1 00:00:02,520 --> 00:00:04,586 Welcome back. Again, I'm Nathan Parrish and this is 2 00:00:04,586 --> 00:00:07,766 Linear Circuits. Today we're going to be talking about 3 00:00:07,766 --> 00:00:10,122 voltage. The aims of this class are to allow us to 4 00:00:10,122 --> 00:00:13,650 modify voltages to reflect voltage references, to describe how a chemical 5 00:00:13,650 --> 00:00:19,130 battery works, and then to identify if a battery is charging or discharging. 6 00:00:20,700 --> 00:00:23,242 From our previous class, we talked a little bit about charge and current as 7 00:00:23,242 --> 00:00:27,060 well as electric fields, the way that these charges interact with each other. 8 00:00:27,060 --> 00:00:30,080 We've also looked at current as being the flow of charge. 9 00:00:31,370 --> 00:00:33,802 And so the way that this, kind of, fits in is we've already talked about charge 10 00:00:33,802 --> 00:00:36,386 and current, today we talk about voltage which does have some impact on charge and 11 00:00:36,386 --> 00:00:39,350 current. Next time we'll talk about power and 12 00:00:39,350 --> 00:00:43,190 energy and then we go on and we'll actually look at some actual diagrams. 13 00:00:44,760 --> 00:00:47,847 The lesson objectives today, first of all, we want to calculate voltage from 14 00:00:47,847 --> 00:00:52,185 the energy gained or consumed as a charge moves through an electric field. 15 00:00:52,185 --> 00:00:56,215 We want to be able to correctly specify voltages as reference directions change. 16 00:00:56,215 --> 00:00:59,880 We want to describe the operation of a chemical battery. 17 00:00:59,880 --> 00:01:02,718 And identify battery is charging or discharging based upon the direction that 18 00:01:02,718 --> 00:01:05,780 the current is flowing and the references on the battery. 19 00:01:06,860 --> 00:01:10,175 So, first of all let's kind of describe what voltage is and give a definition for 20 00:01:10,175 --> 00:01:13,534 it. If I take a charge and I move it through 21 00:01:13,534 --> 00:01:17,340 an electric field, like this, there is a voltage. 22 00:01:17,340 --> 00:01:20,970 And the way that we measure that voltage is by calculating the amount of energy, 23 00:01:20,970 --> 00:01:24,490 that is either gained or lost as the charge moves through the electric field, 24 00:01:24,490 --> 00:01:30,230 and then taking that as the ratio of the amount of charge that was moving. 25 00:01:30,230 --> 00:01:35,885 So if this is one coulomb of charge, that is going from the top to the bottom, and 26 00:01:35,885 --> 00:01:41,192 in so doing it consumed one joule of energy then we take 1 joule divided by 1 27 00:01:41,192 --> 00:01:48,420 coulomb , giving us 1 volt. And the variable you're going to use for 28 00:01:48,420 --> 00:01:51,290 voltage is a v. It's measured in units of volts, which is 29 00:01:51,290 --> 00:01:55,830 named after Alessandro Volta, who was one of the first people to study batteries. 30 00:01:56,910 --> 00:02:00,090 It also turns out that you can do this the opposite way, if we have a charge 31 00:02:00,090 --> 00:02:03,323 that is going backwards through and electric field, what it's going to be 32 00:02:03,323 --> 00:02:08,348 doing is generating energy. And so now it's generating one joule of 33 00:02:08,348 --> 00:02:12,301 energy, and if it's one coulomb of charge that is doing it, again one volt from the 34 00:02:12,301 --> 00:02:18,460 bottom to the top. But where does this voltage come from? 35 00:02:18,460 --> 00:02:21,450 What is the origin of it. Voltage is essentially coming from these 36 00:02:21,450 --> 00:02:26,116 electric fields that we talked about. And these electric fields are established 37 00:02:26,116 --> 00:02:30,100 because of a difference in density of charge. 