Single Bulb Circuits
Materials:
 Flashlight battery
 Light bulb
 Wire
We begin our study of electric circuits by connecting a battery and a bulb together and
observing what happens. We investigate the conditions under which the bulb lights brightly,
dimly or not at all.
Obtain one battery, one light bulb, and one wire. Connect these in as many ways as you can.
Sketch each arrangement on the next page. On one side of the page, list arrangements in which
the bulb lights. On the other side of the page, list arrangements in which the bulb does not light.
You should make sketches of at least four different arrangements that light the bulb. How are
they similar? How are they different from arrangements in which the bulb fails to light?
Use the space below to state what requirements must be met in order for a bulb to light.
Discuss as a class.
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ARRANGEMENTS THAT
LIGHT THE BULB
ARRANGEMENTS THAT
DON’T LIGHT THE BULB
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A Model for the Electric Circuit
Materials:
 6V Battery
 Light bulbs
 Heavy hookup wires
 Thin wire
 Coffee Stirrers
 Straws
 Cup of Water
An arrangement of a bulb, battery and wire that allows the bulb to light is said to be a closed
electric circuit. The terms complete circuit, or just circuit are also used. The word “circuit”
was originally used to mean “a circular route or course”.
In working with electric circuits, you may have noticed some regularity in the way they
behave. Perhaps you have begun to form a mental picture, or a model, that helps you think
about what is happening in a circuit. In this experiment, we will begin the process of
developing a scientific model for an electric circuit.
A scientific model is a set of rules that applies to a particular system that makes it possible to
explain and predict the behavior of that system. We would like to build such a model for
electric circuits that will enable us to predict the behavior of any circuit of batteries and bulbs.
If we connect several bulbs and batteries together in a circuit, we would like to be able to
predict which bulbs will light, which will be brightest, dimmest and so forth.
Activity 1: Briefly connect the terminals of a battery with the thin wire until the wire feels
warm. Don’t maintain this connection for more than 5 seconds at a time since the wire
will get very hot. Does the wire seem to be the same temperature along its entire length or are
some sections warmer than other? What might this observation suggest about what is
happening in the wire at one place compared to another?
Discuss as a class.
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Now make a single bulb circuit using the 6V battery and hook-up cables you have been
provided. Sketch your circuit below. Show the working circuit (with the bulb lit) to the
instructor before moving on. Note the brightness of the bulb.
When a wire or a light bulb is connected across a battery, we have evidence that something is
happening in the circuit. The wire becomes warm to the touch; the bulb glows. In constructing
a model to account for what we observe, it is helpful to think in terms of a flow around a
circuit. We can envision the flow in a continuous loop from one terminal of the battery,
through the rest of the circuit, back to the other terminal of the battery, through the battery, and
back around the circuit. We have found that a light bulb included in this circuit will light.
We shall assume that the brightness of the bulb is an indicator of the amount of flow
through the bulb. Brighter means more flow (though “twice as bright” does not
necessarily mean twice the flow).
The assumptions that something is flowing through the entire circuit (including the bulb) and
that a light bulb can be used as an indicator of the flow are both consistent with our
observations.
Follow-Up Question:
1. Can you tell from your observations thus far the direction of the flow through the
circuit? Why or why not? Can you think of any way you might be able to figure this
out?
Discuss as a class.
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There is a standard way to draw diagrams that represent electrical circuits. Shown below is a
circuit diagram for a bulb attached to a battery:
wire
bulb
battery
wire
Notice that the diagram indicates the way in which things are connected: A wire goes from
one side of the battery to one terminal on the bulb. A second wire goes from the other side of
the battery to the other terminal on the bulb. Wire up a circuit in this way and verify that the
bulb lights up.
Consider the following dispute between two students:
Student 1: “When the bulb is lit there is a flow of electric current
from the battery to the bulb. There is also an equal flow of
electric current from bulb back to the battery.”
Student 2: “There is only flow from the battery to the bulb. We
know this is so because a battery can light a bulb, but a bulb
can’t do anything without a battery.”
