Download I. Complete circuits

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Investigation 13
Electric Circuits
In this investigation, we construct a model for electric current that we can use to predict and explain the
behavior of simple electric circuits.
I.
Complete circuits
A.
Obtain a battery, a light bulb, and a one piece of wire. Connect these in a variety of ways. 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.
THERE MUST BE A COMPLETE PATH FOR THE CURRENT FROM, ONE END OF THE BATTERY
TO THE OTHER. THE COMPLETE PATH MUST TRAVEL THROUGH THE BULB FROM THE
BASE TO THE SIDE SCREW PART OF THE BULB.
B.
When a student briefly connects a wire across the terminals of a
battery, the wire feels warm. Why does the wire get warm?
THE WARMTH IS DUE TO THE HEATING OF THE WIRE
CAUSED BY THE CURRENT FLOWING THROUGH THE WIRE.
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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 is happening in the wire at each
place compared to the others?
THE HEAT IS DUE TO THE CURRENT. IF THE HEATING IS THE SAME AT POINTS 1, 2, AND 3,
THEN THAT MEANS THAT THE CURRENT IS THE SAME AT THOSE POINTS.
C.
Light a bulb using a battery and a single wire. Observe and record the brightness of the bulb when
objects made out of various materials are inserted into the circuit. Try materials such as paper, coins,
pencil lead, eraser, your finger, etc.
What is similar about the objects that let the bulb light?
THE MATERIALS THAT LET THE BULB LIGHT ARE ALL METALS.
D.
Carefully examine a bulb. Two wires extend from the filament of the bulb into
the base. You probably cannot see into the base, however, you should be able
to make a good guess as to where the wires are attached. Describe and draw
where the wires attach. Explain based on your observations in parts A to C.
THE CURRENT MUST PASS THROUGH THE FILAMENT. THE CURRENT
IS DELIVERED TO THE BASE OF THE BULB, TRAVELS THROUGH THE
FILAMENT, THEN EXITS AT THE SIDE OF THE BULB.
On the basis of the observations that we have made, we will make the following assumptions:
1.
When an unbroken path exists from one terminal of the battery, through the rest of the circuit, and
then back to the other terminal of the battery, we have a complete and continuous path for the flow
of electrons. We call this flow electric current.
2.
The brightness of a bulb can be used as an indicator of the amount of current through that bulb: the
brighter the bulb, the greater the current.
We will use these assumptions when examining and deducing the characteristics of various circuits.
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II.
Bulbs in series
(In this section and in section III, use two batteries in the holder as your
source.)
Set up 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.
Compare the brightness of the two bulbs with each other. (Pay attention only
to large differences in brightness. You may notice minor differences because
two “identical” bulbs are, in fact, not quite identical.)
Use the assumptions we have made in developing our model for electric current to answer the
following questions:
1.
Is the current the same through both bulbs? How do you know?
CURRENT IS THE SAME THROUGH BOTH BULBS.
2.
Do you think that switching the order of the bulbs might make any difference in the brightness
of the bulbs? Check your answer.
NO DIFFERENCE
3.
If you were to unscrew one of the bulbs from the circuit, what do you think will happen? Did
it? Why?
THE BULBS GO OUT.
B.
A
Compare the brightness of each of the bulbs in the two-bulb circuit with that of a
bulb in a single-bulb circuit. (A quick way to connect a single-bulb circuit is to
disconnect the wire from point A, and attach it to point B. The upper bulb is no
longer “in the circuit.” There is no complete path for current to flow.)
B
THE BULBS ARE DIMMER IN A TWO-BULB CIRCUIT.
Use the assumptions we have made in developing our model for electric current
to answer the following questions:
1.
How does the current through a bulb in a single-bulb circuit compare with the current through the
same bulb when it is connected in series with a second bulb in a two-bulb circuit?
CURRENT IN THE SINGLE-BULB CIRCUIT IS GREATER – THE BULB IS BRIGHTER.
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2.
What does your answer to question 1 imply about how the current coming from the battery in a
single-bulb circuit compares to the current coming from the battery in a two-bulb series circuit?
