Travis Lick
Physics Paper #2
Part I:
During the onset of this class, I was not too sure about what I knew and what I
didn’t know regarding electricity and current. I did know that I was shocked while
installing outlets in my brother’s house by an electrician who found it funny to turn on
the power to items I was currently working on. The problem was, at the time I was
simply following his instructions about what needed to connect where and I didn’t
understand why it was the case. Electricity and current seemed too obscure for me to feel
comfortable understanding. However, after a few classes of inquiry based experiments, a
light bulb went off over my head and the concepts of circuits and current have become
much more tangible and easier to understand.
A: During the first class, we discovered that there are several components of a
circuit that must be connected properly in order for the circuit to be considered complete.
First, there needs to be a power source and a conductor in order to have a complete
circuit. A conductor is an object or material that allows electric current to freely flow
through it. Through investigation, we found that most metals are conductors, some better
than others and objects such as wood, plastic and porcelain are not very good at
conducting current. These objects that don’t allow electrical current to flow through
them are called insulators.
In experiment 1.1 on the first day of class, we were asked to connect a bulb, a
single wire and a battery in order to make the bulb light. The first reaction was that it
could not be done because a second wire would be needed to connect the “other” terminal
of the battery. My understanding at this point was that each end of the battery, each
terminal, would need to have a wire from it to the metal portion of the bulb. However,
we figured out that the bulb could be directly connected to a battery terminal and the wire
could be used to connect the other terminal. We then discovered that there was a
difference where on the bulb the wire was connected. This lead us to the conclusion that
there were also two “terminals” on the bulb itself. Below is a diagram (picture 1) of one
of the ways we figured out to light the bulb. We discovered that the rivet at the bottom of
the bulb was one terminal and the threads along the sides were another. As long as each
of them was connected to a different terminal on the battery, the bulb would light.
In this picture, the rivet at the bottom of the bulb is in direct contact
with the top battery terminal and the thread of the bulb is connected to the bottom
terminal of the battery. This diagram shows a complete circuit in which the bulb would
light.
After creating this closed circuit we looked more closely at the bulb itself and saw
that there were two posts that held each of the filaments that lit when connected properly.
Through inspection and trial and error, we found that one post was directly connected to
the rivet at the bottom of the bulb and the other post was connected to the threads along
the side of the bulb. This confirmed that the bulb itself had two terminals that needed to
be connected properly in order for it to light.
From this point on, by looking at a picture of a circuit diagram containing bulbs,
batteries and wires, a determination could be made regarding if the bulb would light or
not. In order for the bulb to light, there needed to be a connection from each battery
terminal to each bulb terminal. This constituted a closed circuit in which the current
could flow from one end of the battery through a conductor or series of conductors and
back to the other terminal of the battery. By seeing the lighted bulb, this showed us that
the current was flowing through it in a circular pattern, from one terminal to the other.
However, we could not determine at this point which direction the flow was going we
could only tell that it was circular.
B: The next idea for the electrical model that we observed was that the current
was not be “used up” during its travels through the circuit. We observed this in
experiment 2.4 when we added an additional wire and an additional battery to our model.
In this experiment we observed that each bulb had the same brightness and therefore must
be receiving the same amount of current flow. Picture 2 below shows this model setup
and displays both bulbs being connected in a series.
Both bulbs in picture 2 are lit with equal brightness leading to our conclusion that
current is not used up but passes through the first bulb, into the second and back through
to the battery.
C: Our next task was to set up bulbs in a formation that was called parallel.
Based on what we had observed thus far, my understanding was that both bulbs would
have the same brightness as two bulbs in a series circuit. When constructing this model,
we found that bulbs arranged in a parallel circuit were equally as bright as a single bulb
circuit but brighter than two bulbs in a series circuit! This was one of the biggest
revelations that was made during this class because it showed us that there was something
going on with the current flow that we hadn’t observed previously and hadn’t anticipated
happening. Picture 3 below is a parallel circuit containing two identical bulbs.
Picture 3 was a tremendously important model to build because the results were
very unexpected. The bulbs in this model were brighter than the bulbs in picture 2 even
though identical bulbs, wires and battery were used. This lead to the conclusion that the
output of the battery was not constant! I am using an exclamation point in that sentence
because after experiment 2.6 was done in class, it lead to this quote with asterisks and
exclamation points around it, “Current output is not constant!” This was a big
breakthrough in understanding because it was unexpected and it lead us to the
understanding of resistance in circuits.
Through continued experimentation and model preparation, we found that circuits
that are setup in “series,” cause greater resistance and therefore less output by the battery.
The result of these series circuits is dimmer bulbs than that of single bulb setups.
Conversely, circuits that are setup in parallel, give less resistance to the battery and
therefore greater output of current. This greater current output causes an increase in bulb
brightness.
Part II:
In this section of the paper, I am going to elaborate on the topics that were
covered earlier based on additional research that I conducted.
A: In portion 1 of the paper, the requirements for a complete circuit were discussed and
the types of materials that comprise them. First, a powers source is needed in order to
send current through a conductor. This is necessary because, according to Ohms’ Law,
“to produce an electric current in a circuit, a difference in potential is required”
(Gaincoli, 2005). The two types of current are called AC and DC current and they can
both be found in common everyday applications. AC stands for alternating current while
DC stands for direct current. Alternating current is what most household appliances use
because it is safer to handle and doesn’t cause sever injuries like direct current.
Alternating current is comprised of electrons that move in both directions very rapidly as
opposed to direct current electrons that travel in one direction only. The sinusoidal path
of the AC current is different from the direct path that a DC current will produce. These
current need to flow through a material that allows this type of transport called a
conductor. A conductor is a material that has loosely bound electrons that tend to move
freely throughout it but cannot be easily removed from the object. Conversely, insulators
are made up of electrons that are bound very closely together around the nucleus and
don’t have as much mobility. When current is added to a conducting material, the charge
from the battery is sent through the material from one battery terminal to the other, this
charge excites the free moving electrons and forces them to move through the conductor.
