- [Voiceover] Some students want to know what gets
used up in an incandescent lightbulb when it is in series
with a resistor: current, energy, or both.
They come up with the following two questions.
In one second, do fewer electrons leave the bulb
than enter the bulb?
Does the electric potential energy of electrons change
while inside the bulb?
So this first question is really a measure of current
because current is how much charge per second
is going through a particular part of our circuit,
so charge per second.
You could also think of electrons per second.
So current's going to measure the first one,
and the electric potential energy of the electrons,
well that's going to be voltage.
Does the voltage change?
Is there a voltage drop when we go
from one side of the bulb to another?
The students have an adjustable power source,
insulated wire, lightbulbs, resistors,
switches, voltmeters, ammeters,
and other standard lab equipment.
Assume the power supply and voltmeters
are marked in tenths of a volt increments
and ammeters are marked in hundredths
of an amp increments.
Describe an experimental procedure
that could be used to answer questions one
and two above.
In your description, state the measurements
you would make and how you would use
the equipment to make them.
Include a neat, labeled diagram of your setup.
All right, so let's do part A right over here.
So part A, so we're going to have
a power source.
I think they said it was a variable power source.
Did they say that?
Yeah, the students have an adjustable power source,
insulated wire, lightbulbs, resistors,
switches, and voltmeters.
They want to know an incandescent bulb,
lightbulb, when it is in series with a resistor.
So an incandescent lightbulb
when it is in series with a resistor.
So let's make an attempt at drawing this.
So just the circuit, the circuit only,
that they care about,
we could do our adjustable power source.
So let me draw that.
So you could do that like this.
I'll do a couple here.
So like this.
So that would be a power source
where this is the positive end,
this is the negative end,
and then let me make my circuit
before any measurement tools.
Then we'll add the measurement tools.
So I'm going to make it in series
with a resistor.
So let's put a resistor here
of some resistance.
Then let's keep going with our circuit.
So we're going to keep going
with our circuit.
And now let's put our incandescent lightbulb.
The symbol for an incandescent lightbulb,
there's actually several,
I'll use one where I do a little bump here
and then I'll continue
and I'm actually gonna put a circle around that bump.
So it would be just like I'm almost there.
I'm trying to do it neatly
because they're telling us to do it
with a neat diagram.
All right, let me draw it a little bit better than that.
It would look something like this.
And then let me put a circle around this.
So putting a circle around this,
this is our incandescent lightbulb.
This is our power source.
To show it's a variable power source,
I can put an arrow across it like this.
That shows us that it is a variable power source.
So this is a circuit that I've just set up.
But I wanna use some ammeters and voltmeters
in order to measure what's happening
as our electrons are going through the lightbulb.
The standard convention is to show current
going from the positive terminal to the negative terminal,
but we know, and if you don't know
I encourage you to watch the Khan Academy
videos on it, what's actually happening
is you have electrons traveling
from the negative terminal
to the positive one.
But in general, if we just wanted to talk
about the current, you would denote it,
the convention is that the current
goes from the positive direction
to the negative direction.
You could view it as they're the positive gaps
of electrons or however you want to,
but the electrons are actually moving
in the other direction.
So the first question is do you have
a different number of electrons
moving per second before entering the lightbulb
then when you come out of the lightbulb?
Well the way you can measure that
is by measuring the current
on either side of the lightbulb.
The way we can measure the current
on either side of the lightbulb
is we can insert ammeters on either side.
Ammeters have to be inserted in series.
So I'm clearing up some space
so I can insert my ammeters.
So that is one ammeter.
It's going to measure current
right through that part.
So I'll put A there.
And then that is another ammeter
right over there, ammeter.
So these are going to measure current
on either side of our lightbulb.
So current measures current
on either side.
Or I could say measuring current
entering and exiting lightbulb.
Current entering and exiting
the lightbulb.
Entering the lightbulb,
or maybe I'll say exiting the bulb.
We also care about the voltage drop.
So we could put a voltmeter,
and the voltmeter can be put in parallel
with the lightbulb.
So let me draw the voltmeter here.
So I'm trying to draw it neatly.
So this is the voltmeter.
It's going to measure the voltage drop
from one side
to another.
Just connect this there.
So this measures the voltage drop.
Measures voltage.
Measures voltage drop.
So I've drawn my diagram.
Let's see, what else do I need to do?
So they say describe an experimental procedure
that could be used to answer questions
one and two above.
In your description, state the measurements
you would make and how you would use
the equipment to make them.
Include a neat, labeled diagram of your setup.
Okay, so I guess my description,
I could say I'd put two ammeters
in series
with bulb.
One before bulb.
One after.
If current same on both,
then same number of electrons
entering and exiting bulb.
If the current
is the same
on either side,
then electrons
per second
entering and exiting will be the same.
If the currents are different,
then the rate of electrons is different,
then rate of electrons
passing
are different.
All right, so that's the first part,
the first statement, to try to go
for this first statement for statement one.
In one second, do fewer electrons
leave the bulb than enter the bulb?
If the ammeters are measuring the same current,
well then you have the same number
of electrons per second entering
and leaving the bulb.
If the ammeter measures different currents,
well then you've got different numbers of electrons.
All right, now for statement two,
so let me write this,
this is statement one.
That is my procedure right over there.
Now for statement two,
statement one test I guess I could say,
and then statement two test,
I think you guys get the idea
by this point,
but I'm just writing it out
because you would have to
if you were taking this AP test,
I would say put voltmeter,
I could lowercase voltmeter,
put voltmeter
in parallel
with, let me write it out,
with bulb.
If measures voltage drop,
or I could say if and only if,
I'll say if measured voltage drop,
then, how did they actually phrase the statement?
Let's see, does the electric potential energy
of electrons change?
Then electric potential energy
of electron changes.
Did they say electric potential
energy of electron changes?
Then electric potential
energy
of electrons,
I don't normally write this much,
changes.
Otherwise, it does not.
If no voltage drop,
then no change in potential energy.
If no voltage drop,
then no change
in potential energy,
electric potential energy you could write.
And there you go.
That is part A
where I've set up my neat diagram.
I'm measuring the current entering
and exiting the lightbulb.
And actually, if you view it
from the electrons point of view,
this one is measuring the electrons
going that direction,
this is electrons entering,
electrons exiting, but either way,
the electrons per second
would affect current.
Same current?
Well then you have the same electrons per second.
Different current, then you have different
electrons per second entering
and exiting the bulb.
This measures the electric potential energy
across the lightbulb.
If you actually measure a voltage here,
then that means that the potential energy
is changing from one side
of the bulb to another.
And so I'll stop there.
Then I will,
well, actually, I think I answered part B too.
Explain how data from the experiment
you described can be used
to answer question one above.
Explain how the data from the experiment
you described can be used
to answer question two above.
So this was really parts A and B.
So actually let me write this down.
So this is A plus B right over here.
We have the diagram.
If you're taking the test,
depending on how your time pressure is,
you might want to label this more,
but if you're running out of time,
then you might not have time
to describe it in as much detail.