CYCLE 5
Developing Our Ideas
ACTIVITY 3: Electric Circuits and Current
Purpose
In Cycles 1 and 2 you learned that there are two ways that scientists describe
interactions: either in terms of energy transfer and energy changes, or in
terms of forces. When thinking about an electric circuit interaction, there are
also two ways of describing it. In the previous activity you learned how to
describe the interaction in terms of the transfer of electrical energy from the
battery (or other source) to the receiver. In this activity you will learn of
another way of describing what happens in an electric circuit, and that is in
terms of electric current.
How can we describe an electric circuit
interaction in terms of electric current?
Does Electricity
Circuit?
Flow
One-Way
or
Two-Ways
in
the
Two students both believed there is a flow
of electricity in a simple circuit consisting
of a battery and bulb. But they differed in
thinking about whether the electricity flows
one way or two ways.
Kristen says: “I think electricity
flows out of each end of the
battery, meets at the bulb, and
causes the bulb to light.”
© 2005 PET
Amara says: “I think electricity flows in
one direction around the circuit. It
comes out of one end of the battery,
goes through the bulb, and then into
the other end of the battery.”
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Cycle 5: Modified by Dr. Armen Kocharian
Let us refer to Kristen’s model as the “two flow” model, and to Amara’s
model as the “one-flow” model.
Do you agree with the one-flow or two-flow model, or with both? What
is your reasoning?
To provide some evidence that might help settle the issue, you should
perform the following experiment.
Experiment #1: How do the two ends of the battery need to
be connected to the two sides of the bulb?
The evidence from Activity 1
suggests that one side of the bulb
needs to be connected to the
positive end of the battery, and the
other side of the bulb needs to be
connected to the negative end of
the battery.
But, do the two sides of the bulb need to be connected to the positive and
negative ends of the same battery? Consider the following arrangement:
Do you think the bulb in the above arrangement will light? Explain your
reasons.
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Activity 3: Electric Circuits and Current
Get two batteries, two hook-up wires and a bulb in a socket. Hook up the
arrangement shown above.
Does the bulb glow?
In Activity 2, Experiment #2, step 2, you hooked a battery to the motor/fan,
then figured out how to make the fan blades rotate in the opposite direction.
You may wish to go back and look at what you did in that experiment.
Based on your observations in the experiment with the two separate
batteries, and the experiment from Activity 2 with the motor/fan, what
would you now claim about the one-flow versus two-flow model? Why?
B Participate in a whole class discussion.
Beginning to Think About Electric Current
Back in Cycle 2 you learned that the rate of change of speed of an object (also
known as acceleration) depends on two factors: the strength of the
unbalanced force acting on it and the mass of the object. Although an electric
circuit is a much more complicated system than just a single object acted on
by a force, it is helpful to think about an electric circuit in a somewhat similar
way. Scientists believe that inside batteries, wires and all electric devices
there are particles called electric charges. During an electric circuit interaction
between a battery and a bulb the battery exerts a push (force) on these electric
charges, causing them to move around the circuit, from one end of the
battery, through the wires and filament of the bulb and into the other end of
the battery. (Notice this implies that scientists have a one-flow model of
electricity.) The rate of flow of electric charges is called the electric current.
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Cycle 5: Modified by Dr. Armen Kocharian
The greater the rate of flow of electric charges, the greater the value of the
electric current. So, in comparing what happens in an electric circuit with a
battery and bulb to what happens when a force is exerted on an object, the
push provided by the battery is analogous to the force acting on the object.
The value of the electric current in the circuit is analogous to the rate of
change of speed of the object. The property of a bulb that is analogous to the
mass of the object is called the resistance of the bulb.
In the remainder of this activity you will perform experiments to develop and
test ideas about the relationships between the strength of the push provided
by the battery, the amount of resistance of the bulb, and the value of the
electric current in an electric circuit.
Collecting and Interpreting Evidence
In the next experiment you will study the relationship between the strength
of the push from the battery and the value of the electric current. To help you
think about and make sense of this relationship, you will first be introduced
to a simple model of the electric circuit.
Experiment #2: How does the value of the electric current
depend on the strength of the battery push?
