1 00:00:00,820 --> 00:00:11,580 Our next topic is series and parallel circuits, and before we get into the actual circuits and the concepts of series and parallel, we need to say a little bit about schematic diagrams. 2 00:00:12,400 --> 00:00:29,260 Everything that electricians do is represented, or everything that they use is represented with a little symbol, and I've been drawing pictures of a simple circuit made of a battery and a light bulb, and I'll show you how we would represent this circuit schematically. 3 00:00:30,160 --> 00:00:32,539 So let's draw the light bulb and some wires. 4 00:00:39,350 --> 00:00:41,689 Okay, an electrician, if you were making an electrical diagram, 5 00:00:42,090 --> 00:00:44,950 he would not draw a picture like the one I just drew. 6 00:00:47,049 --> 00:00:48,729 There are three things in this picture. 7 00:00:48,850 --> 00:00:52,810 There's the battery, the voltage source, there's the wires, and there's the bulb. 8 00:00:52,810 --> 00:00:54,630 And he would draw the picture like this. 9 00:00:55,429 --> 00:00:59,310 That little set of lines represents the battery. 10 00:01:00,350 --> 00:01:03,850 And the wires would always be drawn with straight lines at right angles. 11 00:01:03,850 --> 00:01:10,909 and the bulb would be drawn like this that little that little loop inside the bulb represents the 12 00:01:10,909 --> 00:01:17,049 filament and then the wire coming down to the other side would look like that and for each 13 00:01:17,049 --> 00:01:23,030 thing like a bulb for example you see that symbol and a battery for example you see that symbol 14 00:01:23,030 --> 00:01:26,870 each thing that you can put in a circuit and there are lots of things besides batteries and 15 00:01:26,870 --> 00:01:32,629 balls but each thing has its own symbol and you also might have a switch in here let me um put a 16 00:01:32,629 --> 00:01:40,010 little gap in the wire and put in a switch something like that that might 17 00:01:40,010 --> 00:01:44,489 correspond to a little switch here in the wire you could have a little device 18 00:01:44,489 --> 00:01:54,239 here with a little switch that you could flip back and forth to allow the current 19 00:01:54,239 --> 00:01:57,659 to flow or not but again you wouldn't draw an actual picture of the switch you 20 00:01:57,659 --> 00:02:02,000 would draw this symbol which represents any switch now another thing that you 21 00:02:02,000 --> 00:02:05,060 commonly see is not just a light bulb in a circuit, but 22 00:02:05,060 --> 00:02:10,060 you see a resistor, which is represented by this zigzag 23 00:02:10,060 --> 00:02:10,439 line. 24 00:02:10,439 --> 00:02:14,139 So this circuit has a voltage source, V, and a resistor, R. 25 00:02:14,139 --> 00:02:15,500 And the resistor could be anything. 26 00:02:15,500 --> 00:02:16,340 It could be a toaster. 27 00:02:16,340 --> 00:02:17,639 It could even be an electric motor. 28 00:02:17,639 --> 00:02:20,139 It could be something that has electrical resistance. 29 00:02:20,139 --> 00:02:23,560 It could be a heater, or it could just be a resistor, an 30 00:02:23,560 --> 00:02:26,740 actual little device that is simply there to limit the 31 00:02:26,740 --> 00:02:28,139 current flow. 32 00:02:28,139 --> 00:02:33,060 It's actually pretty common to have a bulb and a resistor in combination. 33 00:02:33,659 --> 00:02:37,379 So we might have a bulb here and a resistor there. 34 00:02:37,460 --> 00:02:42,319 And the resistor keeps too much current from flowing through the bulb so that it doesn't burn out. 35 00:02:42,580 --> 00:02:46,219 But again, the point here is that we have all these little symbols for the things in the circuits. 36 00:02:46,219 --> 00:02:50,840 And the diagrams are done at right angles like this to keep them neat because they can get large and complicated. 37 00:02:51,860 --> 00:02:56,000 Here's a picture of a circuit diagram for a solar panel regulator. 38 00:02:56,000 --> 00:03:00,919 And there are lots of pictures like this that you can find on the web, on the internet. 39 00:03:01,360 --> 00:03:04,639 If you want to build something, you can see diagrams of how to do it. 40 00:03:05,599 --> 00:03:07,780 And they can get a lot more complicated than this. 41 00:03:09,120 --> 00:03:14,860 And in a more advanced physics or electrical engineering class, you would become very familiar with diagrams like this. 42 00:03:15,340 --> 00:03:16,360 Schematic diagrams. 43 00:03:17,479 --> 00:03:19,439 Now let's talk about series and parallel. 44 00:03:20,300 --> 00:03:24,819 Fundamentally, there are two ways in which resistors can be connected, or anything. 45 00:03:24,819 --> 00:03:26,979 I'm going to be drawing these diagrams with resistors, 46 00:03:27,060 --> 00:03:29,840 but you could be connecting resistors, light bulbs, toasters, anything. 47 00:03:30,219 --> 00:03:32,599 And there's two ways that they can be connected to a battery 48 00:03:32,599 --> 00:03:34,139 or some other voltage source. 