38 00:02:30,100 --> 00:02:33,324 So, if we look at this example, we have a whole bunch of negative charges all 39 00:02:33,324 --> 00:02:36,756 clumped up together, and when we do that, they have a tendency to try and pull all 40 00:02:36,756 --> 00:02:41,678 positive charges towards them. So if I introduce a positive charge to 41 00:02:41,678 --> 00:02:44,942 the system, this positive charge is going to be sucked up by that big collection of 42 00:02:44,942 --> 00:02:48,846 negative charges. Now, if we instead take all of these 43 00:02:48,846 --> 00:02:52,752 negative charges and distribute them evenly throughout the material, our 44 00:02:52,752 --> 00:02:58,276 electric field goes away. It's only by a difference in the charge 45 00:02:58,276 --> 00:03:03,910 density that we establish this voltage. Now, we're going to back and shove them 46 00:03:03,910 --> 00:03:07,880 all back together and reestablish our electric field again. 47 00:03:09,340 --> 00:03:11,890 Now when we're talking about voltage, reference is very important. 48 00:03:11,890 --> 00:03:14,308 If somebody says oh, well it's at 1,000 volts. 49 00:03:14,308 --> 00:03:18,320 Well, 1,000 volts with reference to what? Could be anything. 50 00:03:18,320 --> 00:03:21,890 It's kind of similar to if you wanted to measure how tall a man was. 51 00:03:21,890 --> 00:03:24,904 Now you might say this man is six feet tall. 52 00:03:24,904 --> 00:03:27,818 But generally what we're going to be doing is measuring from the bottoms of 53 00:03:27,818 --> 00:03:32,254 his feet to the top of his head. But if he happened to be a mountain 54 00:03:32,254 --> 00:03:36,082 climber you could just as easily say that this man is 10,000 feet tall, if we were 55 00:03:36,082 --> 00:03:39,562 measuring from the sea, sea floor to the top of his head we might get a very 56 00:03:39,562 --> 00:03:44,194 different value. So keep the reference instructions is 57 00:03:44,194 --> 00:03:47,470 very important, anytime you see a voltage you should be asking yourself. 58 00:03:47,470 --> 00:03:50,440 With reference to what? And so, if we flip the reference 59 00:03:50,440 --> 00:03:52,440 direction. If we decided, instead, to measure a man 60 00:03:52,440 --> 00:03:55,229 from the top of his head to the bottom of his feet. 61 00:03:55,229 --> 00:03:58,600 We might get negative six feet. And that's just fine. 62 00:03:58,600 --> 00:04:00,430 But the important thing is, we're consistent. 63 00:04:00,430 --> 00:04:03,160 We know what our reference is with reference to. 64 00:04:03,160 --> 00:04:08,850 And so the way we do that in si-, in our diagrams, is, we'll use a minus sign. 65 00:04:08,850 --> 00:04:12,940 To say what the bottom is, and the plus sign to say where the top is. 66 00:04:12,940 --> 00:04:16,244 Now if I flip the bottom and the top signs, so the plus is now at the bottom 67 00:04:16,244 --> 00:04:19,660 and minus is now at the top, you take whatever voltage you have and you flip 68 00:04:19,660 --> 00:04:24,178 its direction. Because the field inside of the device 69 00:04:24,178 --> 00:04:28,050 hasn't changed, just what we're referencing, and our direction. 70 00:04:28,050 --> 00:04:31,000 So let's actually do a problem, to practice this. 71 00:04:33,410 --> 00:04:37,240 In this problem, we have, four points. And we want to know, the value, of the 72 00:04:37,240 --> 00:04:41,100 voltage v. And it has its references labeled as 73 00:04:41,100 --> 00:04:43,548 well. Here, we have a minus here, same where 74 00:04:43,548 --> 00:04:47,119 the bottom is, and a plus here, where the top is. 75 00:04:47,119 --> 00:04:52,109 So to do this, we have, some, identified voltages here, here, and here. 76 00:04:52,109 --> 00:04:55,953 This is, this is three volts, this negative six volts, and this is negative 77 00:04:55,953 --> 00:04:59,600 four volts. So let's work our way around. 78 00:04:59,600 --> 00:05:03,520 This is a minus and this is a plus, and this is three volts. 79 00:05:03,520 --> 00:05:07,441 So from this point to this point, we've gone up three volts. 80 00:05:07,441 --> 00:05:11,710 This is a minus and this is a plus. This is a minus 6 volts. 81 00:05:11,710 --> 00:05:15,338 So from this point to this point we've gone down 6 volts. 82 00:05:15,338 --> 00:05:21,160 So up three and down six means overall we've gone down three. 83 00:05:21,160 --> 00:05:25,950 Now here's a plus and here's a minus, and this is minus four volts. 