[Electromagnetic Induction]
JJ: All right, Dan, what kind of setup do
we have here?
Dan: I heard that if you move a magnet through a coil of wire, it generates an EMF,
and if you attach something to it and have a circuit,
you can light up a light bulb or something.
I tried it, it seemed to work, but it was
hard to get quantitative results
so I could see what affected it.
What I did is I put a coil around this tube,
and I can drop the magnet through
so it'll be going through the coil about the same speed every time.
Then I can alter how many loops are in the
coil
to see what effect that is, too, just by winding the wire around.
JJ: It looks like that loop is connected to
this voltage sensor here.
Dan: The voltage acts as a proxy for the EMF
generated, and we can measure that using Capstone.
Let's give it a try.
JJ: Before I start recording data from that
voltage sensor, I'm just going to click zero.
Dan: Yeah, definitely zero it.
JJ: Also, notice the sample rate, the rate
that data is being collected, is really high
because I think that thing is going to go
through pretty quick.
We're going to record 1,000 data points per
second.
Are you ready?
Dan: Yep.
JJ: Click record.
Dan: It definitely did something when it went
through.
Let's expand that, see what that looks like.
JJ: Let me just zoom in here.
Oh, look at that.
There's a little bit of a dip down and then
a dip up as it goes through.
Dan: When it's falling toward the loop, the
voltage is measured to be negative,
and when it's falling away, positive.
That's interesting.
JJ: I think in this experiment, we're going
to want to figure out
what the maximum voltage is as it goes through,
whether it's negative or positive.
We want the absolute value of that, right?
Dan: Yeah. Just take the magnitude of it.
JJ: The magnitude.
Dan: It's like 0.02-something.
These are really tiny numbers.
It'd probably be a good idea to do multiple
trials.
Let's drop it, say, four times through.
JJ: I'm going to delete this run, this was
sort of a test.
I'll just click delete, and then just for
good practice, I'll click zero again,
and then I'll click record.
OK.
JJ: There's one...
Dan: There's one...
JJ: ...two...
JJ: And you're dropping the same end of the
magnet first each time, right?
Dan: Yeah, I don't have it marked North or
South, but I put a little mark just to tell.
JJ: If it was the North, the North end goes
in first every time kind of thing, right?
Dan: There's some variation there.
I think doing multiple trials was a good idea.
Let's read off the maximum and then average
them together for each pair.
JJ: It looks like, for this one, the setup
that we have,
the negative spike has a greater  
 magnitude than the positive spike.
I'm just going to go ahead and use that.
Dan: Every time?
JJ: Yeah.
This one is -0.024, but we want the magnitude
so we're just going to use...
Dan: 0.024.
JJ: 0.024.
Then the next one is 0.024, and then the next
one is 0.023, and then the next one is 0.023.
Dan: You can probably guess that average.
It's 0.0235.
I think we have a data table set up for that.
JJ: That's right.
We've got a data table that shows number of loops
and whatever the average is that we just calculated.
For one loop, our average was 0.0235.
Dan: Yeah, since it's five, we'll put that
extra place there.
Now, let's double the loops.
You want to make sure that you turn it in
the same direction as the first loop
so it's winding around in this direction.
Now I have two loops just about the same area
each, so I've doubled the area of the loops.
JJ: I see.
Because if you took the two loops and spread them out to one big loop,
that's double the area of one loop.
Dan: It'd be similar, yeah.
JJ: I got it.
Dan: This will be easier.
JJ: So two loops now, right?
Dan: But we're still dropping it from same
height, 25 centimeters.
JJ: All right.
Just let me know when you're ready.
Dan: I'm ready.
JJ: OK, go ahead.
JJ: We see the spike is noticeably taller.
Dan: Good.That means I wound it in the wrong direction.
JJ: The right direction.
Dan: I mean, right direction.
Dan: There we go.
JJ: All right.
This one, it looks like the positive spike
is the taller of the two--positive versus negative.
I'm going to use my coordinates tool, and
I'm just going to show this.
We're not actually going to write this down
or calculate the average, but we'll just pop
that there, take that value, do it for those
four runs...
Dan: We got to leave them something to do,
right?
Your job is to figure out which one of those
is the greatest peak for each pair,
and average the four values together for all four pairs
and enter that in the data table for two loops.
Let's do a third loop.
JJ: We keep the loops at the same place in
the tube for all of these runs, right?
Dan: Yeah, it's averaged around 25 centimeters.
JJ: 25 centimeters from the top?
We're dropping the magnet from 25 centimeters
above the loop every time?
Dan: Every time, pretty much.
JJ: Now we've got three loops.
I'm going to go ahead and click record, and
we're ready.
JJ: Yep, this one is taller than the previous.
JJ: That's four. That looks pretty good.
Dan: Three loops, four.
JJ: You're ready?
Dan: Ready.