49 00:03:34,360 --> 00:03:41,210 They could be connected in series, which would look like this. 50 00:03:41,629 --> 00:03:44,870 Here's the battery, and I'll draw a wire coming out here. 51 00:03:45,310 --> 00:03:52,229 And then here's one resistor, and here's another, and here's another. 52 00:03:52,669 --> 00:03:55,849 And the point here is that they're connected all three in a row. 53 00:03:55,849 --> 00:04:01,150 So if you imagine the current flow, the charge comes out of the battery, flows around the circuit, 54 00:04:01,250 --> 00:04:05,110 it goes through one resistor, then through the next one, and then through the third one. 55 00:04:05,610 --> 00:04:07,069 And there's only one path. 56 00:04:07,889 --> 00:04:11,849 It's like a one-lane road, or a one-lane loop road. 57 00:04:12,069 --> 00:04:13,169 There's one way to go. 58 00:04:13,289 --> 00:04:16,470 Every electron that comes out of the battery goes around and follows that wire, 59 00:04:16,810 --> 00:04:21,149 goes through resistor A, and then resistor B, and then resistor C, and then back to the battery. 60 00:04:22,029 --> 00:04:23,850 Now a parallel circuit is different. 61 00:04:25,850 --> 00:04:44,850 In a parallel circuit, the current divides, so I'm going to orient this one a little bit differently, but here's a battery, and then imagine you have a wire coming out here, and then the wire comes over to this point and splits into three paths, so electrons coming out of the wire, they travel around the circuit this way, and then they divide. 62 00:04:44,850 --> 00:04:49,170 Some of them go down this branch, some down that branch, and some down the third branch. 63 00:04:49,350 --> 00:04:52,569 And you could have, on each branch, a resistor. 64 00:04:57,939 --> 00:05:02,180 And then the electrons flow through those resistors, and then they all join back up. 65 00:05:04,759 --> 00:05:07,339 And then the current flows back to the battery. 66 00:05:07,899 --> 00:05:16,759 So the electrons all end up coming back and rejoining at this point, and then flowing back to the battery. 67 00:05:16,759 --> 00:05:24,259 and the essential aspect of a parallel circuit is that the current divides and flows through multiple paths. 68 00:05:24,660 --> 00:05:27,959 It doesn't have to be three, it could be two paths or four or a hundred. 69 00:05:27,959 --> 00:05:33,120 The point is in a series circuit the current goes through one and then the other, all of them in a row 70 00:05:33,120 --> 00:05:35,420 and in a parallel circuit the current divides. 71 00:05:36,019 --> 00:05:42,139 Now one thing to note is that the actual geometrical layout of the resistors in your little circuit project 72 00:05:42,139 --> 00:05:46,660 isn't the determining factor in whether or not the resistors are in series or parallel. 73 00:05:46,759 --> 00:05:49,660 The determining factor is whether the current divides or not. 74 00:05:49,879 --> 00:05:52,819 So in this case, for example, I have a resistor here. 75 00:05:53,620 --> 00:05:55,579 And imagine the electrons flowing through it. 76 00:05:56,079 --> 00:05:58,079 And then they come down and flow through this resistor. 77 00:05:59,360 --> 00:06:00,959 And then they flow through this one. 78 00:06:03,139 --> 00:06:04,819 And then they flow back to the battery. 79 00:06:05,180 --> 00:06:07,740 Look at the actual physical layout on the screen here. 80 00:06:08,120 --> 00:06:12,259 These three resistors are literally parallel to each other. 81 00:06:12,360 --> 00:06:13,759 Those wires are parallel. 82 00:06:13,879 --> 00:06:14,959 They form parallel lines. 83 00:06:14,959 --> 00:06:22,180 but this is not a parallel circuit this is a series circuit and the reason is because the 84 00:06:22,180 --> 00:06:26,899 electrons come out of the battery they flow through one resistor and then the next one 85 00:06:26,899 --> 00:06:33,259 and then the next one all three in a row electrically this is no different from the 86 00:06:33,259 --> 00:06:39,279 one we drew earlier like this the electrons flow through one resistor then another and then another 87 00:06:39,279 --> 00:06:43,519 the electrons don't really know or care how the wires bend or how the circuit is shaped 88 00:06:44,000 --> 00:06:47,279 What does matter is whether the current divides or not. 89 00:06:47,600 --> 00:06:51,060 And if it divides into multiple paths, then it's a parallel circuit. 90 00:06:51,240 --> 00:06:55,800 If the electrons go through one and then another and then another, then it's a series circuit. 91 00:06:58,019 --> 00:07:01,079 Okay, a couple more comments about series and parallel circuits. 92 00:07:01,199 --> 00:07:06,060 I've drawn two pictures here of light bulbs connected to a battery, 93 00:07:06,420 --> 00:07:10,620 one in which they're connected in series and one in which they're connected in parallel. 94 00:07:11,139 --> 00:07:16,139 And you can see in the series circuit, electric current flows through all three bulbs in a row. 95 00:07:16,800 --> 00:07:21,319 And in the parallel circuit, it has to divide and goes through three different branches. 