Do you agree with student 1 or student 2? Explain your reasoning.
Discuss as a class.
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Activity 2: Set up a circuit having two bulbs connected one after
the other, as shown in the diagram to the right. When bulbs are
connected one after the other in this way they are said to be in
series.
Compare the brightness of the two bulbs. Pay attention to large differences only – small
differences may be due to the fact that no two bulbs are exactly the same:
Compare the brightness of each of the bulbs in the above circuit with the brightness of the
bulb in the single bulb circuit of Activity 1.
Switch the order of the bulbs in the above circuit and see if this makes any difference:
If one of the bulbs is unscrewed, what happens to the brightness of the other bulb?
How does the amount of electric current that flows from the battery in the series two bulb
circuit compare with the amount of electric current that flows from the battery in the single
bulb circuit? Explain your reasoning.
Discuss as a class.
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Activity 3: Set up a two-bulb circuit having the terminals of
the bulbs attached together as shown in the diagram to the
right. When bulbs are connected in this way they are said to
be connected in parallel.
Compare the brightness of the two bulbs:
Compare the brightness of each of the bulbs in the above circuit with the brightness of the
bulb in a single bulb circuit:
Switch the order of the bulbs in the above circuit and see if this makes any difference:
If one of the bulbs is unscrewed, what happens to the brightness of the other bulb?
How does the amount of electric current that flows from the battery in the parallel two bulb
circuit compare with the amount of electric current that flows from the battery in the single
bulb circuit? Explain your reasoning
Discuss as a class.
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How does the amount of electric current that flows from the battery in the parallel two bulb
circuit compare with the amount of electric current that flows from the battery in the series
two bulb circuit? Explain your reasoning.
Discuss as a class.
It sometimes helps to think of the flow of electricity in wires and bulbs as being similar the
flow of water through pipes. In this picture, a wire behaves like a big fat pipe through which
water flows very easily, and a bulb is like a skinny pipe through which water flows more
slowly. In the same way, electric current flows very easily through the wires used when
hooking up a circuit, but faces more resistance when flowing through a bulb.
You can think of a battery as a pump. A water pump pushes water through pipes by creating
pressure. A battery pushes electric current through wires and bulbs by creating a voltage. Just
like pressure is a measure of how hard a pump can push water, voltage is a measure of how
hard a battery can push electric current.
big pipe
wire
bulb
battery
narrow
pipe
pump
wire
big pipe
Notice that you need the water pipes to make a complete “circuit” in order for water to keep
flowing. You also can’t have any leaks. These things are true in electric circuits also: You need
a circuit for electricity to flow, and you can’t “leak” electricity anywhere.
To make water flow through a narrow pipe we need pressure. If we increase the pressure we
increase the flow. If we make the narrow pipe longer, it’s harder for the water to move, and the
flow becomes smaller.
In the same way, to make electric current flow through a lamp we need voltage. If we increase
the voltage we increase the flow and the lamp becomes brighter. If we put two lamps one after
the other it becomes harder for current to flow, and the lamps become dimmer.
For electric circuits this is summarized by Ohm’s Law: I = V/R which says that the current I
through any part of a circuit is equal to the voltage V pushing across that part of the circuit
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divided by the resistance R of that part of the circuit. Resistance is just a measure of how hard
it is for current to flow (like the skinniness of the pipe).
We can look at this more directly. Fill your cup with water. Try and drink some of the water
through the regular drinking straw. Now try and drink some through one of the coffee stirrers.
How much water can you get through the coffee stirrer as compared to the drinking straw?
Now, connect two coffee stirrers end-to-end. (This may take a little work to get it so that it
doesn’t have any leaks.) Try and take a drink through the two coffee stirrers end-to-end. Is it
easier or harder than with a single coffee stirrer? Now try the same thing with the two coffee
stirrers side-by-side. Is this easier or harder than a single coffee stirrer? How would this
change if you were to use a bunch of coffee stirrers together, all side-by-side. What do you
observe?
