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Dialogue: 0,0:00:00.00,0:00:03.81,Default,,0000,0000,0000,,>> Hello, this is Dr. Cynthia Furse at the University of Utah.
Dialogue: 0,0:00:03.81,0:00:07.30,Default,,0000,0000,0000,,Today, I'd like to do an Introduction to Capacitors.
Dialogue: 0,0:00:07.30,0:00:11.43,Default,,0000,0000,0000,,Let's talk about what is capacitance and how it relates to current and charges,
Dialogue: 0,0:00:11.43,0:00:13.11,Default,,0000,0000,0000,,what are the effects of the parameters of
Dialogue: 0,0:00:13.11,0:00:16.44,Default,,0000,0000,0000,,the capacitor and what does it do to voltage and current?
Dialogue: 0,0:00:16.44,0:00:20.60,Default,,0000,0000,0000,,A capacitor is a passive element that stores energy in the electric field.
Dialogue: 0,0:00:20.60,0:00:22.38,Default,,0000,0000,0000,,This means that it does not require
Dialogue: 0,0:00:22.38,0:00:25.24,Default,,0000,0000,0000,,an external voltage source in order to act like a capacitor.
Dialogue: 0,0:00:25.24,0:00:27.94,Default,,0000,0000,0000,,Remember that it is actually storing energy.
Dialogue: 0,0:00:27.94,0:00:30.66,Default,,0000,0000,0000,,The capacitance, which you remember from physics,
Dialogue: 0,0:00:30.66,0:00:35.26,Default,,0000,0000,0000,,of a parallel plate is epsilon A over d. Epsilon is the dielectric material.
Dialogue: 0,0:00:35.26,0:00:38.42,Default,,0000,0000,0000,,It is a constant epsilon knot times epsilon r.
Dialogue: 0,0:00:38.42,0:00:40.91,Default,,0000,0000,0000,,The smallest epsilon r is one for air and
Dialogue: 0,0:00:40.91,0:00:43.80,Default,,0000,0000,0000,,a large epsilon r would be something like 80 for water.
Dialogue: 0,0:00:43.80,0:00:46.44,Default,,0000,0000,0000,,Plastics, glasses, et cetera are very close to air.
Dialogue: 0,0:00:46.44,0:00:47.78,Default,,0000,0000,0000,,Maybe there epsilon value is
Dialogue: 0,0:00:47.78,0:00:52.54,Default,,0000,0000,0000,,about two or even four and things that are getting closer to water would be more like 80.
Dialogue: 0,0:00:52.54,0:00:56.38,Default,,0000,0000,0000,,When epsilon increases, their capacitance increases.
Dialogue: 0,0:00:56.38,0:01:00.08,Default,,0000,0000,0000,,A is the area of the two plates that's the part that's actually is parallel between them.
Dialogue: 0,0:01:00.08,0:01:01.73,Default,,0000,0000,0000,,So, as I increase that area,
Dialogue: 0,0:01:01.73,0:01:05.72,Default,,0000,0000,0000,,the capacitance increases and as I decrease the distance of the place,
Dialogue: 0,0:01:05.72,0:01:08.31,Default,,0000,0000,0000,,the capacitance would also increase.
Dialogue: 0,0:01:08.74,0:01:11.92,Default,,0000,0000,0000,,Capacitors effect that flow of charges.
Dialogue: 0,0:01:11.92,0:01:15.52,Default,,0000,0000,0000,,The current is the change of charge dq dt.
Dialogue: 0,0:01:15.52,0:01:19.02,Default,,0000,0000,0000,,So, it's how much charge changes over a period of time.
Dialogue: 0,0:01:19.02,0:01:21.65,Default,,0000,0000,0000,,The way this works is charges flow from
Dialogue: 0,0:01:21.65,0:01:24.54,Default,,0000,0000,0000,,the positive voltage source onto the top of the capacitor.
Dialogue: 0,0:01:24.54,0:01:29.94,Default,,0000,0000,0000,,They are stored there, they get hung up there, and then they force positive charges
Dialogue: 0,0:01:29.94,0:01:32.78,Default,,0000,0000,0000,,away from the bottom plate leaving the bottom plate negatively
Dialogue: 0,0:01:32.78,0:01:35.96,Default,,0000,0000,0000,,charged and the top plate positively charged,
Dialogue: 0,0:01:35.96,0:01:37.31,Default,,0000,0000,0000,,that creates a voltage difference.
Dialogue: 0,0:01:37.31,0:01:39.88,Default,,0000,0000,0000,,That allows us to be able to do work.