96 00:07:22,180 --> 00:07:25,160 And that's the fundamental difference in the series and parallel circuits. 97 00:07:25,459 --> 00:07:31,319 In the series circuit, current could be flowing through all of these bulbs, so they could all be sitting here shining. 98 00:07:35,110 --> 00:07:39,089 But if one bulb burns out, that essentially breaks the circuit. 99 00:07:39,230 --> 00:07:40,350 And it doesn't matter which bulb. 100 00:07:40,610 --> 00:07:43,290 Suppose this bulb in the middle right here burned out. 101 00:07:43,290 --> 00:07:48,149 So I'm going to scratch out the filament like the filament just burned out in that case the bulb will go out and 102 00:07:48,529 --> 00:07:53,949 So will the other two because if this filament is dead if this filament in the middle bulb is 103 00:07:54,790 --> 00:07:59,129 Broken then that's that's part of the path through which the electricity flows 104 00:07:59,129 --> 00:08:06,089 And if you break that path then none of the electricity flows and when the when the current stops flowing the bulbs don't light in 105 00:08:06,490 --> 00:08:14,639 The parallel circuit these three bulbs could be sitting here glowing all three of them are lit 106 00:08:14,639 --> 00:08:20,399 and if one of them burns out suppose the one in the middle here burns out so I'll 107 00:08:20,399 --> 00:08:24,540 I'll just scratch out the filament and it could break or crack the filament is 108 00:08:24,540 --> 00:08:29,639 no longer conducting electricity that means electrons no longer flow down that 109 00:08:29,639 --> 00:08:35,539 middle branch and so that bulb goes out but the other two bulbs remain lit you 110 00:08:35,539 --> 00:08:40,620 can see that electrons can still form a complete circuit right there and they 111 00:08:40,620 --> 00:08:46,019 can still travel through the third bulb like that right there it's only that 112 00:08:46,019 --> 00:08:50,480 that one branch that's compromised by the breaking of that bulb, the other two will 113 00:08:50,480 --> 00:08:52,080 continue to light. 114 00:08:52,080 --> 00:08:56,460 And there's another effect that you should understand with series and parallel circuits, 115 00:08:56,460 --> 00:08:58,720 and that's the effect of adding more bulbs. 116 00:08:58,720 --> 00:09:04,879 If I start adding more bulbs in the series circuit here, remember that each bulb has 117 00:09:04,879 --> 00:09:10,759 resistance, so adding more bulbs causes a greater total resistance, and that's going 118 00:09:10,759 --> 00:09:16,960 to cause less current flow, and each bulb will burn less brightly. But because there's 119 00:09:16,960 --> 00:09:21,519 less current flow, adding more bulbs causes less current to come out of the battery, and 120 00:09:21,519 --> 00:09:24,799 the battery would end up lasting a little bit longer. And that should make sense too. 121 00:09:24,799 --> 00:09:29,779 If the bulbs burn less brightly, the battery will last a little bit longer. So if they're 122 00:09:29,779 --> 00:09:36,419 in series, adding more bulbs in series increases the resistance and decreases the current. 123 00:09:36,419 --> 00:09:40,399 In parallel, the opposite thing is the case. If you add more bulbs in parallel, suppose 124 00:09:40,399 --> 00:09:47,419 I come over here and add another light bulb over here I've added an additional 125 00:09:47,419 --> 00:09:51,440 path through which current can flow and this is like adding more lanes on the 126 00:09:51,440 --> 00:09:58,159 highway there's now an additional path that the electrons can take and the 127 00:09:58,159 --> 00:10:02,360 result is more traffic you add more lanes in the road you get more traffic 128 00:10:02,360 --> 00:10:07,399 there so adding an additional light bulb in parallel or more light bulbs in 129 00:10:07,399 --> 00:10:11,240 parallel will decrease the overall resistance because you're adding more 130 00:10:11,240 --> 00:10:15,559 paths for current to flow. All the light bulbs will continue to burn just as 131 00:10:15,559 --> 00:10:20,279 brightly. What happens is the decrease in overall resistance results in an 132 00:10:20,279 --> 00:10:24,240 increase in current flow. So you can take a little battery, a little flashlight 133 00:10:24,240 --> 00:10:28,039 battery, and you can hook up 10 or 20 bulbs to it in parallel and they'll all 134 00:10:28,039 --> 00:10:32,299 glow very brightly, but what happens is your battery will drain more quickly 135 00:10:32,299 --> 00:10:36,360 because you're literally drawing more current out of the battery. If you hook 136 00:10:36,360 --> 00:10:40,299 up 10 or 20 little flashlight light bulbs to a 137 00:10:40,299 --> 00:10:44,799 a battery in series it's there's going to be too much resistance you won't see any glow at all 138 00:10:44,799 --> 00:10:48,399 but if you come hook them up in parallel they'll all glow just as bright as normal 139 00:10:48,399 --> 00:10:53,120 but you'll have decreased resistance and increased current flow