WEBVTT 00:00:05.660 --> 00:00:08.400 >> In this video, we'll introduce some of 00:00:08.400 --> 00:00:11.445 the practical aspects of capacitors and inductors. 00:00:11.445 --> 00:00:13.020 As usual in the lab videos, 00:00:13.020 --> 00:00:15.390 I'll assume that you're getting the theoretical information about 00:00:15.390 --> 00:00:18.090 these devices from the textbook and the lecture videos. 00:00:18.090 --> 00:00:20.475 We'll spend our time in this video then, 00:00:20.475 --> 00:00:23.250 looking at some physical capacitors and inductors. 00:00:23.250 --> 00:00:26.310 Talking a bit about how these devices store energy, 00:00:26.310 --> 00:00:30.380 show you how to identify capacitance and inductance values on physical parts, 00:00:30.380 --> 00:00:34.140 and show you how to measure capacitances using your DMM. 00:00:34.140 --> 00:00:39.500 Sadly, most affordable DMMs do not have an inductance measurement capability. 00:00:39.500 --> 00:00:40.820 So, for this course, 00:00:40.820 --> 00:00:45.290 we'll just have to believe the nominal inductance values for any inductors that we use. 00:00:45.290 --> 00:00:47.614 First, we'll talk about capacitors; 00:00:47.614 --> 00:00:50.030 Typical capacitor construction consists of 00:00:50.030 --> 00:00:56.075 two conductive elements separated by a non-conductive material, called a dielectric. 00:00:56.075 --> 00:00:59.120 The dielectric prevents current flow from one element 00:00:59.120 --> 00:01:02.095 to the other and is characterized by its permitivity, 00:01:02.095 --> 00:01:05.275 designated by the Greek letter, epsilon. 00:01:05.275 --> 00:01:09.305 If we apply a voltage difference between the two plates, 00:01:09.305 --> 00:01:12.170 charges will accumulate on the upper and lower plates. 00:01:12.170 --> 00:01:15.845 These will create an electric field between the plates. 00:01:15.845 --> 00:01:19.580 The capacitor stores energy in this electric field. 00:01:19.580 --> 00:01:24.410 Capacitance is a quantity which tells us how much energy a capacitor can store. 00:01:24.410 --> 00:01:27.140 For a capacitor consisting of two parallel plates, 00:01:27.140 --> 00:01:30.230 like the one we saw in the previous slide, the capacitance is, 00:01:30.230 --> 00:01:33.680 A times epsilon over d. Where A, 00:01:33.680 --> 00:01:35.840 is the cross-sectional area of the plates, 00:01:35.840 --> 00:01:38.660 d is the spacing between the plates, 00:01:38.660 --> 00:01:41.995 and epsilon is the permittivity of the dielectric. 00:01:41.995 --> 00:01:46.415 So, we can increase the capacitance by increasing the cross-sectional area, 00:01:46.415 --> 00:01:49.910 decreasing the spacing, or increasing the permittivity. 00:01:49.910 --> 00:01:53.180 Now, let's take a look at a few physical capacitors, 00:01:53.180 --> 00:01:56.575 measure their capacitance to get a feeling for these relationships. 00:01:56.575 --> 00:01:59.405 This is a variable parallel plate capacitor. 00:01:59.405 --> 00:02:02.360 The two plates actually consist of multiple plates each. 00:02:02.360 --> 00:02:05.340 So, these fins all create one plate, 00:02:05.340 --> 00:02:06.990 they're all electrically connected. 00:02:06.990 --> 00:02:09.590 The other plate is created by these fins. 00:02:09.590 --> 00:02:12.860 The two sets of fins are separated by air gaps between them, 00:02:12.860 --> 00:02:14.915 so the dielectric is simply air. 00:02:14.915 --> 00:02:17.660 If I turn the knob on the side of the capacitor, 00:02:17.660 --> 00:02:20.600 I can increase or decrease the capacitance, 00:02:20.600 --> 00:02:24.545 by increasing or decreasing the area of overlap of the conductors. 00:02:24.545 --> 00:02:26.810 Let's measure the capacitance and verify that, 00:02:26.810 --> 00:02:28.940 that's what actually happens. 00:02:28.940 --> 00:02:31.700 To measure capacitance using my DMM, 00:02:31.700 --> 00:02:35.420 I plug the leads into the COM and Volt/Ohm ports. 00:02:35.420 --> 00:02:40.050 I also connect the leads to the two terminals of the capacitor. 