1 00:00:00,000 --> 00:00:03,810 >> Hello, this is Dr. Cynthia Furse at the University of Utah. 2 00:00:03,810 --> 00:00:07,305 Today, I'd like to do an Introduction to Capacitors. 3 00:00:07,305 --> 00:00:11,430 Let's talk about what is capacitance and how it relates to current and charges, 4 00:00:11,430 --> 00:00:13,110 what are the effects of the parameters of 5 00:00:13,110 --> 00:00:16,440 the capacitor and what does it do to voltage and current? 6 00:00:16,440 --> 00:00:20,600 A capacitor is a passive element that stores energy in the electric field. 7 00:00:20,600 --> 00:00:22,380 This means that it does not require 8 00:00:22,380 --> 00:00:25,245 an external voltage source in order to act like a capacitor. 9 00:00:25,245 --> 00:00:27,945 Remember that it is actually storing energy. 10 00:00:27,945 --> 00:00:30,660 The capacitance, which you remember from physics, 11 00:00:30,660 --> 00:00:35,265 of a parallel plate is epsilon A over d. Epsilon is the dielectric material. 12 00:00:35,265 --> 00:00:38,415 It is a constant epsilon knot times epsilon r. 13 00:00:38,415 --> 00:00:40,910 The smallest epsilon r is one for air and 14 00:00:40,910 --> 00:00:43,795 a large epsilon r would be something like 80 for water. 15 00:00:43,795 --> 00:00:46,435 Plastics, glasses, et cetera are very close to air. 16 00:00:46,435 --> 00:00:47,780 Maybe there epsilon value is 17 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. 18 00:00:52,535 --> 00:00:56,375 When epsilon increases, their capacitance increases. 19 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. 20 00:01:00,080 --> 00:01:01,730 So, as I increase that area, 21 00:01:01,730 --> 00:01:05,720 the capacitance increases and as I decrease the distance of the place, 22 00:01:05,720 --> 00:01:08,310 the capacitance would also increase. 23 00:01:08,740 --> 00:01:11,915 Capacitors effect that flow of charges. 24 00:01:11,915 --> 00:01:15,515 The current is the change of charge dq dt. 25 00:01:15,515 --> 00:01:19,025 So, it's how much charge changes over a period of time. 26 00:01:19,025 --> 00:01:21,650 The way this works is charges flow from 27 00:01:21,650 --> 00:01:24,535 the positive voltage source onto the top of the capacitor. 28 00:01:24,535 --> 00:01:29,945 They are stored there, they get hung up there, and then they force positive charges 29 00:01:29,945 --> 00:01:32,780 away from the bottom plate leaving the bottom plate negatively 30 00:01:32,780 --> 00:01:35,960 charged and the top plate positively charged, 31 00:01:35,960 --> 00:01:37,310 that creates a voltage difference. 32 00:01:37,310 --> 00:01:39,875 That allows us to be able to do work. 33 00:01:39,875 --> 00:01:44,360 Displacement current is the current that appears between the two plates. 34 00:01:44,360 --> 00:01:46,340 You're more familiar with conduction current. 35 00:01:46,340 --> 00:01:48,440 Conduction current is actually the flow of charges. 36 00:01:48,440 --> 00:01:52,550 The fact that these charges are flowing or moving that's conducting electricity. 37 00:01:52,550 --> 00:01:55,100 But then right here between the two plates, 38 00:01:55,100 --> 00:01:58,970 it looks like there should be no current because there are no charges that can move. 39 00:01:58,970 --> 00:02:01,265 But the electric field forcing the charges 40 00:02:01,265 --> 00:02:04,160 makes a current that we called displacement current. 41 00:02:04,160 --> 00:02:07,010 It's the appearance that acts like there's a flow of charges, even though 42 00:02:07,010 --> 00:02:10,205 there aren't actually charges flowing in between the two plates. 43 00:02:10,205 --> 00:02:13,130 Again remember, that the conduction current is 44 00:02:13,130 --> 00:02:17,600 calculated as the change of charge divided by time. 45 00:02:17,600 --> 00:02:21,200 So, what does it mean when we talk about epsilon A over d? 46 00:02:21,200 --> 00:02:23,825 Of course a larger A can store more charges. 47 00:02:23,825 --> 00:02:28,580 Epsilon means that the attraction between the two charges or the ability to set up 48 00:02:28,580 --> 00:02:31,820 this strong electric field increases with larger epsilon and 49 00:02:31,820 --> 00:02:36,130 d of course means that it reduces the attraction between the two charges. 50 00:02:36,130 --> 00:02:38,220 How can we build with this? 51 00:02:38,220 --> 00:02:40,700 So, here's a simple parallel plate capacitor. 52 00:02:40,700 --> 00:02:43,970 Let's suppose that we wanted to be able to use these as a sensor. 53 00:02:43,970 --> 00:02:47,060 Let's consider two metal pieces with foam in between them. 54 00:02:47,060 --> 00:02:49,340 If we switched them or push them together, 55 00:02:49,340 --> 00:02:52,175 we'd be changing d, there would be change in the capacitance. 56 00:02:52,175 --> 00:02:53,870 If we split them back and forth, 57 00:02:53,870 --> 00:02:56,060 that would be changing the effective area because 58 00:02:56,060 --> 00:02:58,820 the area is only this where they are overlapping. 59 00:02:58,820 --> 00:03:01,580 Then, if wet them off with some other material between it, 60 00:03:01,580 --> 00:03:03,125 we'll be changing the epsilon. 61 00:03:03,125 --> 00:03:06,115 That's how we can use capacitors as sensors. 62 00:03:06,115 --> 00:03:10,490 So basically, we talked about what is capacitance and how does it relate to 63 00:03:10,490 --> 00:03:14,930 current and charges and the effect of epsilon A and d. In the next video, 64 00:03:14,930 --> 00:03:19,270 we're going to talk about what this does to current and voltage.