Requirements of a Circuit
Suppose that you were given a small light bulb,
an electrochemical cell and a bare copper wire and were asked to find the four
different arrangements of the three items that would result in the formation of
an electric circuit that would light the bulb. What four arrangements would
result in the successful lighting of the bulb? And more importantly, what does
each of the four arrangements have in common that would lead us into an
understanding of the two requirements of an electric circuit?
The activity itself is a worthwhile activity and if not
performed before, one ought to try it before reading further. Like many lab
activities, there is power in the actual engagement in the activity that cannot
be replaced by simply reading about it. When this activity is performed in the
physics classroom, there are numerous observations that can be made by watching
a class full of students eager to find the four arrangements. The following
arrangements are often tried and do not result in the lighting of the bulb.
After a few minutes of trying, several healthy chuckles, and
an occasional exclamation of how hot the wire is getting, a couple of students
become successful at lighting the bulb. Unlike the above attempts, the first
successful attempt is 
characterized by the production of a complete conducting loop from the
positive terminal to the negative terminal, with both the battery and the light
bulb being part of the loop. As shown in the diagram at the right, the base of
the light bulb connects to the positive terminal of the cell and the wire
extends from the ribbed sides of the light bulb down to the negative terminal
of the cell. A complete conducting loop is made with the light bulb being part
of the loop. A circuit exists and charge flows along the complete conducting
path, lighting the bulb in the process. Compare the arrangement of the cell,
bulb and wire at the right to the unsuccessful arrangements shown above. In
attempt A, the wire does not loop back to the negative terminal of the cell. In
attempt B, the wire does form a loop but not back to the negative terminal of
the cell. In attempt C, there is no complete loop at all. Attempt D resembles
attempt B in that there is a loop but not from the positive terminal to the
negative terminal. And in attempt E, there is a loop and it does go from
positive terminal to negative terminal; this is a circuit but the light bulb is
not included as part of it. CAUTION: Attempt E will cause your fingers to get
hot as you hold the bare wire and charge begins to flow at a high rate between
the positive and negative terminals.
Light
Bulb Anatomy
Once one group of students successfully lights the bulb, many other lab
groups quickly follow suit. But then the question emerges as to what other ways
that the cell, bulb and bare wire can be arranged in such a manner as to light
the bulb. Often a short light bulb anatomy lesson prompts the lab groups into a
quick discovery of one or more of the remaining arrangements.
A light bulb is a relatively simple device consisting of a
filament resting upon or somehow attached to two wires. The wires and the
filament are conducting materials that allow charge to flow through them. One
wire is connected to the ribbed sides of the light bulbs. The other wire is
connected to the bottom base of the light bulb. The ribbed edge and the bottom
base are separated by an insulating material that prevents the direct flow of
charge between the bottom base and the ribbed edge. The only pathway by which
charge can make it from the ribbed edge to the bottom base or vice versa is the
pathway that includes the wires and the filament. Charge can either enter the
ribbed edge, make the pathway through the filament and exit
out the bottom base; or it can enter the bottom base, make the pathway through
the filament and exit out the ribbed edge. As such, there are two possible
entry points and two corresponding exit points.
The successful means of lighting the bulb as shown above
involved placing the bottom base of the bulb on the positive terminal and
connecting the ribbed edge to the negative terminal using a wire. Any charge
that enters the light bulb at the bottom base exits the bulb at the location
where the wire makes contact with the ribbed edge. Yet the bottom base does not
have to be the part of the bulb that touches the positive terminal. The bulb
will light just as easily if the ribbed edge is placed on top of the positive
terminal and the bottom base is connected to the negative terminal using a
wire. The final two arrangements that lead to a lit light bulb involve placing
the bulb at the negative terminal of the cell, either by making contact to it
with the ribbed edge or with the bottom base. A wire must then connect the
other part of the bulb to the positive terminal of the cell.
The
Requirement of a Closed Conducting Path
There are two requirements that must be met to establish an
electric circuit. The first is clearly demonstrated by the above activity.
