WEBVTT 00:00:00.000 --> 00:00:03.810 >> Hello, this is Dr. Cynthia Furse at the University of Utah. 00:00:03.810 --> 00:00:07.305 Today, I'd like to do an Introduction to Capacitors. 00:00:07.305 --> 00:00:11.430 Let's talk about what is capacitance and how it relates to current and charges, 00:00:11.430 --> 00:00:13.110 what are the effects of the parameters of 00:00:13.110 --> 00:00:16.440 the capacitor and what does it do to voltage and current? 00:00:16.440 --> 00:00:20.600 A capacitor is a passive element that stores energy in the electric field. 00:00:20.600 --> 00:00:22.380 This means that it does not require 00:00:22.380 --> 00:00:25.245 an external voltage source in order to act like a capacitor. 00:00:25.245 --> 00:00:27.945 Remember that it is actually storing energy. 00:00:27.945 --> 00:00:30.660 The capacitance, which you remember from physics, 00:00:30.660 --> 00:00:35.265 of a parallel plate is epsilon A over d. Epsilon is the dielectric material. 00:00:35.265 --> 00:00:38.415 It is a constant epsilon knot times epsilon r. 00:00:38.415 --> 00:00:40.910 The smallest epsilon r is one for air and 00:00:40.910 --> 00:00:43.795 a large epsilon r would be something like 80 for water. 00:00:43.795 --> 00:00:46.435 Plastics, glasses, et cetera are very close to air. 00:00:46.435 --> 00:00:47.780 Maybe there epsilon value is 00:00:47.780 --> 00:00:52.535 about two or even four and things that are getting closer to water would be more like 80. 00:00:52.535 --> 00:00:56.375 When epsilon increases, their capacitance increases. 00:00:56.375 --> 00:01:00.080 A is the area of the two plates that's the part that's actually is parallel between them. 00:01:00.080 --> 00:01:01.730 So, as I increase that area, 00:01:01.730 --> 00:01:05.720 the capacitance increases and as I decrease the distance of the place, 00:01:05.720 --> 00:01:08.310 the capacitance would also increase. 00:01:08.740 --> 00:01:11.915 Capacitors effect that flow of charges. 00:01:11.915 --> 00:01:15.515 The current is the change of charge dq dt. 00:01:15.515 --> 00:01:19.025 So, it's how much charge changes over a period of time. 00:01:19.025 --> 00:01:21.650 The way this works is charges flow from 00:01:21.650 --> 00:01:24.535 the positive voltage source onto the top of the capacitor. 00:01:24.535 --> 00:01:29.945 They are stored there, they get hung up there, and then they force positive charges 00:01:29.945 --> 00:01:32.780 away from the bottom plate leaving the bottom plate negatively 00:01:32.780 --> 00:01:35.960 charged and the top plate positively charged, 00:01:35.960 --> 00:01:37.310 that creates a voltage difference. 00:01:37.310 --> 00:01:39.875 That allows us to be able to do work. 00:01:39.875 --> 00:01:44.360 Displacement current is the current that appears between the two plates. 00:01:44.360 --> 00:01:46.340 You're more familiar with conduction current. 00:01:46.340 --> 00:01:48.440 Conduction current is actually the flow of charges. 00:01:48.440 --> 00:01:52.550 The fact that these charges are flowing or moving that's conducting electricity. 00:01:52.550 --> 00:01:55.100 But then right here between the two plates, 00:01:55.100 --> 00:01:58.970 it looks like there should be no current because there are no charges that can move. 00:01:58.970 --> 00:02:01.265 But the electric field forcing the charges 00:02:01.265 --> 00:02:04.160 makes a current that we called displacement current. 00:02:04.160 --> 00:02:07.010 It's the appearance that acts like there's a flow of charges, even though 00:02:07.010 --> 00:02:10.205 there aren't actually charges flowing in between the two plates. 00:02:10.205 --> 00:02:13.130 Again remember, that the conduction current is 00:02:13.130 --> 00:02:17.600 calculated as the change of charge divided by time. 00:02:17.600 --> 00:02:21.200 So, what does it mean when we talk about epsilon A over d? 00:02:21.200 --> 00:02:23.825 Of course a larger A can store more charges. 00:02:23.825 --> 00:02:28.580 Epsilon means that the attraction between the two charges or the ability to set up 00:02:28.580 --> 00:02:31.820 this strong electric field increases with larger epsilon and 00:02:31.820 --> 00:02:36.130 d of course means that it reduces the attraction between the two charges. 00:02:36.130 --> 00:02:38.220 How can we build with this? 00:02:38.220 --> 00:02:40.700 So, here's a simple parallel plate capacitor. 00:02:40.700 --> 00:02:43.970 Let's suppose that we wanted to be able to use these as a sensor. 00:02:43.970 --> 00:02:47.060 Let's consider two metal pieces with foam in between them. 00:02:47.060 --> 00:02:49.340 If we switched them or push them together, 00:02:49.340 --> 00:02:52.175 we'd be changing d, there would be change in the capacitance. 00:02:52.175 --> 00:02:53.870 If we split them back and forth, 00:02:53.870 --> 00:02:56.060 that would be changing the effective area because 00:02:56.060 --> 00:02:58.820 the area is only this where they are overlapping. 00:02:58.820 --> 00:03:01.580 Then, if wet them off with some other material between it, 00:03:01.580 --> 00:03:03.125 we'll be changing the epsilon. 00:03:03.125 --> 00:03:06.115 That's how we can use capacitors as sensors. 00:03:06.115 --> 00:03:10.490 So basically, we talked about what is capacitance and how does it relate to 00:03:10.490 --> 00:03:14.930 current and charges and the effect of epsilon A and d. In the next video, 00:03:14.930 --> 00:03:19.270 we're going to talk about what this does to current and voltage.