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- [Voiceover] Some students
want to know what gets

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used up in an incandescent
lightbulb when it is in series

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with a resistor: current, energy, or both.

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They come up with the
following two questions.

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In one second, do fewer
electrons leave the bulb

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than enter the bulb?

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Does the electric potential
energy of electrons change

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while inside the bulb?

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So this first question is
really a measure of current

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because current is how
much charge per second

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is going through a particular
part of our circuit,

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so charge per second.

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You could also think of
electrons per second.

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So current's going to
measure the first one,

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and the electric potential
energy of the electrons,

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well that's going to be voltage.

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Does the voltage change?

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Is there a voltage drop when we go

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from one side of the bulb to another?

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The students have an
adjustable power source,

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insulated wire, lightbulbs, resistors,

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switches, voltmeters, ammeters,

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and other standard lab equipment.

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Assume the power supply and voltmeters

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are marked in tenths of a volt increments

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and ammeters are marked in hundredths

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of an amp increments.

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Describe an experimental procedure

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that could be used to answer questions one

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and two above.

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In your description,
state the measurements

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you would make and how you would use

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the equipment to make them.

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Include a neat, labeled
diagram of your setup.

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All right, so let's do
part A right over here.

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So part A, so we're going to have

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a power source.

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I think they said it was
a variable power source.

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Did they say that?

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Yeah, the students have an
adjustable power source,

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insulated wire, lightbulbs, resistors,

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switches, and voltmeters.

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They want to know an incandescent bulb,

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lightbulb, when it is in
series with a resistor.

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So an incandescent lightbulb

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when it is in series with a resistor.

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So let's make an attempt at drawing this.

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So just the circuit, the circuit only,

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that they care about,

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we could do our adjustable power source.

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So let me draw that.

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So you could do that like this.

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I'll do a couple here.

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So like this.

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So that would be a power source

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where this is the positive end,

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this is the negative end,

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and then let me make my circuit

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before any measurement tools.

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Then we'll add the measurement tools.

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So I'm going to make it in series

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with a resistor.

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So let's put a resistor here

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of some resistance.

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Then let's keep going with our circuit.

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So we're going to keep going

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with our circuit.

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And now let's put our
incandescent lightbulb.

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The symbol for an incandescent lightbulb,

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there's actually several,

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I'll use one where I do a little bump here

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and then I'll continue

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and I'm actually gonna put
a circle around that bump.

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So it would be just like I'm almost there.

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I'm trying to do it neatly

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because they're telling us to do it

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with a neat diagram.

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All right, let me draw it a
little bit better than that.

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It would look something like this.

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And then let me put a circle around this.

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So putting a circle around this,

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this is our incandescent lightbulb.

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This is our power source.

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To show it's a variable power source,

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I can put an arrow across it like this.

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That shows us that it is
a variable power source.

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So this is a circuit
that I've just set up.

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But I wanna use some
ammeters and voltmeters

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in order to measure what's happening

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as our electrons are going
through the lightbulb.

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The standard convention is to show current

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going from the positive terminal
to the negative terminal,

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but we know, and if you don't know

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I encourage you to watch the Khan Academy

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videos on it, what's actually happening

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is you have electrons traveling

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from the negative terminal

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to the positive one.

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But in general, if we just wanted to talk

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about the current, you would denote it,

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the convention is that the current

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goes from the positive direction

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to the negative direction.

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You could view it as
they're the positive gaps

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of electrons or however you want to,

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but the electrons are actually moving

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in the other direction.

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So the first question is do you have

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a different number of electrons

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moving per second before
entering the lightbulb

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then when you come out of the lightbulb?

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Well the way you can measure that

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is by measuring the current

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on either side of the lightbulb.

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The way we can measure the current

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on either side of the lightbulb

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is we can insert ammeters on either side.

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Ammeters have to be inserted in series.

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So I'm clearing up some space

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so I can insert my ammeters.

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So that is one ammeter.

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It's going to measure current

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right through that part.