00:02:40.580 --> 00:02:43.910 To measure capacitance, I turn my knobs so that 00:02:43.910 --> 00:02:47.105 the indicator lines up with the little capacitor symbol. 00:02:47.105 --> 00:02:50.450 Currently, there's little or no overlapping area between 00:02:50.450 --> 00:02:53.870 these fins and I get about 0.1 nanofarads. 00:02:53.870 --> 00:02:55.400 By turning the knob, 00:02:55.400 --> 00:02:59.435 I can increase the overlapping area and increase the capacitance. 00:02:59.435 --> 00:03:01.865 Now, it's at about 0.4 nanofarads. 00:03:01.865 --> 00:03:05.480 If the fins are entirely overlapped, 00:03:05.480 --> 00:03:08.470 I get about 0.7 nanofarads. 00:03:08.470 --> 00:03:12.065 Now, let's take a look at a very simple homemade capacitor. 00:03:12.065 --> 00:03:16.010 My capacitor consists of two wires which are approximately parallel. 00:03:16.010 --> 00:03:21.295 They make two conductive elements with a dielectric between them which is currently air. 00:03:21.295 --> 00:03:25.500 I can use this property to measure water level. 00:03:25.500 --> 00:03:27.800 If I change the permitivity between these, 00:03:27.800 --> 00:03:29.435 it'll change the capacitance. 00:03:29.435 --> 00:03:33.895 If I insert my wires into this tube of water, 00:03:33.895 --> 00:03:36.960 then when I change the water level, 00:03:36.960 --> 00:03:40.115 I will also change the capacitance between these wires. 00:03:40.115 --> 00:03:41.570 Let's connect our DMM, 00:03:41.570 --> 00:03:43.955 change the water level and see how it works. 00:03:43.955 --> 00:03:46.699 Now, when the water level is low, 00:03:46.699 --> 00:03:50.165 our capacitance is about a half a nanofarad. 00:03:50.165 --> 00:03:51.740 Adding some water to this, 00:03:51.740 --> 00:03:56.850 increases the water level and should cause the capacitance to go up. 00:04:03.170 --> 00:04:08.180 Now, let's look at some typical capacitors which we'll use to create electric circuits. 00:04:08.180 --> 00:04:09.665 The capacitors I'll show you, 00:04:09.665 --> 00:04:11.840 are from the digital analog parts kit. 00:04:11.840 --> 00:04:15.095 The general principles are applicable to most capacitors. 00:04:15.095 --> 00:04:17.600 Capacitors are generally described by the type of 00:04:17.600 --> 00:04:20.195 dielectric material and their overall construction. 00:04:20.195 --> 00:04:22.445 Most of the capacitors in our parts kit 00:04:22.445 --> 00:04:24.335 are disc-shaped, like this one. 00:04:24.335 --> 00:04:26.150 On these types of capacitors, 00:04:26.150 --> 00:04:30.770 the nominal capacitance is encoded as three numbers printed on the capacitor. 00:04:30.770 --> 00:04:35.855 These numbers provide a capacitance in picofarads in exponential notation. 00:04:35.855 --> 00:04:38.060 The first two numbers are the mantissa of 00:04:38.060 --> 00:04:40.610 the number and the third number is the exponent. 00:04:40.610 --> 00:04:45.310 For example, this capacitance has the digits 103 printed on it. 00:04:45.310 --> 00:04:50.075 This means that its capacitance is 10 times 10 to the third picofarads. 00:04:50.075 --> 00:04:53.515 A picofarad is one times 10 to the minus 12 farads. 00:04:53.515 --> 00:05:00.395 Therefore, the capacitance of this capacitor is 10 times 10 to the third picofarads, 00:05:00.395 --> 00:05:02.990 which is times 10 to the minus 12, 00:05:02.990 --> 00:05:10.775 which is equal to 10 times 10 to the minus 9, or 10 nanofarads. 00:05:10.775 --> 00:05:15.440 Our actual measured capacitance is about 11 and a half nanofarads. 00:05:15.440 --> 00:05:18.590 Electrolytic capacitors are also fairly common. 00:05:18.590 --> 00:05:23.075 In electrolytic capacitors, one or both of the conductive plates are not metallic. 00:05:23.075 --> 00:05:27.305 These capacitors tend to have a relatively high capacitance for their size. 00:05:27.305 --> 00:05:33.100 The electrolytic capacitors in your parts kits are can-shaped rather than disc-shaped. 