84 00:05:25,950 --> 00:05:28,950 And this is the same thing as if this were a minus, and this were a plus, and 85 00:05:28,950 --> 00:05:34,314 this was a positive four volts. So from here to here, we've actually gone 86 00:05:34,314 --> 00:05:40,400 up four volts. So we go up three, down six and up four. 87 00:05:40,400 --> 00:05:43,485 Which means from here to here, we've actually gone up one. 88 00:05:43,485 --> 00:05:46,620 And so this is a way that we can find voltages, by taking a look at other 89 00:05:46,620 --> 00:05:51,505 voltages and just adding them together. So now we've specified our reference 90 00:05:51,505 --> 00:05:53,928 directions, and we've combined them in a way that makes sense. 91 00:05:53,928 --> 00:05:58,284 Now let's look at an actual application of voltage being generated, and see how 92 00:05:58,284 --> 00:06:03,136 it works in an actual application. And this is an application that is still 93 00:06:03,136 --> 00:06:06,080 used today, lead-acid battery, you might be familiar with the way that these 94 00:06:06,080 --> 00:06:09,410 batteries operate in your vehicles, in your cars. 95 00:06:11,080 --> 00:06:15,568 For lead-acid battery, we have what's called a cathode here, and an anode here, 96 00:06:15,568 --> 00:06:20,819 when we are discharging current. So this arrow here is indicating that we 97 00:06:20,819 --> 00:06:27,000 see current flowing this direction. There's a kind of mnemonic device, acid, 98 00:06:27,000 --> 00:06:32,600 where anode, we have current into device, ACID, anode, current into device, so here 99 00:06:32,600 --> 00:06:37,760 we see the current flowing into the device. 100 00:06:37,760 --> 00:06:39,390 So that means this is going to be our anode. 101 00:06:40,660 --> 00:06:43,020 The battery is going to be labeled plus and minus, to let us know what the 102 00:06:43,020 --> 00:06:45,690 reference directions for the battery happen to be. 103 00:06:45,690 --> 00:06:48,512 And anytime you look at a battery you'll see that they're labeled appropriately. 104 00:06:48,512 --> 00:06:54,872 In lead-acid battery this negative terminal is just a pure lead bar, and 105 00:06:54,872 --> 00:07:03,610 then in this other terminal, the positive terminal, it's lead oxide. 106 00:07:03,610 --> 00:07:08,170 Inside of this middle area there is hydro-, or there's sulfuric acid and 107 00:07:08,170 --> 00:07:11,924 water. And what happens is the sulfuric acid in 108 00:07:11,924 --> 00:07:15,360 the water is spread out. So, we get these ions, that are kind of 109 00:07:15,360 --> 00:07:20,144 swimming in a soup. Now, when we're discharging a battery, 110 00:07:20,144 --> 00:07:23,856 what happens is we have some ions here, sulphide ion that's going to come over 111 00:07:23,856 --> 00:07:29,003 here to this lead bar. And it's going to combine, and this is in 112 00:07:29,003 --> 00:07:34,475 a reduction. Type of chemical reaction, and then in a 113 00:07:34,475 --> 00:07:38,501 reduction electrons are generated. And so these electrons are now able to 114 00:07:38,501 --> 00:07:40,515 flow. Well, they get pushed through, and 115 00:07:40,515 --> 00:07:43,765 remember the current's flowing this direction, so these electrons are going 116 00:07:43,765 --> 00:07:47,210 to be flowing against our traditional current flow. 117 00:07:47,210 --> 00:07:51,850 But when electrons hit this side it now makes an oxidation reaction possible. 118 00:07:51,850 --> 00:07:56,360 Because an oxidation reaction wants extra electrons for it to occur. 119 00:07:56,360 --> 00:08:00,560 So these two electrons are going to mix, then, with this lead oxide, some 120 00:08:00,560 --> 00:08:06,645 hydrogen, and some sulphate. And all together, we get two water atoms, 121 00:08:06,645 --> 00:08:11,018 or two water molecules. As well as some lead sulphate, that, is 122 00:08:11,018 --> 00:08:13,032 over here. So you notice that we now have lead 123 00:08:13,032 --> 00:08:15,670 sulphate on both sides. And this is what happens as the battery 124 00:08:15,670 --> 00:08:18,584 discharges. Now, if the battery continues to 125 00:08:18,584 --> 00:08:22,016 discharge, what will happen is these two plates are then going to be coated with 126 00:08:22,016 --> 00:08:27,065 this lead sulfate, and the battery dies. But what we're going to make sure is that 127 00:08:27,065 --> 00:08:31,506 we don't completely kill our battery. You might be aware, lead-acid batteries 128 00:08:31,506 --> 00:08:35,650 can recharge, and so we're going to take a look at that reaction next. 129 00:08:35,650 --> 00:08:39,082 But because of the electrons that are being generated over here and pushed over 130 00:08:39,082 --> 00:08:42,358 here, that's what's causing this displacement, this difference in charge 131 00:08:42,358 --> 00:08:46,905 density. And that's why a battery has a voltage. 