You will need:
•
•
•
•
Straw
Sheet of 8.5 x 11 inch paper
Two pieces of tape
Stand to mount paper on table
A simple model of an electric circuit involves
blowing air through a straw. Each member of
your team will need a regular straw. In using
this model we will assume your mouth and
lungs are analogous to the battery. The air
inside your mouth, lungs and straw is
analogous to the electric charges inside the battery, bulb and wires of the
circuit. The straw is analogous to the bulb. (More specifically, the straw is
analogous to the filament of the bulb. You observed in Activity 1 that only
the filament part of the bulb glows.)
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Activity 3: Electric Circuits and Current
Blow through the straw and observe the effect at the other end. By feeling
the moving air pushing against your hand you get a sense of the rate of flow
of the air, which we will call the air current.
The air current in the straw is analogous to the electric current in the circuit.
However, this analogy is not perfect, in part because in a circuit the wire
doesn’t end, but instead is connected to the other end of the battery. But even
though this analogy (like all analogies) is not perfect, we can still use the
model to help us make sense of some properties of electric current and
circuits. We will call this the blowing-through-straw model for an electric
circuit.
Blow harder and harder through the straw. Does the value of the air current
increase, decrease or remain constant?
It will be useful in experiments to
follow to have some simple and
qualitative way to measure the
value of the air current. Take an
8.5-inch x 11-inch piece of paper
and draw a large dot in its center.
Then tape the upper left and right
corners of the paper to the rod so
the paper hangs freely.
See
picture on right.
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Cycle 5: Modified by Dr. Armen Kocharian
Hold the straw horizontally with its
end almost touching the large dot.
Blow gently and quickly. Then
blow harder and harder. Another
member of your group should
observe each time about how far
the paper moves away from its
initial position.
What seems to be the general relationship between how hard you blow
through the straw and how far the bottom of the paper moves?
The harder you blow through the straw, the higher is the value of the air
current (the rate at which air moves through the straw). The hanging paper
can be used as a very simple air current meter, allowing you to get a
qualitative measure of the rate at which air moves through the straw (and
comes out the other end). Do not put away the air current meter. You will
use it again later in the activity.
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Activity 3: Electric Circuits and Current
Now consider the electric circuit. A single battery cell provides a constant
push on the electric charges in the circuit. The strength of the battery push
(analogous to how hard you blow through the straw) is determined by the
voltage of the battery. The greater the battery voltage, the greater the
strength of the battery push on the electric charges. Each D-cell battery has a
voltage of 1.5 volts. If you combine two D-cells together (in the right way),
the total voltage is 3.0 volts; three together gives a total voltage of 4.5 volts,
and so on.
What do you predict would be the relationship, if any, between the strength
of the battery push and the value of the electric current in a circuit? Why do
you think so?
To test your prediction, you will use a computer simulator. The EM Devices
simulator allows you to set up circuits with batteries and bulbs. The
simulator also has a device to measure the rate at which electric charges move
through the circuit; that is, a device to measure electric current. The device is
called an ammeter. (It is analogous to your air current meter, although it
works quite differently.) The units for electric current are amperes, or more
commonly milli-amperes (mA), which are thousandths of an ampere.
To the right is a picture from the
simulator of a simple electric
circuit consisting of a battery,
ammeter and bulb. The table
below summarizes the analogous
relationships between the electric
circuit and the blowing-throughstraw model.
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Cycle 5: Modified by Dr. Armen Kocharian
Analogy between Electric Circuit and Blowing-through-straw Model
Electric Circuit
Battery
Strength of battery push (measured
in volts)
Light bulb filament
Electric charges in battery, bulb,
ammeter and connecting wires
Electric current is rate of flow of
electric charges
Ammeter measures value of electric
current
Value of electric current measured in
amps (milla-amps)
Blowing-through-straw Model
Lungs and mouth
Strength of blowing through straw
Straw
Air in lungs, mouth, straw and
hitting paper
Air current is the rate of flow of air
Air current meter (hanging paper)
measures value of air current
Value of air current measured by
how far end of paper moves from its
original position
Open the EM Devices simulator by going to the PET simulator index page
and click on Cycle 5 Activity 3 Setup #1. Then read How To Use the EM
Devices Simulator at the end of this activity. This will tell you how to set up a
circuit. After reading the instructions, continue below.
