Download Investigation 14

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Investigation 14
Electric Circuits
I.
Complete circuits
A.
Use one battery, one light bulb, and one piece of wire. Connect these together in a variety of ways
such that the bulb lights. Sketch each arrangement below.
Arrangements that do light the bulb
Arrangements that do not light the bulb
You should have found four different arrangements that light the bulb. Examine the arrangements
that light and do not light the bulb. State the requirements that must be met in order for the bulb to
light.
B.
When a student briefly connects a wire across the terminals of a
battery, the wire feels warm. Why does the wire get warm?
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1
2
The student finds that the wire seems to be equally warm at points 1, 2, and 3. Based on this
observation, what might you conclude about the amount of current flow at each place in the wire?
C.
Light a bulb using a battery and a single wire (One of your connections in part I, A.). Observe and
record the brightness of the bulb when objects made out of various materials are inserted between the
bulb and the battery. Try materials such as paper, various coins, pencil lead, eraser, your finger, etc.
Material
bulb lights
bulb does not light
Coin
Piece of paper
What is similar about the materials that let the bulb light?
D.
filament
Carefully examine a bulb. Two wires extend from the filament of the bulb into
the base. You cannot see into the base, however, you should be able to make a
good guess as to where the wires of the filament are attached to the base.
Draw where the wires attach.
base of
bulb
II.
Single Bulb Circuit
A.
The diagram shows a circuit with a single battery and a single bulb. Note that a
complete and unbroken path is present that allows electrons to flow from the
negative end of the battery, through the bulb, then back to the battery entering
the positive end. Because there is only one path for the electrons, the electron flow
or the electric current is the same at all points in the circuit. That is, the amount of
current flowing past point A is the same as flowing past point B, and is also the
same flowing through the bulb.
glass or
ceramic
A
B
B.
The strength of the current depends on the impeding effect of the bulb – its resistance. If the
resistance increases, then what happens to the current flow?
C.
If the current through the bulb is large, then that means a lot of energy is being delivered to the bulb
by the electric current. Do you think that the light from the bulb be brighter when the current is
large or when the current is small? Explain.
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A
III. Bulbs in series
A two-bulb circuit with identical bulbs connected one after the other as shown.
Bulbs connected in this way are said to be connect in series.
A.
B
Notice that there is still only one path for the electrons to take going from the
negative end of the battery, through the bulbs, and then back to the battery.
1.
In this circuit, is the current the same flowing past points A, B, and C?
C
B.
2.
As a result of inserting a second bulb, do you think that the resistance to the flow of current
increases, decreases, or remains the same?
3.
In this case, will the current increase, decrease, or remain the same?
4.
Will the brightness of each of the two bulbs be the same? Why?
5.
If you were to compare the brightness each of the two bulbs in this circuit to the brightness
when only one bulb is connected (as in II-A above), then would each of the two bulbs be
brighter, dimmer, or the same brightness as the single bulb?
6.
If one of the bulbs were to burn out (the filament breaks), what will happen? Why?
We know that a bulb causes an impeding effect (or resistance) to the current in the circuit.
1.
Thinking of the bulb in this way, would adding more bulbs in series cause the impeding effect
(or total resistance) to increase, decrease, or remain the same as before.
2.
Formulate a rule for predicting how the current coming from the battery would change (i.e.,
whether it would increase, decrease, or remain the same) if the number of bulbs connected in series
were to be increased.
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IV. Bulbs in parallel
A
A two-bulb circuit with identical bulbs so that their terminals are
connected together is shown. Bulbs connected together in this way
are said to be connected in parallel.
A.
V.
A.
Notice that as the electron current leaves the negative end of the
battery, it reaches point B and divides, part going through the left
bulb and part going through the right bulb. Because the bulbs are
identical, we can assume the electron current through both bulbs is
the same and, therefore, the bulbs will be of the same brightness.
B
1.
If you were to unscrew the right bulb from the circuit, then notice that the circuit with just the
left bulb looks just like the single bulb circuit in section II-A. It is also of the same brightness.
2.
If you screw the right bulb back in, the brightness of the left bulb does not change, and the
brightness of both bulbs will be the same. This means that the current in each of the bulbs is the
same, and that the battery must now be delivering twice the amount of current than when
lighting only the left bulb.
3.
So, again, how does the amount of current coming from the battery when connected to a single
bulb compare to the current coming from a battery when connected to a two bulbs in parallel?
(More current delivered, less current delivered, or the same amount of current delivered?)
4.
How does the current coming from the battery change (i.e., whether it would increase, decrease, or
remain the same) if the number of bulbs connected in parallel were increased?
5.
How does the resistance of the circuit change (i.e., whether it would increase, decrease, or remain
the same) if the number of bulbs in parallel were increased?
Circuits – Conclusions
As we connect more and more bulbs in series to a battery, we know that the total resistance of the
circuit ____________________________(increases, decreases, or remains the same), and the current
delivered by the battery _______________________(increases, decreases, or remains the same).
B.
As we connect more and more bulbs in parallel to a battery, we know that the total resistance of the
circuit _____________ _______________(increases, decreases, or remains the same), and the current
delivered by the battery _________________________(increases, decreases, or remains the same).
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VI. Application of These Ideas to Your Home Electrical Circuits
The batteries in the circuits described above act very much like the voltage sources supplied to your
home by the electric company, except that the batteries deliver DC and the electric company delivers
AC. This voltage supplies the energy needed to power the electrical devices in your home. The
bulbs in the above investigation act like the electrical devices in your home.
A.
From what you found in this investigation, do you think that the electrical devices in your home are
connected in series or parallel? How do you know?
B.
Why is it dangerous to connect too many lights and electric devices to one electrical circuit?
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