the product of the voltage across a bulb and the current
flowing through it.
Final Clean-up
Please disconnect all alligator leads and reattach them to the clip card. Replace all
equipment to the carts.
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Voltage and Current
Current and voltage are the two quantities used to describe the behavior of electricity in a
circuit.
Voltage
The total electric potential (“voltage”) across a circuit cannot exceed the voltage of the
power source (battery). The power source provides stored electrical energy ready to power
a circuit. Voltage, like pressure in a fluid, is applied across two points in a circuit. Voltage
does not “flow through a circuit”.
The voltage across a single component depends on three factors: the component itself (the
electrical resistance of the component), the resistance of all the other components in the
circuit, and the total voltage of the power source.
When you measure the voltage across a light bulb (a device with resistance) the voltage is
high. When you measure the voltage across a length of wire (very low resistance), the
voltage is very low. Ohm’s Law relates current, voltage and resistance.
Current
Current is the flow of electric charge from one point to another within a circuit.
Current depends on the electric potential of the power source and the challenge the
circuit presents to the flow of current known as electrical resistance. The current is the
same at any point in a series circuit. The current is not the same everywhere in a parallel
circuit; it divides between the parallel paths.
Current is a conserved quantity. Thus, in a parallel circuit the total current at the source
(the positive pole of the battery) is equal to the sum of the current flowing in all the parallel
branches:
I total  I1  I 2  I 3 
This of course must also be the total current returned to the negative pole of the battery.
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