Start with one battery, close the switch, and record the value of the electric
current in Table 1. Fill in the value of the battery voltage. Then add a second
battery, a third battery, and a fourth battery. (Make sure the batteries are
connected together with the minus side of one battery connected to the plus side
of the next battery.) In each case record the ammeter reading in Table 1, fill in
the battery voltage, and also note what happens to the brightness of the bulb.
Table 1: Battery Voltage versus Electric Current
Number of Batteries
1
2
3
4
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Battery Voltage (volts)
Electric Current (mA)
Activity 3: Electric Circuits and Current
What is the relationship between the battery voltage (strength of battery
push) and the value of the electric current?
Does this relationship confirm or disconfirm your prediction?
What is the relationship between the value of the electric current and the
brightness of the bulb?
This observed relationship between the electric current value and bulb
brightness suggests that the brightness of the bulb can be used as a measure
of the value of the electric current. However, it is only a qualitative measure,
depending on the observer’s judgments of brightness. The ammeter provides
a quantitative measure (using measured numbers rather than subjective
judgments) and, for that reason, is generally more useful when studying
electric circuits.
As you have seen, it is the filament wire in the bulb that actually glows. So
far you have always used identical bulbs, with identical filament wires. What
would happen if the filament wire changed in some way? Would the value
of the electric current in the circuit change, or remain the same? The purpose
of the next experiment is to help you answer that question.
Experiment #3: How does the value of the electric current
in a circuit depend on the properties of the bulb
filament?
You will need the following materials:
•
•
•
•
2 coffee stirrers
Regular straw
Air current meter
Scissors
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Cycle 5: Modified by Dr. Armen Kocharian
Consider two properties of a filament wire that could be different from one
bulb to the next: its length and thickness. Before doing an experiment to
investigate what happens in electric circuits you will first work with straws
and stirrers and apply the blowing-through-straw model to make predictions
about electric circuits.
First, cut the regular straw so it is the same length as the full-length stirrer.
Since both the straw and stirrer have the same length, what is the main
difference between the two?
Blow through the straw, and then blow through the stirrer. Which one seems
more difficult to blow air through?
Next, cut one of the coffee stirrers into a 1/3-length piece and a 2/3-length
piece. Put aside the 2/3-length piece.
Blow through the 1/3-length stirrer, and then blow through the full-length
stirrer. Which one seems more difficult to blow air through?
Scientists would use the term resistance to refer to the property of the straw
or stirrer that determines how difficult it is to blow air through it. Fill in the
following:
The longer the stirrer/straw, the __________________ (greater, smaller) its
resistance to the flow of air though it.
The thinner the stirrer/straw, the __________________ (greater, smaller) its
resistance to the flow of air though it.
Next, make sure the air current meter is set up.
Hold the 1/3-length stirrer with its end very close to the dot on the paper,
blow quickly through it, and notice how far the bottom of the paper moves
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Activity 3: Electric Circuits and Current
away. Then hold the full-length stirrer with its end the same distance from
the dot, and blow equally hard and as quickly. In which case does the paper
move away further?
Repeat using the full-length straw. When you blow equally quick and hard
through the same length stirrer and straw, in which case does the paper move
further away?
The distance the paper moves is related to the value of the air current. Fill in
the following:
For the same blowing strength, the longer the stirrer/straw, the
__________________ (higher, lower) the value of the air current.
For the same blowing strength, the thinner the stirrer/straw, the
__________________ (higher, lower) the value of the air current.
For the same blowing strength, what seems to be the relationship between
the amount of resistance of the stirrer/straw and the value of the air current
in a stirrer/straw?
Check your answer to the above question with another group. If there are
differences in your answer, together you should repeat one or more of the
previous observations.
Having worked with the straws/stirrers, you are now in a position to use the
blowing-through-straw model to help you think about actual bulbs and
batteries. The filaments of different types of bulbs generally have different
lengths and/or thicknesses. Scientists use the term resistance to refer to the
property of a wire that determines how difficult it is for electric charges to
move through it. The amount of resistance depends (in part) on its length
and thickness. In analogy with the stirrers/straws:
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Cycle 5: Modified by Dr. Armen Kocharian
•
The longer the filament, the greater its resistance to the flow of electric
charges though it.
