L 5-1(part 1 of 2)

Introduction to capacitors for ECE1250 at the University of Utah

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

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

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

what are the effects of the parameters of

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

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

This means that it does not require

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

Remember that it is actually storing energy.

The capacitance, which you remember from physics,

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

It is a constant epsilon knot times epsilon r.

The smallest epsilon r is one for air and

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

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

Maybe there epsilon value is

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

When epsilon increases, their capacitance increases.

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

So, as I increase that area,

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

the capacitance would also increase.

Capacitors effect that flow of charges.

The current is the change of charge dq dt.

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

The way this works is charges flow from

the positive voltage source onto the top of the capacitor.

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

away from the bottom plate leaving the bottom plate negatively

charged and the top plate positively charged,

that creates a voltage difference.

That allows us to be able to do work.

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

You're more familiar with conduction current.

Conduction current is actually the flow of charges.

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

But then right here between the two plates,

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

But the electric field forcing the charges

makes a current that we called displacement current.

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

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

Again remember, that the conduction current is

calculated as the change of charge divided by time.

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

Of course a larger A can store more charges.

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

this strong electric field increases with larger epsilon and

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

How can we build with this?

So, here's a simple parallel plate capacitor.

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

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

If we switched them or push them together,

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

If we split them back and forth,

that would be changing the effective area because

the area is only this where they are overlapping.

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

we'll be changing the epsilon.

That's how we can use capacitors as sensors.

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

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

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