That is, is the current coming from the battery in a single-bulb circuit greater than, less than, or the
same as the current coming from the battery in a two-bulb series circuit?
CURRENT COMING FROM THE BATTERY IN A SINGLE-BULB CIRCUIT IS GREATER THAN
THE CURRENT IN A TWO-BULB CIRCUIT.
C.
We may think of a bulb as causing 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.
ADDING BULBS WILL INCREASE THE RESISTANCE.
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 or decreased.
AS THE NUMBER OF BULBS INCREASE, TE CURRENT COMING FROM THE BATTERY
DECREASES. THERE IS MORE RESISTANCE IN THE CIRCUIT.
III. Bulbs in parallel
Set up a two-bulb circuit with identical bulbs so that their terminals
are connected together as shown. Bulbs connected together in this
way are said to be connected in parallel.
A.
Compare the brightness of the bulbs in this circuit. Are they the
same brightness? (Remember that minor differences do not matter.)
THE BULBS HAVE THE SAME BRIGHTNESS.
1.
From your observation of the brightness of the bulbs, what can you conclude about the amount
of current through each bulb?
THE CURRENT THROUGH EACH BULB IS THE SAME.
2.
If you were to unscrew one of the bulbs from the circuit, what do you think will happen to the
other bulb? Why?
THE BRIGHTNESS DOES NOT CHANGE. THE CURRENT DOESN’T CHANGE.
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B.
Compare the brightness of each of the bulbs in the two-bulb circuit with that of a bulb in a singlebulb circuit. (When you unscrew one bulb from the circuit, you now have a single-bulb circuit.)
1.
Did the brightness of the remaining bulb in the single-bulb circuit change (remember
substantially) when the first bulb was unscrewed? What does this say about the current
through the remaining bulb now in comparison to the current through it before?
THE BRIGHTNESS DID NOT CHANGE. THIS SAYS THAT THE CURRENT THROUGH IT
ALSO DOES NOT CHANGE.
2.
Is the brightness of each bulb in the two-bulb parallel circuit greater than, less than, or equal to that
of a bulb in a single-bulb circuit?
SAME BRIGHTNESS
3.
How does the amount of current coming from a battery connected to a single bulb compare to the
current coming from a battery connected to a two-bulb parallel circuit? (More current delivered,
less current delivered, or the same amount of current delivered?)
THERE IS LESS CURRENT COMING FROM THE BATTERY IN THE SINGLE-BULB CIRCUIT
COMPARED WITH THE TWO-BULB CIRCUIT.
C.
Based on your observations above, 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 parallel were increased or decreased.
THE CURRENT COMING FROM THE BATTERY WILL INCREASE AS THE NUMBER OF BULBS
INCREASE.
What can you infer about the total resistance of a circuit as the number of bulbs connected in parallel
is increased or decreased?
AS THE NUMBER OF BULBS IN PARALLEL IS INCREASED, THE RESISTANCE OF THE
CIRCUIT DECREASES.
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IV. Circuits – Conclusions
Circuits consisting of bulbs (resistances) can be connected to batteries in series or in parallel (or in any
combinations of series and parallel).
As we connect more and more bulbs in series to a battery, we know that the total resistance of the
circuit ______INCREASES______________, and the current delivered by the battery
____DECRESASES__________.
As we connect more and more bulbs in parallel to a battery, we know that the total resistance of the
circuit ____DECREASES________________, and the current delivered by the battery
___INCREASES____________.
V.
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?
CONNECTED IN PARALLEL. WHEN A BULB GOES OUT OR AN APPLIANCE IS
DISCONNECTED, ALL THE OTHERS DO NOT GO OUT.
B.
Why is it dangerous to connect too many lights and electric devices to one electrical circuit?
CONNECTING MORE LIGHTS OR ELECTRICAL DEVICES WILL CAUSE THE CURRENT TO
INCREASE. THE INCREASE CAN BE SUCH THAT A LOT OF HEAT IS PRODUCED POSSIBLY
CAUSING A FIRE.
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