[Capacitor Fundamentals]
Dan: Hey, JJ.
What are you doing with the aluminum foil?
Are you going to do some baking?
JJ: [laughs] [jokingly] I'm going to bake.
No, I'm making a capacitor.
Just like a parallel plate capacitor in our textbooks, you can make a parallel plate capacitor
using pieces of tin foil.
These are the plates basically.
Dan: That's not a capacitor.
That's a capacitor.
How can you do it out of that?
How does that work?
JJ: The capacitor that you have in your hand is actually just like this except it's a super-narrow
strip that's been cut and rolled up into a tube.
Dan: OK.
If we roll this into a tube, it'd look like this.
Got it.
JJ: The trick is you've got two conducting plates that are separated by some sort of
a dielectric.
In this case in our capacitor, the dielectric is just a sheet of paper.
Dan: Then the orange and blue thing there?
JJ: This is a capacitance meter.
Our capacitor has some capacitance.
It's really small.
We need a meter that can measure it really small, which is what we have here.
When we touch these probes onto these tabs that we have sticking out here, we can measure
the capacitance of our capacitor.
In this experiment, what we're going to do is we're going to test the effect of a couple
things on how much capacitance we can have here.
One of them is the area.
This aluminum sheet has some area.
Dan: These are about the same.
I'm just going to measure the length and width of one of them and get the area from that.
That's like 23.5 and 19.5.
I'll enter that in centimeters.
Oh, the area just calculated it for me.
JJ: [laughs]
Dan: I was all ready with my calculator.
JJ: Before we can measure the capacitance of our capacitor, we got to make sure there's
a dielectric in between -- that's the piece of paper -- and that they're lined up.
Dan: Isn't there some air in between, too?
JJ: There is air.
Actually, we'll see in the second part of this experiment how changing the distance
between them changes the capacitance.
To help us get rid of the air and make sure that the thickness is just one paper thickness
between the plates of our capacitor, we're just going to take a textbook and put it on there.
It's going to smash things nice and flat.
Dan: Well, that's not just any textbook, JJ.
JJ: [laughs]
Dan: That's the PASCO Essential Physics textbook.
JJ: That's right.
Dan: You can get it online, too.
JJ: I'm going to go ahead and just put the textbook on there.
It's going to press those plates together.
We're going to have a thickness of one sheet of paper between them.
Our tabs are still sticking out here.
We can take the capacitance meter and then just connect them to those tabs.
We can measure the capacitance of our capacitor.
It looks like it's about 2.96 nanofarads.
Dan: To change the area, we could cut it and stuff.
Is there a simpler, faster way there?
JJ: In fact, the fastest way is just fold it in half.
We'll cut the area in half.
I'll tell you what.
I'll fold one on the top plate.
You fold the bottom plate.
We'll just go lengthwise or this way, like that.
Just get it to the bottom.
Dan: Does it matter if we have the shiny side out or not?
JJ: No, it shouldn't.
Dan: Good, because we're different.
I'll just put the length now as half of what it was.
That'd be 11.25.
The width is the same.
JJ: There's our bottom plate.
Dan: That's not right.
It's 11.75.
There we go.
JJ: There you go.
We've taken the same original area.
We fold it in half.
This is our new area because the aluminum foil is conducting.
When we smash it flat again, the top sheet touches the bottom sheet.
The whole thing is now effectively one plate but half the area.
We'll do our one-paper-thick.
Make sure the tab sticks out on the end.
Then we'll take this one, and we'll put it on there.
We don't want these tabs touching each other.
I'll put them close but not touching.
Dan: I would guess with less area, it'll be less capacitance.
JJ: Let's take a look.
I'm going to smush it flat again so we get that one-paper-thick.
Let's bring the mirror over and adjust these contacts.
Now our capacitance is 1.59 nanofarads.
Dan: It's definitely less.
Now fold the...
JJ: You can take the top one off of the bottom one.
Dan: With the long side.
JJ: Fold it in half again.
[background noise]
Dan: The length stays the same.
Then it's 9 point...
JJ: Oh, I have the bottom.
That's right.
We'll put that there and put this on top.
Make sure the tabs aren't touching again.