00:05:33.100 --> 00:05:35.175 Electrolytic capacitors are polarized, 00:05:35.175 --> 00:05:37.340 that means they don't really work the same way if you 00:05:37.340 --> 00:05:39.755 switch the polarity of the voltage at their terminals. 00:05:39.755 --> 00:05:43.805 The polarity is indicated by the length of the leads on the capacitor. 00:05:43.805 --> 00:05:47.780 The anode has a longer wire than the cathode. 00:05:47.780 --> 00:05:52.655 It is intended that the anode always be at a higher voltage than the cathode. 00:05:52.655 --> 00:05:54.200 If you reversed the polarity, 00:05:54.200 --> 00:05:56.390 the capacitor properties will be different. 00:05:56.390 --> 00:05:58.580 In fact, if you apply a voltage in which 00:05:58.580 --> 00:06:00.950 the cathode is at a higher voltage than the anode, 00:06:00.950 --> 00:06:02.705 the capacitor can explode. 00:06:02.705 --> 00:06:04.115 If you do this, by the way, 00:06:04.115 --> 00:06:06.095 I recommend that you wear eye protection. 00:06:06.095 --> 00:06:08.360 I've personally never blown up a capacitor by 00:06:08.360 --> 00:06:11.315 reversing its polarity, but there's a first time for everything. 00:06:11.315 --> 00:06:13.580 The electrolytic capacitors in our kit, 00:06:13.580 --> 00:06:15.830 are physically large enough so that the capacitance is 00:06:15.830 --> 00:06:19.180 printed directly on the side of the capacitor. 00:06:19.180 --> 00:06:21.380 This capacitor, for example, 00:06:21.380 --> 00:06:24.455 is nominally 220 microfarads. 00:06:24.455 --> 00:06:28.400 The printing on the side of the capacitor also provides additional information. 00:06:28.400 --> 00:06:32.030 The cathode is indicated by a white stripe with a minus sign on 00:06:32.030 --> 00:06:36.065 it and the maximum allowable safe voltage is also printed out on the side. 00:06:36.065 --> 00:06:39.290 This capacitor's rated voltage is 10 volts. 00:06:39.290 --> 00:06:41.720 Now, let's talk a bit about inductors. 00:06:41.720 --> 00:06:44.895 Inductors like capacitors store electrical energy. 00:06:44.895 --> 00:06:49.085 Unlike capacitors, inductors store energy in a magnetic field. 00:06:49.085 --> 00:06:54.599 Typical inductors are constructed by winding a conductive wire around a central core. 00:06:54.599 --> 00:06:56.520 When current runs through the coil, 00:06:56.520 --> 00:06:58.515 a magnetic field is created. 00:06:58.515 --> 00:07:01.190 The inductance of the inductor is indicative 00:07:01.190 --> 00:07:03.920 of the amount of energy that can be stored by the inductor. 00:07:03.920 --> 00:07:06.980 The inductance is typically related to the number of turns 00:07:06.980 --> 00:07:10.165 the coil takes around the central core and the core material. 00:07:10.165 --> 00:07:14.585 Ferrite core materials typically result in relatively high inductance. 00:07:14.585 --> 00:07:16.760 Here's a very simple homemade inductor, 00:07:16.760 --> 00:07:19.205 I've simply wrapped wire around a carriage bolt. 00:07:19.205 --> 00:07:21.440 If I connect a voltage source to the wire, 00:07:21.440 --> 00:07:25.475 current will flow through the wire and a magnetic field will be created. 00:07:25.475 --> 00:07:28.340 To create a fairly large magnetic field, 00:07:28.340 --> 00:07:29.645 I'll need a high current. 00:07:29.645 --> 00:07:32.480 So, I'll use the six-volt batteries as my power source. 00:07:32.480 --> 00:07:35.000 I know I have a magnetic field because I can pick up 00:07:35.000 --> 00:07:38.345 screws with my newly created electromagnet. 00:07:38.345 --> 00:07:41.645 Inductors come in a wide range of shapes and sizes. 00:07:41.645 --> 00:07:45.740 This is a large high current inductor, about 120 millihenries. 00:07:45.740 --> 00:07:48.500 The analog parts kit contains two inductors; 00:07:48.500 --> 00:07:51.715 a one millihenry inductor and a one microhenry inductor. 00:07:51.715 --> 00:07:55.220 The millihenry inductor is encoded with the numerals 102 on 00:07:55.220 --> 00:07:59.300 the side, and the microhenry inductor has the numerals 1R0. 00:07:59.300 --> 00:08:02.670 These are both ferrite core inductors.