The drinking straw represents a low-resistance object, like a wire. The coffee stirrers
represent a high-resistance object, like the light bulb. Your mouth is acting like the battery,
and the water is acting like the flow of electricity. When you had the two coffee stirrers endto-end, that was like the two light bulbs in series. You notice that it was harder to pull the
water through the two coffee stirrers, so as a result, there was less flow. Similarly, the two
light bulbs in series were dimmer then the single light bulb. The two coffee stirrers side-byside were easier to drink through. This was like the two light bulbs in parallel. You were able
to pull more water through, but each stirrer only got half of the water. Even though there was
about twice as much water flow, each stirrer only got half of it. Similarly, with the two light
bulbs in parallel, there was twice as much flow, but the two bulbs had to share it, so they were
each as bright as the single bulb by itself had been.
Discuss as a class.
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Activity 4: You are about to set up the three-bulb circuit
shown in the diagram to the right.
A
Before connecting the circuit, think carefully and answer the
following questions:
B
C
1) Predict what you expect brightness of each of the three
bulbs (A, B and C) will be compared to each other.
A and B:
A and C:
B and C:
Hook up the circuit, observe the brightness of the three bulbs, and compare them:
A and B:
A and C:
B and C:
Explain in words how you think the electric current flows through your circuit in order to
explain your observations. It may help to think of the water analogy.
How does the amount of electric current that flows from the battery in the three bulb circuit
compare with the amount of electric current that flows from the battery in the single bulb
circuit? Explain your reasoning.
Discuss as a class.
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Lighting a Light Bulb with a Balloon
Materials:
 Latex Balloon
 Compact Fluorescent Light Bulb
 Piece of Wool
The forces that actually move the charged electrons inside an electric circuit are carried by
electric fields. We can see the effects from electric fields in many different ways. We will
look at one of these in this next activity.
Blow up your balloon to full size and tie it off. After it is fully inflated, rub the balloon with
the wool cloth for a few minutes. Make sure to rub the same part of the balloon back and
forth rather than all over the whole balloon. It also works if you have longer dry hair to rub
the balloon on your hair. After rubbing the balloon, try sticking the balloon on the wall or the
side of your head. What happens? Why does this happen?
Discuss as a class.
By rubbing the balloon, you gave it an electric charge. Electrons jumped from the wool onto
the balloon, giving the balloon a negative charge (and the wool a positive charge). When an
object has an electric charge, it generates an electric field. Sometimes this field can be very
strong. (This is what allows you to shock people with a jolt of static electricity after rubbing
your feet on the carpet during the winter.) This electric field pulls positive and negative
charges together. Since all matter is made of positive and negative charges, this is why the
balloon (negative charge) is pulled in toward your hair (positive charges). (This is also why
your clothes can stick together in the dryer if you don’t use a dryer sheet. The clothes rubbing
together build up an electric charge and stick together. A dryer sheet uses some special
chemicals that help keep the charges from jumping from one piece of clothing to another and
can help neutralize the charges if they do jump over.)
Electric fields can move charges for other interesting effects too. This is how a fluorescent
light bulb works. Make sure that your balloon is fully charged. (You will need the classroom
lights off to be able to see this next part.) Now, take the fluorescent light bulb and wave it
near the charged part of the balloon. (If you don’t see anything, try touching different parts of
the bulb to the balloon.) What happens? Write down why you think happens.
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Does it matter what part of the balloon you have near the bulb? Why or why not?
Discuss as a class.
What’s going on:
A fluorescent light bulb works because it has a special gas on the inside. Strong electric fields
can accelerate the atoms, causing them to collide and excite the electrons in the atoms in the
gas or even pull some of the electrons off of the atoms. When these electrons de-excite or
recombine with the atoms, they emit light. (This is the same reason you can see lightning or
static electricity sparks. When electrons accelerate or when they recombine with or de-excite
in atoms, they emit light.) Normally, the voltage that we place across a fluorescent light bulb
in a household circuit provides the electric field to make the light bulb glow. However, any
charged object gives off an electric field. Even though the field is much weaker, even our
charged balloon has an electric field, just like you’d have in a household circuit. This electric
field can also accelerate the charges in the gas in the light bulb. We see the result as light.