Dialogue: 0,0:01:39.88,0:01:44.36,Default,,0000,0000,0000,,Displacement current is the current that appears between the two plates.
Dialogue: 0,0:01:44.36,0:01:46.34,Default,,0000,0000,0000,,You're more familiar with conduction current.
Dialogue: 0,0:01:46.34,0:01:48.44,Default,,0000,0000,0000,,Conduction current is actually the flow of charges.
Dialogue: 0,0:01:48.44,0:01:52.55,Default,,0000,0000,0000,,The fact that these charges are flowing or moving that's conducting electricity.
Dialogue: 0,0:01:52.55,0:01:55.10,Default,,0000,0000,0000,,But then right here between the two plates,
Dialogue: 0,0:01:55.10,0:01:58.97,Default,,0000,0000,0000,,it looks like there should be no current because there are no charges that can move.
Dialogue: 0,0:01:58.97,0:02:01.26,Default,,0000,0000,0000,,But the electric field forcing the charges
Dialogue: 0,0:02:01.26,0:02:04.16,Default,,0000,0000,0000,,makes a current that we called displacement current.
Dialogue: 0,0:02:04.16,0:02:07.01,Default,,0000,0000,0000,,It's the appearance that acts like there's a flow of charges, even though
Dialogue: 0,0:02:07.01,0:02:10.20,Default,,0000,0000,0000,,there aren't actually charges flowing in between the two plates.
Dialogue: 0,0:02:10.20,0:02:13.13,Default,,0000,0000,0000,,Again remember, that the conduction current is
Dialogue: 0,0:02:13.13,0:02:17.60,Default,,0000,0000,0000,,calculated as the change of charge divided by time.
Dialogue: 0,0:02:17.60,0:02:21.20,Default,,0000,0000,0000,,So, what does it mean when we talk about epsilon A over d?
Dialogue: 0,0:02:21.20,0:02:23.82,Default,,0000,0000,0000,,Of course a larger A can store more charges.
Dialogue: 0,0:02:23.82,0:02:28.58,Default,,0000,0000,0000,,Epsilon means that the attraction between the two charges or the ability to set up
Dialogue: 0,0:02:28.58,0:02:31.82,Default,,0000,0000,0000,,this strong electric field increases with larger epsilon and
Dialogue: 0,0:02:31.82,0:02:36.13,Default,,0000,0000,0000,,d of course means that it reduces the attraction between the two charges.
Dialogue: 0,0:02:36.13,0:02:38.22,Default,,0000,0000,0000,,How can we build with this?
Dialogue: 0,0:02:38.22,0:02:40.70,Default,,0000,0000,0000,,So, here's a simple parallel plate capacitor.
Dialogue: 0,0:02:40.70,0:02:43.97,Default,,0000,0000,0000,,Let's suppose that we wanted to be able to use these as a sensor.
Dialogue: 0,0:02:43.97,0:02:47.06,Default,,0000,0000,0000,,Let's consider two metal pieces with foam in between them.
Dialogue: 0,0:02:47.06,0:02:49.34,Default,,0000,0000,0000,,If we switched them or push them together,
Dialogue: 0,0:02:49.34,0:02:52.18,Default,,0000,0000,0000,,we'd be changing d, there would be change in the capacitance.
Dialogue: 0,0:02:52.18,0:02:53.87,Default,,0000,0000,0000,,If we split them back and forth,
Dialogue: 0,0:02:53.87,0:02:56.06,Default,,0000,0000,0000,,that would be changing the effective area because
Dialogue: 0,0:02:56.06,0:02:58.82,Default,,0000,0000,0000,,the area is only this where they are overlapping.
Dialogue: 0,0:02:58.82,0:03:01.58,Default,,0000,0000,0000,,Then, if wet them off with some other material between it,
Dialogue: 0,0:03:01.58,0:03:03.12,Default,,0000,0000,0000,,we'll be changing the epsilon.
Dialogue: 0,0:03:03.12,0:03:06.12,Default,,0000,0000,0000,,That's how we can use capacitors as sensors.
Dialogue: 0,0:03:06.12,0:03:10.49,Default,,0000,0000,0000,,So basically, we talked about what is capacitance and how does it relate to
Dialogue: 0,0:03:10.49,0:03:14.93,Default,,0000,0000,0000,,current and charges and the effect of epsilon A and d. In the next video,
Dialogue: 0,0:03:14.93,0:03:19.27,Default,,0000,0000,0000,,we're going to talk about what this does to current and voltage.