•
The thinner the filament, the greater its resistance to the flow of electric
charges though it.
For the same battery voltage, predict the relationship between the amount of
resistance of the bulb filament and the value of the electric current in a
circuit? How does the blowing-through-straw model guide your prediction?
You can use the EM Devices simulator to test your prediction.
Go to the PET simulator index page and click on Cycle 5 Activity 3 Setup #2.
In the simulator window you will see the two identical circuits already setup.
Each circuit includes a battery, ammeter and bulb. The battery voltage (1.5
volts) is the same in both cases.
If you double click on the upper bulb, you
will open its property window. Each
bulb starts with an amount of resistance
equal to 30 ohms. (The ohm is the unit of
resistance.) Change the amount of
resistance of the upper bulb to 16 ohms.
Then click OK. In the two circuits the
upper bulb has a resistance of 16 ohms,
the lower bulb a resistance of 30 ohms.
(Since the resistance of the bulb filament depends on its length and thickness,
changing the amount of resistance would normally require changing its
length and/or its thickness. The simulator, however, doesn’t allow you to
change length and/or thickness, only the total amount of resistance.)
Turn on the simulator. How does the brightness of the 16-ohm bulb compare
with the brightness of the 30-ohm bulb?
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Activity 3: Electric Circuits and Current
In the following table, for the two values of bulb resistance record the
corresponding values of the electric current.
Table 2: Electric Current versus Bulb Filament Resistance
Bulb Filament
Resistance (ohms)
Measured Electric Current
(mA)
16
30
Try several other values for the resistance and record them in Table 2. For
each new value, observe what happens to the bulb brightness and record the
measured values of the electric current. (If the electric current gets too high
the bulb burns out. In that case, delete the burned out bulb and replace it with
a new bulb from the palette to the right of the setup area.)
For the same battery voltage, as the resistance of the bulb filament increases,
does the value of the electric current in the circuit increase, decrease, or
remain essentially constant?
How does this result compare with your
prediction?
As the resistance of the bulb filament increases, what happens to the
brightness of the bulb?
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Cycle 5: Modified by Dr. Armen Kocharian
Does the bulb always glow, or are there values of electric current for which
the bulb does not seem not to glow noticeably?
Resistance and Thermal Energy
What is the reason why materials like bulb filaments have a certain amount of
resistance to the flow of electric charges? Scientists have theories about the
origin of resistance that can answer this question, but the theories are much
too complicated to describe here. Instead, we will try to think about it in a
much simpler way, using a familiar idea. In Cycle 2 you learned that
whenever an object slides over a surface, there is a friction force that opposes
the motion of the object and tends to slow the object down. In the case of an
electric circuit we can imagine that when the battery pushes electric charges
through the metal filament of the bulb, a similar thing happens. There are
friction-like interactions between the moving electric charges and the atoms
that make up the material of the filament wire. These interactions tend to
decrease the rate of flow of the electric charges. This is the origin of the
resistance of the wire. The actual amount of resistance will depend, in part, on
the strength of the friction-like interactions. As you learned in the previous
section, the amount of resistance will also depend on the length and thickness
of the wire.
Why does the bulb glow when there is an electric current in the filament wire?
To help answer this question, again consider the analogy of the object sliding
over a surface. When the block slides over a surface, mechanical energy is
transformed into thermal energy in the surface and this causes the surface to
warm up. In the case of the electric circuit, when the charged particles are
pushed through the bulb filament (by the battery), electrical energy is
transformed into thermal energy of the filament material, and this causes the
material to warm up. When the material warms up sufficiently it will begin
to glow, and as it becomes even hotter it will glows more brightly.
How hot the material becomes, and how brightly it glows, depends both on
the value of the electric current and the amount of resistance. However,
scientists have found that the brightness depends more on the value of the
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Activity 3: Electric Circuits and Current
electric current than it does on the amount of resistance. We can represent
this brightness-resistance-current idea in the following diagrammatic way:
As the amount of resistance , the brightness of the filament
As the amount of resistance
, the brightness of the filament
As the value of electric current , the brightness of the filament
As the value of electric current
, the brightness of the filament
Let’s see how your results from Experiments #2 and #3 are consistent with
this idea.