(This also shows why this doesn’t work with a traditional incandescent bulb – They work on a
different principle.)
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Magnets
Warning: The two neodymium magnets you will be given are very strong. You should
keep them away from credit cards and mechanical watches or these may not work real
well. The magnets are quite brittle and will break if dropped.
Investigating Your Magnets
Materials:
 Two neodymium magnets for each student (you keep these)
 Bar Magnet
 Compass
Activity 1: Place a bar magnet on the table and make sure all other magnets are at least six
feet away from it. You will use the cheap little compass provided to map out the magnetic
field lines of the bar magnet, drawing your results on the diagram on page 2. Place the
compass on the table, up against the bar magnet at one end.
On the diagram, draw an arrow in the following way:
- The position of the arrow should show the position of the compass relative to the bar
magnet.
- The direction of the arrow should be the direction of compass needle, with the head of
the arrow representing the red side of the needle.
Move the compass all around the bar magnet in ½ inch steps. The compass should be
touching the magnet. For each step draw another arrow. Draw all of the arrows the same
length - between ½ and 1 inch or so.
Now move the compass about 1 inch away from the magnet. Move the compass around the
magnet in 1 inch steps, keeping it about an inch away from the magnet at each step, and draw
an arrow for each step. Repeat the procedure using 2 inch and 3 inch distances & steps.
When your diagram is complete, share your findings with others and discuss as a class.
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N
S
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Activity 2: Do the same thing as Activity 1 but map out the field of one of your neodymium
magnets rather than the bar magnet. You should stand your magnet on end and keep it in place
using a piece of tape. Move the compass in circles 1, 2, 3 inches away from the magnet disc
using steps that are small enough that you can the change in the direction if the compass
needle is not too big between adjacent locations. Draw your results on the “view from above”
diagram below.
Describe a way that you can figure out which side of each of your neodymium magnets is
“North”. Color the north sides of your magnets with a marker for future reference.
When your diagram is complete, share your findings with others and discuss as a class.
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The Earth is a Big Beautiful Magnet
Materials:
 Neodymium magnets
 Compass
 Thread
Use the compass provided to find “North” (making sure no other magnets are nearby to mess
this measurement up). Keep this direction in mind during the following exercises.
Working individually, hang one of your neodymium magnets from a string a couple of feet
long. Make sure you keep away from any other magnets. Move around a bit. What do you
observe?
Which side of your hanging magnet is attracted to the north pole of the earth? Is this
consistent with the way magnets attract each other?
Discuss as a class.
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Making a Magnet
Materials:
 1 meter length of wire
 Battery with leads & clips
 Neodymium magnets
 Thread
 Compass
 Razor blade
 Tape
Working as a group, take a length of wire and wrap it
repeatedly around a ¾” cylindrical object until only an
inch or two are left at each end (see sketch to the right).
Using a razor blade scrape about ¼” of the insulation off
each end of the wire. Tie the loops together with string in
two or three places so that they stay together as you handle
it.
Tie with string
Scrape insulation off ends
Activity 1: Stand the loop on its side and hold it in place with some tape. Hook the positive
side of the battery up to one of the leads and the negative side to the other lead. The battery is
now driving an electric current through the loop of wire. Move the compass around near the
loop of wire. What do you see?
Investigate the magnetic field of this loop in exactly the same way that you did for the
neodymium magnet in activity 2 of experiment 1. How does the field of this loop compare to
that of the neodymium magnet?
Discuss as a class.
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Activity 2: Suspend a neodymium magnet from a string and dangle it
near the loop. Investigate the forces between the magnet and the loop
when the battery is driving a current through the loop. Which side of
the loop is “North”?
Table
Switch the way the leads from the battery are hooked up to the loop of wire. Now which side
of the loop is “North”?
Disconnect the battery from the loop. Describe the magnetic field produced by the loop now.
