Physics 215
Physics for Elementary Education
Instructor: Dr. Mark Haugan
Office: PHYS 282
haugan@purdue.edu
TA: Mayra Cervantes
Office: PHYS 222
mcervant@purdue.edu
TA: Jordan Kendall
Office: PHYS 222
kendallj@purdue.edu
TA: Daniel Whitenack
Office: PHYS 136
dwhitena@purdue.edu
Office Hours: If you have questions, just email us to make an
appointment. We enjoy talking about teaching
and learning physics!
Electric Circuit Interactions
Q1. Consider the following three arrangements of battery, bulb and wire(s). Circle the
arrangement(s) where you predict the bulb would glow. [ In the space below the
pictures, explain why you think so. If you do not think any of the bulbs would glow,
explain why not.]
A.
The tip of the bulb
touches the positive end
of the battery, on the
knob. A wire touches the
negative end of the
battery and the flat part
of the positive end of the
battery.
* B.
The screwy side of the
bulb touches the negative
end of the battery. A wire
touches the bottom tip of
the bulb and the flat part
of the positive end of the
battery.
C.
The bottom tip of the
bulb touches the negative
end of the battery. There
are two wires. One wire
touches the screwy side
of the bulb and the
negative end of the
battery. The other wire
touches the negative end
of the battery and the
knob on the positive end
of the battery.
You need a complete circuit that includes the bulb for the bulb to have an electric circuit
interaction with the battery!!!
As you discovered during your lab activities, you need to understand the
structure of bulbs like the ones in the previous question to understand when
a bulb completes a circuit.
Is there a complete circuit including
the bulb’s filament in this case?
Yes, there is a complete circuit loop
because of connections within the bulb.
Another thing you discovered is that such loops must be completed using
materials that are conductors.
When the switch is closed, this circuit is complete
and the bulb will light because iron is a
conductor.
When the switch is closed, this circuit would not
be complete if the iron nail was replaced by a
wooden stick (an insulator). The bulb would not
light in that situation.
As you saw in the case of incandescent light bulbs, both conductors and
insulators are used to construct electrical devices.
The bulb shown here would not operate
as expected if the glass bead holding
the filament supports was a conductor
or if the black stuff separating bulb’s
tip and threads was a conductor.
If they were conductors, we would have three parallel complete circuits instead
of a single circuit loop and little current would pass through the filament.
Parallel and Series Circuits
If the bead holding the filament supports
and the black stuff between the bulb’s
threads and tip were conductors we
would have additional connections in
our bulb battery circuit. These are shown
by new connecting lines in this figure.
bead
connection
blackstuff
connection
The result is a parallel circuit with three loops. Here are examples with two loops
for comparison.
equivalent to
first-bulb
connectors
and to case with connections for second loop
made at the first bulb’s connectors.
Seeing the equivalence of all these 2-loop
circuits makes it easier to see the new bulb
circuit above as a 3-loop parallel circuit.
When several bulbs or other electrical devices, e.g., a space heater, are
arranged in a single-loop circuit, we have an series circuit
1
2
The two bulbs in the circuit above and the two bulbs in this parallel circuit
constructed with an identical battery behave quite differently.
3
4
Q2. Which of the following describes the
behavior you would observe in these circuits?
A) all four bulbs equally bright, i.e., bulb1 = bulb 2 = bulb 3 = bulb 4
B) bulb 1 > bulb 2 and bulb 1 > bulb 3 = bulb 4 > bulb 2
C) bulb 2 > bulb 1 and bulb 2 > bulb 3 = bulb 4 > bulb 1
D) bulb 1 = bulb 2 > bulb 3 = bulb 4
* E) bulb 1 = bulb 2 < bulb 3 = bulb 4
Modeling Current Flow and
Energy-Based Explanations of Circuit Behavior
During the last two lab activities you found that we could build a model of
current flow using an idea from our model of electric interactions between
charged and neutral objects.
Specifically, we can use the idea that materials contain charged components
that can move around a bit inside the material to explain insulators, conductors
and current flow.
Insulators are materials in which the charged components can only shift a bit
inside the material while conductors are materials through which the charged
components can move more freely (though not completely freely, as you found).
When connected in a circuit with a battery, generator, … electric interactions
impart kinetic energy and a direction of flow to the charged components in a
conductor. This is what we call current
flow.
An ammeter measures how many charged components of a conductor flow
though part of a circuit per second.
By measuring current flow in simple circuits
like this one with batteries of different
stength (Voltage) and with bulbs whose
filaments offer different resistance (Ohms)
you concluded that
current is proportional to voltage
that current is inversely proportional to restance
These are consistent with the quantitative relationship called Ohm’s law that
scientists find relate these quantities.
Through your work with real circuits and, then, with circuit simulators you also
discovered that the flow of current is the same at every point within a single
loop of a circuit.
An ammeter measures how many charged
components of a conductor flow though
part of a circuit per second.
When you studied this circuit you found that
the same current flowed through all three
ammeters. So, the idea that current getting
“used up” in the bulbs cannot explain why the bulbs light. The current is not
“used up”. It is the same everywhere in the circuit.
So, how can we understand and explain why the bulbs light and why they light
in the ways they do in different circuits like those we considered in Q2?
We use energy-based ideas that have helped us explain other kinds of physical
behavior.
Energy Conservation in Circuits
Our model of current as moving charged objects within conductors allows us
to use ideas from our earlier work interacting, moving objects in this new
context.
A battery (or generator, …) is clearly
the source of energy in a circuit. It must
transform its chemical energy (or
mechanical energy, …) into kinetic energy
of the charged objects in the circuit’s current
by interacting with them.
Our observation that circuit components, for example, bulbs, get warm and
emit light suggests that a mechanical interaction between the moving current
components analogous to friction transforms their kinetic energy into thermal
energy. The warm bulb can then output energy to its surroundings in the
form of thermal, IR and light energy via heat conduction, IR interaction and
light interaction [more about this last category in this week’s lab activities].
We can use source/receiver and input/output diagrams
to give complete explanations of what is happening in
simple circuits like this in much the same way as we
did for other physical situations.
For example, here is I/O diagrams
for the battery and for the bulb
and here is a S/R diagram representing
the electric-circuit interaction between the
battery and bulb.
We found that energy is conserved in
these interactions, as in the others we’ve
studied.