In Experiment #2 (and Table 1) you found that when the resistance was held
constant (same bulb), and you increased the battery voltage the value of the
electric current ______________________ (increased, decreased, remained the
same), and the brightness of the bulb _________________ (increased,
decreased, remained the same).
How are these results consistent with the brightness-resistance-current idea?
In Experiment #3 (and Table 2) you found that when the battery voltage was
held constant (one battery), and you increased the resistance of the bulb, the
value of the electric current ______________________ (increased, decreased,
remained the same), and the brightness of the bulb _________________
(increased, decreased, remained the same).
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Cycle 5: Modified by Dr. Armen Kocharian
How are these results consistent with the brightness-resistance-current idea?
As mentioned above, one of the factors that determine the amount of
resistance of a wire is the strength of the friction-like interactions. The
strength of the friction-like interactions between the moving electric charges
and the atoms of the material depends both on the temperature of the
material and the type of material. Consider the temperature effect first. For
the same type of material, the strength of the interactions, and hence the
amount of resistance of the wire, increases when the wire gets hotter.
Tungsten filaments are used in bulbs because tungsten can get white-hot
without melting. When a bulb is first turned on, the resistance of its filament
changes drastically as it heats up from room temperature (low resistance) to
its working temperature (high resistance).
Next let’s consider an example of how the strength of the friction–like
interactions (and hence the resistance) depends on the type of material. The
heater elements of most toasters are made from a special nickel-chromium
alloy called (not very creatively) "Nichrome". You would rarely find heater
elements to be made of copper. The reason is that the structure of copper is
very different from the structure of nichrome. The consequence of this
difference in structure is that the friction-like interactions occurring when
electric charges move through copper is much, much weaker than the
friction-like interactions occurring when the electric charges move through
nichrome alloy. This means that the amount of resistance of a copper wire is
much smaller than the amount of resistance of a nichrome wire of the same
length, thickness and temperature. This is why the heater elements of a
toaster get hot enough to glow, while the copper power cord hardly warms
up at all. In the next experiment, however, you will explore a very special
situation where regular copper wires do warm up noticeably.
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Activity 3: Electric Circuits and Current
Experiment #4: What happens when a copper wire is
connected directly to both sides of a battery?
You will need:
• Fresh battery (without a battery holder) and bulb (without a bulb
holder)
• Three pieces of bare copper wire, each at least 10-cm long. (You will
not use the regular plastic covered hook-up wires for this experiment.)
Using the battery and two bare copper wires, make the bulb glow.
While the bulb is glowing, feel the two bare copper wires near where they
are in contact with the battery. Also feel the third bare copper wire that is
not connected to anything. Do the bare copper wires in the circuit feel
warmer, cooler, or about the same temperature as the stand-alone copper
wire?
Put aside the bulb. Use one piece of bare copper wire and
touch its two ends to the two ends of the battery. Keep it
there for several seconds. Feel the wire near where it is in
contact with the battery and compare its temperature to
one of the other stand-alone copper wires.
Remove the wire from the battery.
Does the bare wire connected directly to the two ends of the battery feel
warmer or cooler than the stand-alone copper wire, or does it feel about the
same temperature? Is this the same result you got above when the bulb was
included in the circuit?
Imagine that you hooked up the following two circuits. Predict how the
value of the current in the battery-ammeter circuit on the right would
compare to the value of the electric current in the battery-ammeter-bulb
circuit on the left. Use your observations above, as well as what you read in
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Cycle 5: Modified by Dr. Armen Kocharian
the previous section, to explain your reasoning. (You can assume that
although the ammeter reads the value of the electric current in a circuit, its
presence does not affect the actual value of the electric current nor how hot
the wires might become.)
Open up the same simulator that you used in Experiment #3. Delete
everything in the setup window. Construct the two circuits shown above.
Turn on the simulator.
Fill in the following values from the simulator.
Value of electric current in the battery-ammeter circuit = __________
Value of electric current in the battery-ammeter-bulb circuit = __________
How do these results compare with what your predictions?