There must be a closed conducting path that extends from the positive terminal
to the negative terminal. It is not enough that there is simply a closed
conducting loop; the loop itself must extend from the positive terminal to the
negative terminal of the electrochemical cell. An electric circuit is like a
water circuit at a water park. The flow of charge through wires is similar to
the flow of water through the pipes and along the slides at a water park. If a
pipe gets plugged or broken such that water cannot make the complete path
through the circuit, then
the flow of water will soon cease. In an electric circuit, all connections must
be made and made by conducting materials capable of carryingcharge.
As the cell, bulb and wire experiment continues, some students explore the
capability of various materials to carry a charge by inserting them in their
circuit. Metallic materials are conductors and can be inserted into the circuit
to successfully light the bulb. On the other hand, paper and plastic materials
are typically insulators and their insertion within the circuit will hinder the
flow of charge to such a degree that the current ceases and the bulb no longer
lights. There must be a closed conducting loop from the positive to the
negative terminal in order to establish a circuit and to have a current.
With an understanding of this first requirement of an
electric circuit, it becomes clear what is happening when an incandescent light
bulb in a table lamp or floor lamp no longer works. Over time, a light bulb
filament becomes weak and brittle can often break or simply become loose. When
this occurs, the circuit isopened and a closed conducting loop no longer
exists. Without a closed conducting loop, there can be no circuit, no charge
flow and no lit bulb. Next time you find a broken bulb in a lamp, safely remove
it and inspect the filament. Often times, shaking the removed bulb will cause a
rattle; the filament has likely fallen off the supporting posts that it
normally rests upon to the bottom of the glass globe. When shook, you will hear
the rattle of the filament hitting the glass globe.
The
Requirement of an Energy Supply
The second requirement of an electric circuit that is common
in each of the successful attempts demonstrated above is that there must be an electric
potential difference across the two ends of the circuit. This is most commonly
established by the use of an electrochemical cell, a pack of cells (i.e., a
battery) or some other energy source. It is essential that there is some source
of energy capable of increasing the electric potential energy of a charge as it moves from the low energy
terminal to the high energy terminal. 
As discussed in Lesson 1, it takes energy to move a positive test
charge against the electric field. As applied to electric circuits, the
movement of a positive test charge through the cell from the low energy
terminal to the high energy terminal is a movement against the electric field.
This movement of charge demands that work be done on it in order to lift it up to the higher energy terminal. An
electrochemical cell serves the useful role of supplying the energy to do work
on the charge in order topump it or move it through the cell from the
negative to the positive terminal. By doing so, the cell establishes an
electric potential difference across the two ends of the electric circuit. (The
concept of an electric potential difference and its application to electric circuits
was discussed in detail in Lesson 1.)
In household circuits, the energy is supplied by a local
utility company that is responsible for making sure that the hot and the neutral plates within the circuit panel box of
your home always have an electric potential difference of about 110 Volts to
120 Volts (in the United States). In typical lab activities, an electrochemical
cell or group of cells (i.e., a battery) is used to establish an electric
potential difference across the two ends of the external circuit of about 1.5
Volts (a single cell) or 4.5 Volts (three cells in a pack). Analogies are often
made between an electric circuit and the water circuit at a water park or a roller
coaster ride at an amusement park. In all three cases, there is something that
is moving through a complete loop - that is, through a circuit. 
And in all three cases, it is essential that the circuit include a
section where energy is put into the water, the coaster car or the charge in
order to move it uphill against its natural direction of
motion from a low potential energy to a high potential energy. A water park
ride has a water pump that pumps the water from ground level to the top of the
slide. A roller coaster ride has a motor-driven chain that carries the train of
coaster cars from ground level to the top of the first drop. And an electric
circuit has an electrochemical cell, battery (group of cells) or some other
energy supply that moves the charge from ground level (the negative terminal)
to the positive terminal. By constantly supplying the energy to move the charge
from the low energy, low potential terminal to the high energy, high potential
terminal, and a continuous flow of charge can be maintained.
By establishing this difference in electric potential, charge
is able to flow downhill through the external circuit. This motion of the
charge is natural and does not require energy. Like the movement of water at a
water park or a roller coaster car at an amusement park, the downhill motion is
natural and occurs without the need for energy from an external source. It is
the difference in potential - whether gravitational potential or electric
potential - that causes the water, the coaster car and the charge to move. This
potential difference requires the input of energy from an external source. In
the case of an electric circuit, one of the two requirements to establish an
electric circuit is an energy source.