132 00:08:46,905 --> 00:08:50,188 Well, let's learn what happens if we do it the other direction, if we're actually 133 00:08:50,188 --> 00:08:53,906 charging the battery. In this case, we still see that we have 134 00:08:53,906 --> 00:08:58,165 the lead sulfate on both plates. That are kind of coating the outsides. 135 00:08:58,165 --> 00:09:04,250 We have our water molecules still here. And then this soup of sulfuric acid and 136 00:09:04,250 --> 00:09:08,350 water. But if we take these two water out 137 00:09:08,350 --> 00:09:12,022 molecules and mix them over here, again we're going to be doing another oxidation 138 00:09:12,022 --> 00:09:16,640 or, another reduction reaction. Electrons are provided, and they go this 139 00:09:16,640 --> 00:09:19,012 way. And notice that our current is now 140 00:09:19,012 --> 00:09:22,246 flowing the opposite direction because electrons are going this way, from the 141 00:09:22,246 --> 00:09:25,883 anode to the cathode. And notice our anode and cathode have 142 00:09:25,883 --> 00:09:29,725 flipped places. Because current is going into the anode. 143 00:09:29,725 --> 00:09:32,539 And now the anode is going to be the plus terminal instead of the minus terminal of 144 00:09:32,539 --> 00:09:35,688 the battery. Now, these electrons that have come over 145 00:09:35,688 --> 00:09:37,916 here. We see the same reaction happening where 146 00:09:37,916 --> 00:09:41,264 we get a sulfate coming back into the soup this becomes pure lead again, and 147 00:09:41,264 --> 00:09:45,930 the battery goes back to its original configuration. 148 00:09:45,930 --> 00:09:49,606 And so that's how a battery work. So to emphasize some points, if we have 149 00:09:49,606 --> 00:09:57,460 our current going from the minus to the plus that means the battery is charging. 150 00:09:57,460 --> 00:09:59,569 If we have the current going from the plus to the minus, the battery is 151 00:09:59,569 --> 00:10:02,639 discharging. Now we can see that it's actually these 152 00:10:02,639 --> 00:10:06,863 chemical reactions that are happening within the battery to make this voltage 153 00:10:06,863 --> 00:10:13,722 happen and make batteries work. To summarize, charges create electric 154 00:10:13,722 --> 00:10:16,962 fields, and voltage is the amount of energy that's gained or released as 155 00:10:16,962 --> 00:10:22,105 charges move through an electric field. We also described how voltage originates 156 00:10:22,105 --> 00:10:25,320 from differences in charge density. And, we did a case study by looking at 157 00:10:25,320 --> 00:10:28,420 how lead-acid batteries work, and we emphasized that the direction of the 158 00:10:28,420 --> 00:10:31,620 current flow with reference to the two terminals let us know if the battery is 159 00:10:31,620 --> 00:10:35,715 charging or if the battery is discharging. 160 00:10:35,715 --> 00:10:40,673 So, for next class, we will take a closer look at this way that current and voltage 161 00:10:40,673 --> 00:10:44,528 interact. And we'll take a look at electric power 162 00:10:44,528 --> 00:10:47,810 and electric energy. And we'll do our first circuit analysis. 163 00:10:47,810 --> 00:10:50,210 Now I want to remind you about the forums, if you have any questions on the 164 00:10:50,210 --> 00:10:52,850 material that was covered in this class, please go to the forums and post there, 165 00:10:52,850 --> 00:10:57,200 it's the quickest way that you'll be able to get answers to your questions. 166 00:10:57,200 --> 00:11:01,040 And also go and help the other students. That they're taking this class and answer 167 00:11:01,040 --> 00:11:03,665 their questions if there is something that you understand. 168 00:11:03,665 --> 00:11:06,673 And so, we look forward to seeing you on the forums, as well as seeing you in the 169 00:11:06,673 --> 00:11:09,510 class next time. And until then, cheers.