Whenever a copper wire makes direct contact with the two sides of a battery,
or any source of electricity, the situation is called a short circuit. Even if there
is an ammeter in the circuit, the situation is still called a short circuit. The
battery-ammeter circuit above is an example of a short circuit. When you
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Activity 3: Electric Circuits and Current
touched the copper wire directly to the two ends of the battery without
including a bulb in the circuit, you made a short circuit. In a short circuit the
copper wire can get very hot because the value of the electric current in that
wire gets very high. Short circuits in household wiring and appliances can be
very dangerous. The wires can get hot enough to set fire to surrounding
materials. The function of fuses and circuit breakers is to break a connection
in the circuit if the value of the electric current gets too high.
Summarizing Questions
S1. For a constant amount of resistance (same bulb), how does the value of
the electric current depend on the size of the battery voltage?
S2. For the same battery voltage, how does the value of the electric current
depend on the amount of resistance of the bulb?
S3. How does the length and thickness of a bulb filament wire each influence
the amount of resistance of the wire? Using the blowing-through-straws
model, explain why this makes sense.
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Cycle 5: Modified by Dr. Armen Kocharian
S4. In this activity you learned the idea that the brightness of a bulb depends
both on the amount of its resistance and on the value of electric current in
it: the more resistance, the brighter the bulb; the more current, the
brighter the bulb. However, in Experiment #3 you observed that as the
resistance of the bulb was increased, the bulb became dimmer, not
brighter! This result seems to contradict the above idea. How can you
reconcile this apparent contradiction?
S5. Two students are discussing what they think a battery does that is the
same in different circuits.
Student #1: A battery provides the same amount of voltage. A battery always
provides the same strength push on the electric charges in the circuit, and push is
voltage.
Student #2: A battery provides the same amount of electric current. A battery
always pushes the electric charges at the same rate through a circuit, which
means that the electric current is always the same.
Do you agree with either student #1 or student #2, or do you have a
different interpretation of what a battery does that is the same, regardless
of the circuit? Explain your reasoning.
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Activity 3: Electric Circuits and Current
S6. Consider the following results you observed in Experiment #4. How
might you extend the blowing-through-straw model to explain why it
makes sense that the value of the electric current in the battery-ammeter
circuit is much higher than its value in the battery-ammeter-bulb circuit.
B Participate in a whole class discussion.
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Cycle 5: Modified by Dr. Armen Kocharian
How to Use the EM Devices Simulator
To start, open Cycle 5 Act 3 Setup. You will see a large setup area, and to the right is
the tools palette.
1. Place a battery, ammeter and bulb in
the setup window by first clicking on
each device in the palette, then clicking
in the setup area.
2. Practice opening and closing the switch.
Click the Select button, and then move the
cursor to the top of the handle. Drag the
handle down to close the switch. Drag it
up to open the switch. Leave the switch in
the open (handle up) position.
3. For the circuit to work, all the devices
must be wired together. To draw a wire
between the battery and bulb holder, place
the cursor arrow at one end of the battery,
then click and start dragging the cursor
towards one end of the bulb holder (top
picture).
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Switch OPEN
Switch CLOSED
Activity 3: Electric Circuits and Current
As you get near, the clip will "flash" a
small yellow rectangle (middle picture).
At that point, you can let go of the cursor
and the wire will connect itself (bottom
picture).
4. Wire together the rest of the circuit as
shown here.
5. RUN the Simulator by clicking on the
Run (Play) button.
6. Close the switch. The bulb should light
and the ammeter should show a reading of
the value of the electric current in mA.
Record the value of the current in the
Table in the activity, and note the
brightness of the bulb. Then open the
switch and STOP the simulator.
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Cycle 5: Modified by Dr. Armen Kocharian
7.
To add a second battery (thus
increasing the battery voltage from 1.5
volts to 3.0 volts), first click on the wire
between the negative end of the battery
and the bulb. This will select that wire.
Then press on the Delete key on your
keyboard to delete the wire.
8. Add another battery (from the palette)
and connect it with wires between as
shown in the picture to the right. Turn on
the simulator, close the switch, and record
the value of the electric current in the
activity table.
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