ELECTRIC CURRENT
Name(s) __________________________
The primary purpose of this activity is to decide upon a model for electric current. As is the
case for all scientific models, your electricity model should be able to explain observed phenomena
and to predict the outcome of experiments.
Part 1 CREATING A MODEL OF ELECTRICITY
1)
Obtain a light bulb, a battery, and as many wires as you wish. Determine how to make the
light bulb turn on, and draw a picture of your circuit below.
Arrange your circuit components in two different ways, similar to what you just did, only this time
arrange the components so that the bulb does not turn on.
2) Draw a picture of each of your two new circuits which do not work below. Underneath each
picture, explain why you think the bulb did not go on.
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FOUR MODELS FOR ELECTRIC CURRENT
Four different models for electricity flow in a circuit lighting a light bulb are presented below.
These models represent the four most common models of electricity suggested by students. (See,
for example, Learning in Science: The Implications of Children’s Science, Osborne, R., and
Freyberg, P., Heinemann, Aukland, New Zealand, 1985.) Analyze these four models, and decide
which one seems to be the best. In order to help you decide, you should try various combinations
of bulbs and batteries and test each of the four models. You may use multiple numbers of bulbs
and batteries in any given circuit in order to carry out your tests.
In the space provided below and on the next page, make arguments and drawings showing how you
came to your conclusion.
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EN
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Model A: Charge comes out of one end of the
battery (either the positive or negative side),
goes into the bulb, and produces light.
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11
Model B: Positive charge comes from the
positive side of the battery, and negative charge
comes from the negative side of the battery.
These two different charges meet in the bulb
and produce light.
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EN
EN
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Model C: Charge comes from one end of the
battery (either positive or negative). Some of
this charge gets used up in the bulb to produce
light, and the rest goes back into the battery.
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Model D: Charge comes from one end of the
battery (either positive or negative). All of this
charge goes back into the other end of the
battery.
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DO NOT PROCEED BEYOND THIS POINT UNTIL AFTER YOU HAVE DISCUSSED THE
MODELS FOR ELECTRIC CURRENT AS A CLASS.
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PART 2
APPLICATIONS OF THE MODEL
In a sentence or two, describe a model for electric current in a circuit containing a source of power
(a battery) and a light bulb.
1)
Obtain a cardboard card with holes. Some of the holes in the card are connected to other
holes by strips of aluminum foil that are hidden under the cardboard. Without
disassembling the card, use a battery, a bulb, and perhaps some wire and determine which
holes are connected. Show your answer on the diagram below.
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A
D
2)
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B
C
Make a string of at least three identical light bulbs such that if one of the bulbs is unscrewed,
then all of the bulbs will go out. Draw a diagram of your circuit below. (You may want to
use more than one battery for this activity if your bulbs are too dim with just a single
battery.)
Note: These lights are said to be in series. What is an obvious disadvantage to this arrangement
of lights?
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3)
Arrange three identical light bulbs with one battery such that even if any one of the bulbs are
removed, the other two bulbs will not go out. Draw a diagram of your circuit below.
Note: These lights are said to be in parallel. The advantage to this arrangement of lights is
obvious. Compare the brightness of these bulbs to the brightness of the bulbs when they were in
series. Can you imagine a possible disadvantage of putting too many lights in parallel?
Experimenting with switches
Several different kinds of switches will be needed to answer questions 4 through 6.
4)
Find a way to put a switch in a circuit such that a bulb goes on when the switch in the circuit
is "closed" and off when the switch is "open". Draw a diagram of this circuit that clearly
shows the working of the switch.
5)
Make a circuit using one battery and two bulbs such that one bulb is on and the other bulb
off when a switch is in one position, and where the opposite bulb goes on and the first one
goes off when the switch is in the opposite position. Draw a diagram of this circuit that
clearly shows the working of the switch.
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6)
Make a circuit with one battery, one light bulb, and two switches such that both switches can
turn the bulb on or off no matter what position the other switch is in. This is a common
circuit found in homes. Draw a diagram of this circuit that clearly shows the workings and
positions of the two switches. (This is a hard problem! If you don’t solve it within about 15
minutes, you probably should go on to something else.)
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Electricity Content Overview
The very simplest electrical circuit is composed of three elements:
1. A source which causes electrical charges to move (This might be a battery or an electrical
power plant, for example.)
2. A conductor or carrier of the electric current (This element is the pathway through which the
electrical charges flow. It is usually a metal wire.)
3. A user - something which turns the electrical energy into some other form of energy (such as a
light bulb, an electric motor, etc.) [The concept of energy is covered more fully in Units 3 and
4.]
In the activities preceding this discussion, the batteries caused the charges to move. The charges
moved through the conductors (wires), which caused the light bulbs to turn on.
Shorthand Notations
There are standard ways of drawing electric circuits which help everyone identify the
components making up the circuit. These schematic representations can be used to save time.
1001
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EN
or a source of electrical energy =
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=
wire =
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on / off switch
=
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EN
is the same as
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ELECTRICITY CONTENT OVERVIEW
The term electrical current refers to the electric charges flowing through the conductors in a
circuit. According to the best model for electrical current, the charges moving in an electrical circuit
move in a circuitous path (i.e., they eventually return to where they started), they do not get “used
up” in the circuit. A diagram depicting this motion is shown below. There is just as much current
(moving charges) at any location within this circuit as there is at any other point in the circuit.
Charges moving away
from energy source
element turning electrical
energy into light
Source of energy to
make charges flow
Charges moving toward
energy source
Charges moving away
from energy source
If there is a branch in a circuit, then the
electrical current “splits”: some of the charges
go one way, and the rest go the other way, but
eventually all the charges come back together
again. In the circuit shown at the right, half of
the current would go through the bulb on the
left, and half of the current would go through
the bulb on the right. At all times there are just
as many charges flowing away from the energy
source as there are charges flowing into the
energy source.
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current
splits here
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Charges moving toward
the energy source
current
recombines
here
All materials contain both positive and negative charges. Therefore, it is not the job of the
battery (or any other source of electrical energy in a circuit) to provide the charges for a circuit.
The battery simply causes the charges already present to start moving.
In almost all electrical circuits, including all the ones used in this course, it is the negative
charges which flow to make the electrical current. However, it is very difficult to prove that this is
the case. In fact, mostly due to historical reasons, most books talk about electrical circuits as if it
were the positive charges that are moving. In order to be consistent with this convention, all the
diagrams presented here have also been drawn as if the positive charges were flowing through the
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ELECTRICITY CONTENT OVERVIEW
circuits. Remember, as far as most applications are concerned, it is impossible to tell whether or not
it is the positive or the negative charges that are flowing. In fact, you may wish to reconsider your
model for electricity in order to show that your model will work equally well no matter which kind
of charge you assume is moving.
There are several analogies that can be thought up to help understand electrical circuits. Some
people find them helpful, others find them confusing. One of the more creative ones is to picture
the electrical charges as little peasants carrying backpacks. A peasant on the “road” (wire) from
battery to bulb has a backpack containing electrical energy. The peasant goes through the bulb and
delivers this energy to the light bulb, and then trundles back toward the other end of the battery for
a refill. Peasants are not consumed at the bulb, they just empty their packs at the bulb. Similarly,
the battery does not create the peasants, but simply fills their packs. If two bulbs are connected in
series, each peasant has to go through both bulbs, and gives up only part of its energy at each. If
two bulbs are connected in parallel, then some peasants go through one bulb (giving up all their
energy), while others go through the other bulb. Of course, this analogy isn’t perfect (no analogy
is), but some find it useful.
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ELECTRICITY CONTENT OVERVIEW
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