This charge is never actually used up which will be discussed later in part II. When this
same current is added to an insulator, the electrons are not drawn to or attracted to it, so
they aren’t elevated to an excited state and therefore don’t accelerate as they did in the
conducive material (Giancoli, 2005). When a power source, such as a battery is
connected to a material that can conduct electrical current this is called a complete
circuit because the current has a path from one terminal of the battery, through the
conductor and back to the opposite battery terminal.
B: During classroom experiments, we observed that if two bulbs were connected by two
wires as shown above, the brightness of the bulbs would be the same. This was not an
expected result as we thought the added resistance would cause the current to be divided
between both bulbs and therefore make them both dimmer than the original had been.
This was observed in trial after trial and was understood to be a constant occurrence
when creating our circuit models. The behavior of this phenomenon was observed and
reported by G.R. Kirchoff in the mid 1800’s when he created his two laws of
conservation of charge. The first rule states that, “At any junction point, the sum of all
the currents entering the junction must equal the sum of all the currents leaving the
junction” (Giancoli, 2005).
image 1
In this illustration, (1), the charges going into the junction, (i2 and i3) are equal to the
charges coming out of the junction (i1 and i4).
The second law that Kirchoff proposed was that “The sum of the changes in potential
around any closed path of a circuit must be zero” (Gaincoli, 2005). The potential energy
put into a circuit/resistor must equal the potential coming out.
C:
image 2
One of the most enlightening experiments was one in which it became apparent
that the two circuits above behaved in vastly different ways regardless of the fact that
they have identical batteries with identical power output potential and also three identical
bulbs. The one difference, though it may appear slight, is the method in which they are
all connected. The first circuit is arranged in a system that is called a series circuit, while
the second is called a parallel circuit. This small difference between the two setups
dramatically changes the way the bulbs and also the power source act in each case.
“When two or more resistors are connected end to end along a single path as
shown in the first figure, they are said to be connected in series” (Gaincoli, 2005). In the
first figure, a series circuit is displayed with three bulbs or resistors, (R1, R2 and R3), and
each is given a potential voltage, (V1, V2 and V3). A series circuit has only one path for
the current to pass through, so in order for the circuit to be closed, there needs to be a
path back to the battery. There are three key items to remember when discussing series
circuits.
1. Components in a series circuit share the same current.
Itotal = I1 + I2 + I3, etc.
2. Total resistance in a series circuit is equal to the sum of the individual resistances,
making it greater than any of the individual resistances.
Rtotal = R1 + R2 + R3, etc.
3. Total voltage in a series circuit is equal to the sum of the individual voltage drops.
Etotal = E1 + E2 + E3, etc.
In the case of the series circuit above, the sum of the charge that comes out of the
battery is going to flow back into the battery and into and out of each of the resistors.
The electrical charge that leaves the battery, enters into the first resistor, R1. During
this time, it loses some of its electrical energy, due to the “work” being done to the
resistor. As the charge enters the next resistor, the same thing happens, the third and
so on. By the time the charge returns to the battery, it has lost the energy that it has
gotten from the battery. “The total of the potential drops across a resistors is the same
as the potential rise across the battery. This demonstrates that a charge can only do as
much work as was done on it by the battery” (Wagon, 1998). According to Ohm’s
law, V = IR, it is stated that because all of the resistors are connected, energy
conservation tells us that the total voltage V is equal to the total of the individual
voltages across each resistor. Finally, the more resistors that are added to the
circuit, the more current decreases through the circuit. This accounts for the
dimness of a series circuit with three bulbs as compared to a parallel circuit with three
bulbs (Giancoli, 2005).
The second figure is showing a parallel circuit with three resistors, (R4, R5 and
R6) as well as the potential voltage for each resistor, (V1, V2 and V3). The parallel
circuit is different from the series in that there is more than one route for the current
to flow towards the resistor, essentially giving the current a choice of which direction
to go. In a parallel circuit, the total current that leaves the battery splits into all of the
available resistors evenly. This causes each resistor to carry the same amount of
voltage from the battery. In a parallel circuit, the “full voltage of the battery is
applied to each resistor” (Giancoli, 2005). Because a parallel circuit is giving the
current multiple options of direction, the total resistance of a parallel circuit is less
than each of the equivalent resistances. Because of this, the more resistors that are
added to a parallel circuit, the less resistance and therefore the greater the
amount of current output. This is what can cause a household circuit to overheat
and burst into flames, the more appliances that are added increases the current output
to a point where it becomes too hot and overheats (Gaincoli, 2005).
References:
Giancoli, Douglas, C. (2005). Physics: Principles with Applications. Upper Sadlle River,
New Jersey: Pearson Prentice Hall.
Wagon, Joy (1998). The Parallel Circuit. from The Parallel Circuit Web site:
http://regentsprep.org/Regents/physics/phys03/bparcir/default.htm
Images:
Image 1: http://physics.about.com/od/electromagnetics/f/KirchhoffRule.htm
Image 2:
http://images.google.com/imgres?imgurl=http://iss.cet.edu/electricity/pages/images/G/g1
_3.jpg&imgrefurl=http://iss.cet.edu/electricity/pages/a16.xml&h=203&w=270&sz=5&hl
=en&start=3&tbnid=GyU8VvSLMyXj3M:&tbnh=85&tbnw=113&prev=/images%3Fq%
3Dseries%2Bcircuit%26gbv%3D2%26svnum%3D10%26hl%3Den%26sa%3DG