JJ: Oops, that went down just a little bit.. There we go.
Dan: I have bubble wrap in here because magnets
are fragile.
We don't want it to get cracked.
JJ: There we go, that's four runs.
No, excuse me. Not four runs, four drops.
Dan: Now for five loops.
This will be the last one before we do our
analysis, see what's going on here.
There we go.
JJ: Five loops. Ready?
Dan: Yep.
JJ: Oops. Do that one again. Go ahead.
Dan: Too fast for you?
JJ: Yeah, too fast for me.
JJ: There we go.
JJ: Four.
Dan: There we go. A little variation, I still think it's a good idea to do multiple trials.
Always is.
JJ: Let's take a look at all the runs together
just to see how the amplitudes changed.
I click this button here, and then I can turn
on all of the previous runs.
You see, run one is blue, two is purple, three
is this other color here,
that's shaded a little bit, and you see the successive runs,
the spike or the magnitude gets a little bit taller.
As we increase the loops, we got more EMF
with them.
Dan: You're going to do some analysis with
the data and create a graph.
If you see a trend, you should be able to
use that trend to predict something.
In this case, you're going to predict what
the voltage would be if we had eight loops.
You're going to predict that and then so you
can see how well your prediction went,
we will collect the eight loops.
But don't cheat.
Don't look at it before you make your prediction
and I think you'll find that your prediction will work out well.
JJ: Now we need eight.
We're at five, so we need to run it three
more times.
Dan: You want to help me count here.
JJ: Sure. Five...
Dan: Make sure we have five to start with.
Five, six, seven, eight.
Always want to be careful counting.
JJ: Make sure you've got eight.
Dan: I put them so their average height, anyway, is 25 centimeters below the top.
JJ: Same as what we used for all the others.
Dan: Again, you're going to predict what will
happen
and here we're going to collect the actual...Ready?
JJ: Yep.
JJ: Looks good. JJ: There we go.
Dan: Use that to test your prediction.
We're noticing there's a little difference
between the reading for the top and bottom.
Was there any trend?
Is the one where the magnets going in higher
or when it's coming out higher?
In other words, the first versus the second.
JJ: Yeah, let's take a look at this one right
here.
Dan: There's such a tiny difference.
JJ: Let's take a look at that one there.
Dan: It's just a little bit different, but
doing all these,
you might notice that the second one is usually higher.
I was thinking that was because the magnet's going a little bit faster as it's leaving
the loop than when it's coming in, but it's
really hard to tell.
Our next investigation is going to be changing
the height of loop.
JJ: If you change the height of the loop,
you're going to still drop the magnets from the top of the tube, right?
Dan: From the top.
JJ: The distance between the drop and the
coil starts off smaller,
then it's going to be going a little bit slower when it goes through, right?
Dan: Now I have it five centimeters from the top
so the magnet will have only fallen a short distance.
It will be going kind of slow compared to
the 25-centimeter.
We're also going to title all these runs so
you'll be able to tell.
The ones we already took, we'll say, "25-centimeter
height," and then the number of loops.
Now we have eight loops, and so we'll have
eight loops and say what the height is.
JJ: Let me, just to be safe, click zero here.
We've got eight loops, five centimeters.
The magnet's going to drop five centimeters.
Dan: Not very far.
JJ: You're ready?
Dan: Yep.
JJ: There we go.
Dan: Almost too fast for you.
Dan: There's still definitely a spike even
though it's going a lot slower.
JJ: There's four.
You mentioned a second ago that one spike has a lower magnitude than the other
and you see here the negative spike goes down just past about 0.05
and this one goes about halfway between point 0.05 and 0.1.
Dan: That difference in speed shows up more
here than down at 25.
Let's bring it to 10, so we're going to double
it...
JJ: Doubling the distance.
Dan: You might wonder what happens if you
double the height?
Is it going to make it more or less and if
so, by what factor?
JJ: Ready?
Dan: Yep.
JJ: Go ahead.
Dan: Definitely more.
Dan: Doesn't look twice as much, though.
You may have to explain that as part of the lab.
Even though you're learning about electromagnetism,
you may have to do a little kinematics review to see what's going on there.
Maybe it's just because it's so small, let's
double it again.
Let's bring it up to 20.
JJ: Now you're dropping it from 20 centimeters above the coil,
so it's going to freefall for 20 centimeters...?
Dan: Yeah, pretty much.
There's a little friction, but it's pretty
much freefall.
JJ: All right. Are you ready?
Let me turn these off.
They're not gone. I just hid them so we don't see them.
All right, 20 centimeters.
Dan: The 
last one.
You're not going to do a graph with this
data,
but you are going to get the average peak voltage from each pair of those spikes,
and then see if you can figure out what's going on with the effect of height,
which is also the effect of the speed of the magnet as it goes through.
JJ: OK, great.
Dan: Good luck.