In conclusion, there are two requirements that must be met in
order to establish an electric circuit. The requirements are
1. There must
be an energy supply capable doing work on charge to move it from a low energy
location to a high energy location and thus establish an electric potential
difference across the two ends of the external circuit.
2. There must
be a closed conducting loop in the external circuit that stretches from the
high potential, positive terminal to the low potential, negative terminal.
Check Your Understanding
1. If an electric circuit could be compared to a water
circuit at a water park, then the ...
... battery would be analogous to the ____.
... positive terminal of
the battery would be analogous to the ____.
... current would be
analogous to the ____.
... charge would be
analogous to the ____.
... electric potential
difference would be analogous to the ____.
Choices:
|
A. water
pressure |
B.
gallons of water flowing down slide per minute |
|
C. water |
D. bottom
of the slide |
|
E. water
pump |
F. top of
the slide |
If an electric circuit could be compared to a water circuit at a
water park, then the ...
... battery would be analogous to the water pump (E).
... positive terminal of the
battery would be analogous to the top of the slide (F).
... current would be analogous to the gallons of water flowing down
the slide per minute (B).
... charge would be analogous
to the water (C).
... electric potential difference would be analogous to
the water
pressure (A).
{^cosymantecnisbfw^}
2. Utilize your understanding of the requirements of an
electric circuit to state whether charge would flow through the following
arrangements of cells, bulbs, wires and switches. If there is no charge flow,
then explain why not.
|
a.
|
b.
|
|
Charge Flow: Yes or No? Explanation: |
Charge Flow: Yes or No? Explanation: |
|
c.
|
d.
|
|
Charge Flow: Yes or No? Explanation: |
Charge Flow: Yes or No? Explanation: |
|
a.
|
b.
|
|
Charge Flow: No Explanation: The
switch is in the open position, so there is no closed conducting loop. |
Charge Flow: No Explanation: There
is no closed conducting loop from + to - terminal. |
|
c.
|
d.
|
|
Charge Flow: Yes Explanation: In
this case, there is a closed conducting loop stretching from the + to the -
terminal. Thus, there is a charge flow. However, the flow does not pass
through the light bulb and so the bulb will not light. |
Charge Flow: Maybe Explanation: If
two cells are pumping charge in opposite directions as indicated by the fact
that there + terminals are connected to each other. If they have different
voltage ratings (e.g., 1.5 V and 9 V), then there will be a charge flow.
However, if their voltage ratings are identical, then there will be no charge
flow. |
3. The diagram at the right shows a light bulb connected to a
12-V car battery. The + and - terminals are shown.
a. As a +
charge moves through the battery from D to A, it ________ (gains, loses)
potential energy and ________ (gains, loses) electric potential. The point of
highest energy within a battery is the ______ (+, -) terminal.
b. As a + charge moves through the external circuit from A to
D, it ________ (gains, loses) potential energy and ________ (gains, loses)
electric potential. The point of highest energy within the external circuit is
closest to the ______ (+, -) terminal.
c. Use >, <, and = signs to compare the electric
potential (V) at the four points of the circuit.
VA 
VB VC VD
a. As a positive charge moves through the battery from D to A,
it gains potential energy
and gains electric potential.
The point of highest energy within a battery is the positive terminal.
b. As a positive charge moves through the external circuit from
A to D, it loses potential energy and loses electric potential.
The point of highest energy within the external circuit is closest to the positive terminal.
c. 
4. In the movie Tango
and Cash, Kurt Russell and Sylvester Stallone escape from a prison by
jumping off the top of a tall wall through the air and onto a high-voltage
power line. Before the jump, Stallone objects to the idea, telling Russell
"We're going to fry." Russell responds with "You didn't take
high school Physics did you. As long as you're only touching one wire and
you're feet aren't touching the ground, you don't get electrocuted." Is
this a correct statement?
Answer: Yes!
In order for
there to be a sustained flow of charge from one location to another, there must
be a difference in electric potential. In this case, there would be a momentary
flow of charge between the wire and the actor until they reach the same
electric potential. Once at the same potential, charge flow would cease and
there would be no electrocution. However, if the actor's feet touched the
ground (an electric potential of 0) or another wire of a different potential,
then there would be a sustained charge flow which likely would lead to
electrocution