Parallel Circuits
What happens when there is more than one path for the current to take?
Materials
For each group
2 batteries
3 light bulbs and sockets
wires
LED
Discussion: We will begin class by discussing the questions from last time:
> In this circuit there are more light bulbs lit when the switch is open, but the light bulbs
are brighter when it is closed. For which case is the power supplied by the battery
greater?
> When all the generators are running at Hoover Dam, it produces 2,000,000,000 Watts.
This power is then sent elsewhere by power lines. The diagram at right is a simplified
version of the situation, where the battery represents the generators
and the light bulb represents the city that uses the power. If the
“battery” is driving the circuit at 120 V (household voltage), what is
the current in the power line?
They use transformers to change the voltage, so that the Hoover Dam power station acts
like a 100,000 Volt battery. Now what is the current in the power lines?
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Definition: A parallel circuit contains “parallel” branches, which share the total current
(they don’t need to actually point in parallel directions). In the circuit at far right, the
current from the battery divides, so that some goes through one light bulb and some
through the other. Then the light bulbs are in parallel with each other.
1. Assemble, one at a time, these three circuits, using two batteries in series.
Compare and contrast the performance of each one, as evidenced by the light bulbs'
behaviors.
In the parallel and series circuits what does one light bulb do when you unscrew the
other?
2. Reassemble the series circuit. Using the appropriate multimeter, measure the current
through points A, B, and C as shown.
A:
B:
C:
What is the relation between the measured currents?
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3. Using the appropriate multimeter, measure the 3 voltages labelled below in the series
circuit:
total voltage across the batteries:
voltage across the 1st light bulb:
voltage across the 2nd light bulb:
What is the relation between the measured voltages?
4. Reassemble the parallel circuit. Using the appropriate multimeter, measure the current
through points A, B, and C as shown.
A:
B:
C:
How are these currents related?
What happens to the current at A when you unscrew light bulb C?
5. Without actually measuring, can you use your results from Q4 to tell what the electric
current is at points D, E, and F?
D:
E:
F:
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6. Using the appropriate multimeter, measure the 3 voltages labelled below in the
parallel circuit:
total voltage across the batteries:
voltage across the 1st light bulb:
voltage across the 2nd light bulb:
What is the relation between the measured voltages?
7. Sketch a siphon system that is analogous to your parallel circuit.
If water level is analogous to voltage, and flow rate is analogous to current, is your
siphon system consistent with the behavior of the series circuit?
Check #1: Show your analogy to an instructor, and discuss how it relates to the currents
and voltages you measured.
8. Consider these circuits. The battery is 3 V (actually 2 batteries in series), and the light
bulbs are all the same type. Compared to the light bulb in the simple circuit above, will
the bulbs be brighter, dimmer, or the same?
Circuit A will be
Circuit B will be
Dimmer
Dimmer
The same
The same
Brighter
Brighter
Circuit C will be
Circuit D will be
Dimmer
Dimmer
The same
The same
Brighter
Brighter
After you have made your predictions, check them by building the circuits.
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9. In Circuit D, replace one of the light bulbs with an LED. Record your observations.
10. Use your results so far (in this and past activities) to predict the voltage that will be
measured across light bulb #1, light bulb #2, and the LED in your working circuit. Use a
multimeter to check and record your predictions.
11. Use your results so far (in this and past activities) to predict the current that will be
measured through light bulb #1, light bulb #2, and the LED in your working circuit. Use
a multimeter to check and record your predictions.
13. How does the behavior of this circuit differ from the behavior of the circuit in #11 of
your Electrical Current lab, where a light bulb and LED were set up in series?
Is the electrical current through a light bulb and LED equal in your new (parallel) circuit?
Check #3: Discuss how the LED affects your parallel circuit with an instructor. If none is
available, please continue on.
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14. Some of these circuits have the light bulbs in series. Some have them in parallel.
Please classify the diagrams as series or parallel circuits.
Resistance
Resistance limits the flow of current.
If more than 30 minutes remain in the class period, move on to this short activity;
otherwise, complete your yellow sheet first, then do as much of this activity as time
allows.
Materials
For each group
2 batteries, wires
light bulb and socket
multimeter to measure
current
LED
buzzer
small index card containing 4 resistors
Definitions:
The property of a material that determines the ability of a material to resist a current is
called the resistance. Good conductors have low resistance: it’s easy to drive a large
current through these. Poor conductors (also called insulators) have high resistance. It
takes a high voltage to get even a small current through these.
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Find the resistors in your kit. There is a picture of them at right. When you
apply a voltage to one of these, a current results that is proportional to the
voltage but also determined by the resistance of the device. Keeping the
voltage constant (3 V, for example), we can get very different currents, as
indicated in the table below.
The symbol for a resistor is because the early resistors were long wires which were
folded up to save space. Notice from the table that increasing the resistance decreases the
current.
Resistance
10 Ohm
100 Ohm
1000 Ohm
10000 Ohm
Current
at 3 V
0.3 Amp
0.03 Amp
0.003 Amp
0.0003 Amp
12. Put each resistor in series with a light bulb, a buzzer, or a light-emitting diode and
two batteries, as shown in the diagram. If the current that the resistor permits is larger
than the current the device needs, the device continues to work, and the resistor doesn't
have much effect on it. If the current that the resistor permits is much smaller than the
current the device needs, the device doesn't do anything. If the current that the resistor
permits is about the same as the current the device needs, it will function but not quite as
well (the light bulb is only half as bright, for example. Record in the table below what
happens with each combination of resistor and device, to determine the current that each
device needs.