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So I'll put A there.

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And then that is another ammeter

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right over there, ammeter.

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So these are going to measure current

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on either side of our lightbulb.

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So current measures current

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on either side.

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Or I could say measuring current

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entering and exiting lightbulb.

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Current entering and exiting

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the lightbulb.

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Entering the lightbulb,

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or maybe I'll say exiting the bulb.

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We also care about the voltage drop.

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So we could put a voltmeter,

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and the voltmeter can be put in parallel

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with the lightbulb.

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So let me draw the voltmeter here.

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So I'm trying to draw it neatly.

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So this is the voltmeter.

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It's going to measure the voltage drop

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from one side

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to another.

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Just connect this there.

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So this measures the voltage drop.

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Measures voltage.

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Measures voltage drop.

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So I've drawn my diagram.

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Let's see, what else do I need to do?

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So they say describe an
experimental procedure

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that could be used to answer questions

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one and two above.

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In your description,
state the measurements

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you would make and how you would use

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the equipment to make them.

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Include a neat, labeled
diagram of your setup.

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Okay, so I guess my description,

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I could say I'd put two ammeters

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in series

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with bulb.

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One before bulb.

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One after.

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If current same on both,

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then same number of electrons

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entering and exiting bulb.

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If the current

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is the same

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on either side,

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then electrons

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per second

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entering and exiting will be the same.

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If the currents are different,

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then the rate of electrons is different,

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then rate of electrons

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passing

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are different.

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All right, so that's the first part,

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the first statement, to try to go

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for this first statement
for statement one.

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In one second, do fewer electrons

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leave the bulb than enter the bulb?

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If the ammeters are
measuring the same current,

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well then you have the same number

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of electrons per second entering

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and leaving the bulb.

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If the ammeter measures
different currents,

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well then you've got different
numbers of electrons.

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All right, now for statement two,

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so let me write this,

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this is statement one.

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That is my procedure right over there.

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Now for statement two,

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statement one test I guess I could say,

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and then statement two test,

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I think you guys get the idea

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by this point,

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but I'm just writing it out

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because you would have to

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if you were taking this AP test,

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I would say put voltmeter,

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I could lowercase voltmeter,

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put voltmeter

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in parallel

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with, let me write it out,

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with bulb.

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If measures voltage drop,

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or I could say if and only if,

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I'll say if measured voltage drop,

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then, how did they actually
phrase the statement?

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Let's see, does the
electric potential energy

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of electrons change?

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Then electric potential energy

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of electron changes.

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Did they say electric potential

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energy of electron changes?

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Then electric potential

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energy

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of electrons,

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I don't normally write this much,

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changes.

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Otherwise, it does not.

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If no voltage drop,

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then no change in potential energy.

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If no voltage drop,

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then no change

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in potential energy,

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electric potential energy you could write.

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And there you go.

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That is part A

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where I've set up my neat diagram.

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I'm measuring the current entering

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and exiting the lightbulb.

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And actually, if you view it

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from the electrons point of view,

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this one is measuring the electrons

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going that direction,

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this is electrons entering,

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electrons exiting, but either way,

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the electrons per second

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would affect current.

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Same current?

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Well then you have the
same electrons per second.

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Different current, then you have different

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electrons per second entering

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and exiting the bulb.

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This measures the
electric potential energy

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across the lightbulb.

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If you actually measure a voltage here,

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then that means that the potential energy

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is changing from one side

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of the bulb to another.

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And so I'll stop there.

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Then I will,

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well, actually, I think
I answered part B too.

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Explain how data from the experiment

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you described can be used

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to answer question one above.

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Explain how the data from the experiment

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you described can be used

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to answer question two above.

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So this was really parts A and B.

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So actually let me write this down.

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So this is A plus B right over here.

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We have the diagram.

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If you're taking the test,

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depending on how your time pressure is,

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you might want to label this more,

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but if you're running out of time,

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then you might not have time

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to describe it in as much detail.