1 00:00:00,012 --> 00:00:08,576 Welcome back. We're continuing on with our lesson on 2 00:00:08,576 --> 00:00:13,097 physical resistors and our lab demos. In this particular case, we want to be 3 00:00:13,097 --> 00:00:19,518 looking at real circuits. Previously in this module, we had looked 4 00:00:19,518 --> 00:00:24,686 at abstract concepts and equations. In these two lessons we're trying to look 5 00:00:24,686 --> 00:00:28,440 at physical resistors and their use in circuits. 6 00:00:28,440 --> 00:00:32,024 So the particular lesson objectives are to demonstrate series and parallel 7 00:00:32,024 --> 00:00:35,664 resistance and measure voltage and current using the voltage divider law and 8 00:00:35,664 --> 00:00:40,678 Ohm's Law. Before we show the demo, let's go back 9 00:00:40,678 --> 00:00:44,849 and review a couple of concepts. In the last lesson we learned about 10 00:00:44,849 --> 00:00:47,580 protoboards. Recall that protoboards are used to make 11 00:00:47,580 --> 00:00:51,875 building circuits easier because a lot of the connections are made for you. 12 00:00:51,875 --> 00:00:55,110 So every group of five is connected together. 13 00:00:55,110 --> 00:00:58,591 A wire in this point, in this hole right here is going to be electrically 14 00:00:58,591 --> 00:01:05,910 connected to a wire in that hole there. So, these holes are connected together. 15 00:01:05,910 --> 00:01:08,868 And along the side, side rails, everything along the side rail are, is 16 00:01:08,868 --> 00:01:12,138 connected. So, everything shown in yellow will be 17 00:01:12,138 --> 00:01:15,174 connected. The other two concepts we wanted to 18 00:01:15,174 --> 00:01:20,670 remind you of, resistors in series. Remember that resistors in series add, 19 00:01:20,670 --> 00:01:24,910 and resistors in parallel follow this form right here. 20 00:01:28,580 --> 00:01:32,772 So let's start our lab demo. In this experiment, we will look at 21 00:01:32,772 --> 00:01:38,994 resistors in series and in parallel. One way to connect resistors in series is 22 00:01:38,994 --> 00:01:44,370 to touch the resistors end-to-end. Like this. 23 00:01:44,370 --> 00:01:47,698 Its a little bit hard to, to keep them together that way, and so we usually like 24 00:01:47,698 --> 00:01:51,690 to use the protoboard when we insert these in protoboard. 25 00:01:51,690 --> 00:01:57,888 So if I insert the resistor in the protoboard. 26 00:01:57,888 --> 00:02:04,999 Recall that protoboards, every row of five along the insides of of here. 27 00:02:04,999 --> 00:02:09,210 Every group in a five is connected together. 28 00:02:09,210 --> 00:02:12,632 So, as long as I put the end of this one in the same row as that one they are 29 00:02:12,632 --> 00:02:18,722 connected together. That leaves my hands free to take the 30 00:02:18,722 --> 00:02:24,840 measurements. If I look at these resistors by 31 00:02:24,840 --> 00:02:28,580 themselves, they're supposed to be 1,000 ohm resistors. 32 00:02:28,580 --> 00:02:31,560 Let's actually measure them, 994, this one is 991. 33 00:02:31,560 --> 00:02:40,050 Okay, so they're pretty close to 1,000. They're within the tolerance of these 34 00:02:40,050 --> 00:02:43,908 resistors. If I, if I put them in series, that means 35 00:02:43,908 --> 00:02:48,570 I should sum them up. So this should be about 2000 ohms. 36 00:02:50,950 --> 00:02:53,656 And they do. They sum up to be that. 37 00:02:53,656 --> 00:03:00,397 Let me go ahead and replace one of these 1000 ohm resistors with, the 2000 ohm 38 00:03:00,397 --> 00:03:09,270 resistor and try the same thing. We measure that 2000 ohm resistor. 39 00:03:09,270 --> 00:03:16,290 It is well, close to 2000 ohms. This sum should be about 3000. 40 00:03:16,290 --> 00:03:21,770 Very close, so within tolerance, it's, it's the 3000 ohms. 41 00:03:21,770 --> 00:03:25,470 To connect resistors in parallel, you but both ends in the same rows. 42 00:03:25,470 --> 00:03:34,890 So, let me pull out the 2000, and I want to put the another 1,000 in parallel 43 00:03:34,890 --> 00:03:46,396 with this resistor. So both ends are in the same rows. 44 00:03:46,396 --> 00:03:50,490 Now if you remember what resistor's in parallel are like. 45 00:03:52,610 --> 00:03:56,120 What we'd find is that two resistors that are the same that are in parallel with 46 00:03:56,120 --> 00:03:59,794 one another. The total resistance is actually half of 47 00:03:59,794 --> 00:04:03,008 either one. So if this