Resistance
Current
at 3 V
10 Ohm
0.3 Amp
100 Ohm
0.03 Amp
1000 Ohm 0.003 Amp
10000 Ohm 0.0003 Amp
What the light
bulb does
What the buzzer What the LED does
does
Note that the listed current is what you get with 2 batteries and no other device in your
circuit. What happens to the actual current if other devices are added in series, like you
just did? Make a few test measurements for the 10 and 100 Ohm resistors (with and
without devices added in), to see if your answer is correct.
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13. Based on the amount of current produced by 2 batteries connected to a light bulb,
buzzer, or LED (all by themselves), can you estimate the resistance, in Ohms, of these
devices? (Note that, unlike resistors (approximately) do, these
devices don’t have a constant resistance, so that your estimates will
be just that, only estimates.)
Each group should turn in one copy of this page.
Group members present:
14. The circuit for the lights in a house resembles the circuit drawn at right. The bulbs
are all in parallel. What are the advantages of wiring them this way, instead of putting
them in series?
15. Terry claims there is another advantage: the additional light bulbs don’t cost
anything to operate. He argues that the light bulbs are independent of each other, and so
there is no way for the power company to know how many are turned on. Paula says
there is a way for the power company to tell when a light bulb is turned on. Please
explain who is right.
16. Susan made an extra bright flashlight. It has two batteries
(like most flashlights) but three light bulbs. It gives off three
times as much light, but the batteries don't last very long.
Here is a diagram of Susan's flashlight. Explain the italicized
sentence using concepts of voltage, current, power, and energy as
they pertain to this device.
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Questions from previous exams
1. Here is a diagram of a circuit that contains three identical light bulbs and a 3 Volt
battery.
Are the bulbs all the same brightness, or are some brighter than
others? Explain.
2. Barbara has added a meter to the circuit, as shown.
What physical quantity is she measuring?
3. Based on your experience with such circuits, what is the reading on the meter (give a
number)?
4. Your neighbor has been told that various changes are needed in the wiring of her
house. The electrician says that the house is only wired for 50 Amperes, and that it
should be increased to 200 Amperes.
A) You neighbor doesn't understand why this change should be necessary. The house
worked fine when they bought it, 30 years ago. Please explain why this change is quite
likely to be necessary.
B) Your neighbor wants to know if this change will require that they buy new appliances
that use 200 Amperes. Please answer this question, and explain your answer.
C) Your neighbor is afraid that this change will cause the power bills to increase by a
factor of 4, too. Will it? Please explain.
5. Margie claims that a buzzer does not use very much power. What observations have
you made that support or refute this claim?
6. A single cell of a car battery is only 2 Volts, but the battery itself gives 12 Volts.
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A) Explain how to make a 12 V battery out of 2 V cells.
B) A car headlight draws 5 amperes. What is the power rating for the headlight?
7. Sophie has a nightlight on her bedside table that runs on an AAA battery, but when
she replaced the bulb with a red LED it stopped working Please describe two respects in
which the LED differs from a light bulb that need to be considered, and explain how your
design avoids the problems.
8. The reason that Sophie wants to use a LED is that the flashlight light bulb causes the
battery to run down too fast: it lasted only 40 hours. She wonders how long the batteries
will last in a LED nightlight. Explain how to use the multimeter to estimate this.
What should she measure? Show where to put the multimeter in your circuit diagram.
9. Explain how the measurements with the multimeter determine how long the batteries
will last in the LED nightlight.
10. The electrical circuit for a car uses a 12 V battery. Household circuits are 120 V.
Jeremy is considering using a 60 W household bulb to replace the 60 W headlight in his
car.
Anne says, “This is a bad idea, because the household light bulb has a lower resistance
than a headlight, and so it will be too bright or immediately burn out.”
Barbara says, “This is a bad idea, because the household light bulb has a lower
resistance than a headlight, and so it won’t be bright enough”
Carla says, “This might work, once you figure out how to connect the wires.”
Denise says, “This is a bad idea, because the household light bulb has a higher resistance
than a headlight, and so it will be too bright or immediately burn out.”
Eunice says, “This is a bad idea, because the household light bulb has a higher resistance
than a headlight, and so it won’t be bright enough.”
A As a first step, determine how much current the two kinds of light bulbs use when
they are in their proper circuits.
B. Discuss how the resistances of the bulbs are related.
C. Whose advice for Jeremy is right? Please explain.
11. Numerical answers required The battery pack for an electric car consists of a dozen
12 V batteries connected as shown in the diagram (each box is a 12 V battery). The
largest current that can be drawn from a single battery is 20 Amperes.
A) What is the current in each wire (indicate on the
diagram)?
B) How much power does each battery provide?
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C) How much electrical power can be obtained from the battery pack?
12. Four students have tried to draw the circuit diagram that shows how to wire a two
room house.
This is the diagram they
were given.
Andrew’s diagram
Betty’s diagram
Cindy’s diagram
Debby’s diagram
Did anyone get it right? If not, draw in your version at the top. There could be three
kinds of incorrect diagram: a circuit that lets you turn the lights on and off, but not as
intended; a circuit that doesn’t work because it won’t allow you to turn on the lights in
one of the rooms; a circuit that is dangerous. For each incorrect diagram, explain what
is wrong.
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