They're also over each other and then book on top again.
We get one-paper-thick between them.
Dan: It probably will go down again.
JJ: I'll bring this over here so we can see it.
I'm seeing 1.12 nanofarads.
Dan: Nano is a billion, so this is not a lot of capacitance.
That's why we have this special meter to measure it accurately.
JJ: Are we going again?
Dan: Two more times.
JJ: Two more times, so the tabs are a little fat here to make this work.
What I'm going to do is I'm just going to tear...
Dan: Tear them a little, and flip them as well.
JJ: Tear a little.
Tear a little bit off here.
I guess we can just fold them.
I'm going to tear it and fold it so its halves are a little bit thinner.
You want to do that one?
We'll fold them in half again.
[background noise]
Dan: Looks like we went the same way.
Now that is 5.88.
JJ: The area's going down by a half each time, right?
Dan: So far, yes.
JJ: All right.
Now let's see what we're working with here.
0.61 nanofarads.
Dan: OK, one more.
JJ: One more?
Dan: At some point, it'll be too hard to fold.
JJ: [laughs] You take that one.
I'll take this one.
[background noise]
JJ: The plates are now tiny.
I have the bottom.
You have the top.
There at the bottom.
There at the top.
Make sure the tabs aren't touching.
I'll smush it with the book and get that one paper-thin.
I'll just give it a press to make sure that all the folds get flattened out nice.
This capacitance value is 0.43.
Dan: We have five data points.
You can take this and do the analysis for the effect of area on capacitance.
The other thing we were interested in was the distance between the two.
How are we going to change that?
JJ: Let's get some new capacitor plates.
I guess we could have reused those and tried to carefully unfold them.
Sometimes it's just easier to start with some fresh ones.
There's a bottom plate.
Then here's a top plate.
We're going to make another capacitor.
Now what we're going to do is we're going to change the distance in between the plates
by adding more sheets of paper in between.
Dan: That'll be easier.
JJ: Did we start with one sheet of paper?
Dan: Yeah.
JJ: OK.
Again, I'm going to try to make sure these are lined up.
Dan: It'd be interesting to see how close it is to our first measurement.
They're similar in size to the first one.
JJ: Ah, right.
Dan: This first one was 2.96 with one sheet.
Let's see how close we come to that.
JJ: I'll put the book back on there.
This one is 3.26.
Dan: That's still not too different.
JJ: Not too different, so our plates might not be the exact same area.
Let me put that over there.
Let me add another sheet of paper.
I should be bringing the paper over here.
I'm just going to double the distance in between the plates now by adding another sheet of
paper.
Now, our new capacitance value, 2.40, went down.
Dan: The further apart, it seems like that makes the capacitance go down.
What kind of relationship is that called?
JJ: That's a inverse relationship, right?
Dan: Inverse.
It's looking like with two points you can't really say much now.
JJ: Let's triple the distance from our original data point.
I'm going to try to slide this sheet of paper in between the area there and make sure I
line the plates up properly.
Let me just double-check.
That looks pretty good.
Now, I'll put the book on top.
We've got three sheets of paper spacing between the capacitor plates.
Three sheets of spacing, 1.96 nanofarads.
Dan: A definite trend going down.
JJ: There's three sheets of paper.
Dan: Instead of paper, what would a real capacitor use?
JJ: Capacitors use any kind of dielectric in between the plates.
Some capacitors use air because air acts as a pretty good dielectric.
Some capacitors like the one you have there have a special dielectric material with a
nonconducting glue that holds them together.
Dan: It has to be nonconducting.
JJ: What do we have?
Four sheets?
Dan: Yes.
JJ: Four sheets of paper and the capacitance is 1.65.
Is this our last data point?
Dan: Yeah.
JJ: Five sheets of paper, so the spacing between our plates now is five times what our very
first data point was.
I'm going to put this book on there.
I'll just make sure the book is down.
It gets rid all the air in between it, so we have consistent changes in distance.
That capacitance is 1.46.
Dan: A definite downward trend.
We will leave the rest of the analysis up to you.
You have access to this data file.
You can watch the video again if you need to.
You also have the student handout for this lab to learn more about capacitance.