started out as two 1,000 ohm 48 00:04:03,008 --> 00:04:07,564 resistors and I've put them in parallel, the, they should end up with about 5,000 49 00:04:07,564 --> 00:04:13,366 or 500 and they are very close. And again, it doesn't really matter if 50 00:04:13,366 --> 00:04:16,774 I'm touching it here. Were here they are the same spot 51 00:04:16,774 --> 00:04:21,860 electrically. I get 496 ohms. 52 00:04:21,860 --> 00:04:26,923 If I replace one of these resistors with the 2k resistor, then I've got a 1k 53 00:04:26,923 --> 00:04:33,355 resistor in parallel with the 2k. The total resistance there using our 54 00:04:33,355 --> 00:04:37,496 formula for resistors in parallel should be 666. 55 00:04:37,496 --> 00:04:45,036 Very close, very close. Now let's hook together a circuit with 56 00:04:45,036 --> 00:04:47,932 the voltage source in two resistors in series. 57 00:04:47,932 --> 00:04:56,460 Here's my battery, it's going to be my voltage source. 58 00:04:56,460 --> 00:05:01,298 Let's check the voltage on here. This is, it should be around 400, 4.5, 59 00:05:01,298 --> 00:05:06,260 because I've got three, 1.5 volt batteries. 60 00:05:06,260 --> 00:05:12,958 So, I get 4.63. So, it's a little bit high, but within 61 00:05:12,958 --> 00:05:18,230 range of what we would expect. I want to put that in this protoboard. 62 00:05:18,230 --> 00:05:21,593 But I'm going to add a resistor over here, a wire over here, so that I can 63 00:05:21,593 --> 00:05:25,125 connect up this battery a little bit easier. 64 00:05:25,125 --> 00:05:29,916 Okay. And then when I connect this battery up, 65 00:05:29,916 --> 00:05:36,008 I want to put these, these connections into these same rows. 66 00:05:36,008 --> 00:05:40,488 Okay. So now if I double check this, I should 67 00:05:40,488 --> 00:05:49,780 get that same 4.65. 4.63 volts. 68 00:05:49,780 --> 00:05:53,941 So to connect it up, I put the positive terminal in the same row as that 69 00:05:53,941 --> 00:05:58,686 resistor, and I'm going to add another 1000-ohm resistor, so I have two 1000-ohm 70 00:05:58,686 --> 00:06:06,660 resistors in series with one another. You take this negative terminal, put it 71 00:06:06,660 --> 00:06:13,210 in the same, negatively, put it in the same row as this resistor. 72 00:06:13,210 --> 00:06:17,290 And I want to look at the voltage divider law. 73 00:06:17,290 --> 00:06:22,330 If you look at the schematic of the voltage divider law then the ratio the 74 00:06:22,330 --> 00:06:26,950 voltage on the output is really proportional to the voltage of the 75 00:06:26,950 --> 00:06:33,030 resistance across your what you're measuring. 76 00:06:33,030 --> 00:06:37,818 So in this case if r2 is a larger resistor than the voltage should drop 77 00:06:37,818 --> 00:06:42,990 across r2 would be larger than that across r1. 78 00:06:42,990 --> 00:06:46,439 So larger the resistor the larger the percent voltage drop you have. 79 00:06:47,660 --> 00:06:51,356 When you have two resistors that are in series with one another, that are equal, 80 00:06:51,356 --> 00:06:55,360 that means that each resistor gets half of the voltage. 81 00:06:55,360 --> 00:06:59,585 So look at this case we've got back to the circuit, we've got two resistors that 82 00:06:59,585 --> 00:07:05,460 are equal, normally equal to one another, with intolerance range. 83 00:07:05,460 --> 00:07:11,928 So they should each get half the voltage, and they do, 2.3 2 is about half, and 84 00:07:11,928 --> 00:07:19,096 measured this way, 2.31 is half. Notice that if I switch the terminals, 85 00:07:19,096 --> 00:07:24,030 the polarity on here, so I'm measuring positive to negative here. 86 00:07:24,030 --> 00:07:29,020 If I switched it and did negative to positive, I should negated. 87 00:07:29,020 --> 00:07:33,790 See, I get a negative voltage there. If I took out the 1000 ohm resistor, and 88 00:07:33,790 --> 00:07:40,510 put in 2000 ohm resistor, then the 2000 ohm resistor is bigger, so this is 1 3rd 89 00:07:40,510 --> 00:07:50,600 of the total voltage, and this one is 2 3rds of the total voltage. 90 00:07:50,600 --> 00:07:55,570 So this should get 2 3rds of the current 2 3rds of the, the voltage. 91 00:07:55,570 --> 00:07:59,039 2 3rds of the resistance should get 2 3rds of the voltage. 