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>> Hello, this is Dr. Cynthia Furse at the University of Utah.

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Today, I'd like to do an Introduction to Capacitors.

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Let's talk about what is capacitance and how it relates to current and charges,

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what are the effects of the parameters of

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the capacitor and what does it do to voltage and current?

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A capacitor is a passive element that stores energy in the electric field.

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This means that it does not require

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an external voltage source in order to act like a capacitor.

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Remember that it is actually storing energy.

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The capacitance, which you remember from physics,

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of a parallel plate is epsilon A over d. Epsilon is the dielectric material.

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It is a constant epsilon knot times epsilon r.

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The smallest epsilon r is one for air and

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a large epsilon r would be something like 80 for water.

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Plastics, glasses, et cetera are very close to air.

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Maybe there epsilon value is

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about two or even four and things that are getting closer to water would be more like 80.

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When epsilon increases, their capacitance increases.

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A is the area of the two plates that's the part that's actually is parallel between them.

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So, as I increase that area,

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the capacitance increases and as I decrease the distance of the place,

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the capacitance would also increase.

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Capacitors effect that flow of charges.

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The current is the change of charge dq dt.

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So, it's how much charge changes over a period of time.

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The way this works is charges flow from

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the positive voltage source onto the top of the capacitor.

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They are stored there, they get hung up there, and then they force positive charges

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away from the bottom plate leaving the bottom plate negatively

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charged and the top plate positively charged,

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that creates a voltage difference.

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That allows us to be able to do work.

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Displacement current is the current that appears between the two plates.

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You're more familiar with conduction current.

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Conduction current is actually the flow of charges.

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The fact that these charges are flowing or moving that's conducting electricity.

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But then right here between the two plates,

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it looks like there should be no current because there are no charges that can move.

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But the electric field forcing the charges

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makes a current that we called displacement current.

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It's the appearance that acts like there's a flow of charges, even though

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there aren't actually charges flowing in between the two plates.

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Again remember, that the conduction current is

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calculated as the change of charge divided by time.

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So, what does it mean when we talk about epsilon A over d?

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Of course a larger A can store more charges.

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Epsilon means that the attraction between the two charges or the ability to set up

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this strong electric field increases with larger epsilon and

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d of course means that it reduces the attraction between the two charges.

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How can we build with this?

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So, here's a simple parallel plate capacitor.

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Let's suppose that we wanted to be able to use these as a sensor.

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Let's consider two metal pieces with foam in between them.

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If we switched them or push them together,

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we'd be changing d, there would be change in the capacitance.

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If we split them back and forth,

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that would be changing the effective area because

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the area is only this where they are overlapping.

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Then, if wet them off with some other material between it,

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we'll be changing the epsilon.

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That's how we can use capacitors as sensors.

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So basically, we talked about what is capacitance and how does it relate to

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current and charges and the effect of epsilon A and d. In the next video,

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we're going to talk about what this does to current and voltage.