Discuss as a class.
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Making a Speaker
Materials:
 Paper Cup
 2 meter length of wire
 Audio plug connected to wires
 Neodymium magnets
 AM/FM Radio / CD Player / Laptop / or some other audio source with a standard
headphone jack
Just like a magnet can be used to move a coil of wire which is carrying an alternating current,
a coil of wire can be used to move a magnet.
Take the 2 meter length of wire and wrap it around
the base of the paper cup to form a tight coil about
1 cm or so from the bottom of the cup. This
should be similar to the coil that you used to make
an electromagnet earlier. Once you have a good
coil of wire, connect the two ends of the coiled
wire to the two wires that are connected to the
audio plug. Place the neodymium magnet in the
bottom of the paper cup. Congratulations, you
have just constructed a basic speaker.
Now, plug the audio plug into your radio / CD player / whatever and play some music at a
fairly high volume. (Note that even though you should now have a working speaker, it won’t
be a great speaker, so the more input volume your player can put out, the better your chances
of being able to hear it.) Music or talking with a large dynamic range (changes in volume)
works best. Now, place your ear up to the opening of the paper cup. Can you hear the music?
Why does this happen. Write down your ideas and then discuss as a class.
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What’s going on:
This speaker works about like any other speaker. When the music is played, an electric
current runs through the wire. The direction of this current switches back and forth. As the
volume changes, the flow gets stronger and weaker. As the flow of electricity gets stronger
and weaker, the magnetic field that the wire is generating gets stronger and weaker. This will
pull off and on on the neodymium magnet, causing it to vibrate. The magnet vibrating causes
the drum of the cup to vibrate. This vibration causes waves in the air that you can hear as
sound.
This is the same way a tin can telephone works. As
the speaker speaks into the first tin can, the sound
waves cause the base of the can to vibrate. This
vibration pulls off and on on the string. This string
pulls, in turn, on the base of the other can. (This
also shows why the string has to be pulled taut for
the phone to work.) The pulling of the string on the
base of the second can causes that to vibrate,
producing sound.
Source: http://elderbrief.wordpress.com/
2009/07/28/can-you-hear-me-now-part-2/
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Optional activity [you probably won’t have time to do this in class but you have all of the
needed materials].
Making a Motor
Materials:
 Styrofoam base to hold motor.
 1 meter length of wire
 Battery with leads & clips
 Neodymium magnets
 Thread
 Compass
 Razor blade
 Tape
 Paper clips
Working individually, take a length of wire and wrap it repeatedly around a ¾” cylindrical
object until only an inch or so is left at each end (see sketch below). This time the two leads
should stick straight out at opposite ends of the loop. Tie the loops together with string in two
or three places so that they stay together as you handle it. You may also want to use some tape
to provide extra strength at the places where the leads stick out.
Tie with string
You will again scrape away part of the insulation from the leads that are sticking out, but this
time you will do it a bit differently. Lay the completed loop on the table. Looking down on it
from above it should look like the sketch above. Using the razor blade, remove the insulation
from the top of the leads only. In other words, when you are done the top half of the leads
should be shiny metal, but if you turn the loop over you should still see insulation on the
bottom half, as shown in the “zoomed in” sketch to the right.
If you have any questions or doubts about doing the right thing, ask the instructor before
starting to remove the insulation.
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Using paperclips and the base, make the two vertical supports that will be used to hold your
wire loop as shown in the figure below. (The base doesn’t have to be made out of Styrofoam
– any insulating material that can support the paperclips will do – cardboard works fine).
Place your magnet below the suspended loop as shown. You want the loop to turn just above
the magnet, not much higher. Carefully balance the loop so that it turns easily as it is held by
the supports.
Hook the two wires from the battery up to the two supports and give the loop a little spin to
get it started. If you can’t get it to work after trying for a while, ask for help.
See if you can figure out how the motor works. Explain why stripping the insulation from just
the top half of the leads was the key to getting this motor to spin.
Discuss as a class.
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