92 00:07:59,039 --> 00:08:03,234 And it does. That's about 2 3rds and then this one is, 93 00:08:03,234 --> 00:08:09,695 about 1 3rd. Now, I'm going to take out this 2k 94 00:08:09,695 --> 00:08:16,410 resistor. Leave the 1k, and I'm going to put a 10 95 00:08:16,410 --> 00:08:26,852 ohm resistor in there. So here I've got a thousand ohms, and 96 00:08:26,852 --> 00:08:35,070 here 10 ohms. We can double check that. 97 00:08:36,790 --> 00:08:38,861 Actually double check that resistance here. 98 00:08:38,861 --> 00:08:48,766 9.9 10 ohms very close. So what happens in the in the voltage 99 00:08:48,766 --> 00:08:57,930 divider. 10 ohms is about 1% of the total. 100 00:08:57,930 --> 00:09:04,668 The total between this two is 1,010 ohms. So this resistor here would take about 101 00:09:04,668 --> 00:09:10,700 99% of the voltage, and this only about 1% of the voltage. 102 00:09:10,700 --> 00:09:15,072 So let's look at that. So this should be about 99% of the 103 00:09:15,072 --> 00:09:18,710 voltage. Oh, we'll go back to voltage setting. 104 00:09:18,710 --> 00:09:27,040 But 99% of the voltage, and this about 1% of the voltage. 105 00:09:27,040 --> 00:09:33,040 And that's right. Seeing that, that fact that if I put a 106 00:09:33,040 --> 00:09:40,137 very small resistor in series with a large resistor. 107 00:09:41,700 --> 00:09:47,100 I'm able to use that I'd, that fact to be able to help me to determine the current. 108 00:09:47,100 --> 00:09:52,469 For example, this one, if I want to figure out what the current is through 109 00:09:52,469 --> 00:09:58,475 this loop, I would take this voltage, which is 4 point or, 45 millivolts, and 110 00:09:58,475 --> 00:10:05,709 divide it by 10. I'm using Ohm's Law, divided by 10 ohms, 111 00:10:05,709 --> 00:10:09,853 and I would get 4.5 milliamps. So that's the current that is running 112 00:10:09,853 --> 00:10:14,374 through this circuit right here. The reason that I might, and I often use 113 00:10:14,374 --> 00:10:20,600 this method to tell me what the current is, to record current. 114 00:10:20,600 --> 00:10:23,624 I just put, take my regular circuit, and I put a very small resistor in series 115 00:10:23,624 --> 00:10:26,675 with it. And I used a small resistor because I 116 00:10:26,675 --> 00:10:29,750 don't want to upset the rest of the circuit. 117 00:10:29,750 --> 00:10:32,150 I want to have minimal effect on the rest of the circuit so I put a very small 118 00:10:32,150 --> 00:10:36,486 resistor in there. It takes very small voltage across it and 119 00:10:36,486 --> 00:10:41,694 I read the voltage off of that and divide by the resistance and that gives me 120 00:10:41,694 --> 00:10:46,210 current. So there are actually ways that commonly 121 00:10:46,210 --> 00:10:50,271 we use for measuring current. The one on the right I just mentioned, 122 00:10:50,271 --> 00:10:53,992 where I take R2, and make it a very small resistor in comparison to R1, and I 123 00:10:53,992 --> 00:10:58,140 measure the resistance, the, the voltage across R2, and then use Ohm's Law to find 124 00:10:58,140 --> 00:11:03,064 the current. The other way that I can do current 125 00:11:03,064 --> 00:11:07,518 measurement is to take the resistor out, and this is on the left. 126 00:11:07,518 --> 00:11:13,000 I take the resistor out, and put in ammeter in series with my circuit. 127 00:11:13,000 --> 00:11:20,936 Going back to the circuit. Take it back to, I want to put, I had to, 128 00:11:20,936 --> 00:11:34,313 I have to put this ammeter in series. So that is reading right there 4.5 129 00:11:34,313 --> 00:11:39,276 milliamps, same thing as what I got before. 130 00:11:39,276 --> 00:11:46,564 So in summary, we've looked at resistors in series, resistors in parallel. 131 00:11:46,564 --> 00:11:50,220 The voltage divider lot and measuring current in a circuit. 132 00:11:50,220 --> 00:11:52,902 Thank you. In summary, we've connected physical 133 00:11:52,902 --> 00:11:57,801 resistors in parallel and in series, and we've measured voltages and currents in a 134 00:11:57,801 --> 00:12:03,310 circuit, applying the voltage divider law, and Ohm's law. 135 00:12:04,660 --> 00:12:09,370 In our next lesson, we will use superposition to handle multiple sources. 136 00:12:09,370 --> 00:12:13,335 And I do want to remind you to visit the forum to go ahead and ask questions if 137 00:12:13,335 --> 00:12:18,860 you have any doubts about any of the material, or if you're curious. 138 00:12:18,860 --> 00:12:21,800 And to please try to answer questions that other people have post. 139 00:12:21,800 --> 00:12:23,910 I would really like you to be engaged in this course.