[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.30,0:00:01.50,Default,,0000,0000,0000,,- [Lecturer] Electricity that lights up
Dialogue: 0,0:00:01.50,0:00:05.46,Default,,0000,0000,0000,,above looks very different\Nthan lightning strikes,
Dialogue: 0,0:00:05.46,0:00:08.07,Default,,0000,0000,0000,,but they're actually more\Nsimilar than one might think
Dialogue: 0,0:00:08.07,0:00:10.47,Default,,0000,0000,0000,,because they both have electric current.
Dialogue: 0,0:00:10.47,0:00:12.45,Default,,0000,0000,0000,,So let's understand what\Nelectric current is,
Dialogue: 0,0:00:12.45,0:00:13.80,Default,,0000,0000,0000,,how they're produced,
Dialogue: 0,0:00:13.80,0:00:16.11,Default,,0000,0000,0000,,and also get to understand a\Nlittle bit about lightning.
Dialogue: 0,0:00:16.11,0:00:18.12,Default,,0000,0000,0000,,So what exactly is electric current?
Dialogue: 0,0:00:18.12,0:00:19.14,Default,,0000,0000,0000,,Well think of electric current
Dialogue: 0,0:00:19.14,0:00:23.19,Default,,0000,0000,0000,,as a flow of net charge\Nthrough any given area.
Dialogue: 0,0:00:23.19,0:00:26.52,Default,,0000,0000,0000,,Here's what I mean by flow of net charge.
Dialogue: 0,0:00:26.52,0:00:28.77,Default,,0000,0000,0000,,Well imagine you have a\Ntiny cross-sectional area
Dialogue: 0,0:00:28.77,0:00:30.48,Default,,0000,0000,0000,,through which you have equal amount
Dialogue: 0,0:00:30.48,0:00:31.44,Default,,0000,0000,0000,,of positive charges flowing
Dialogue: 0,0:00:31.44,0:00:33.87,Default,,0000,0000,0000,,to the right and left in any given time.
Dialogue: 0,0:00:33.87,0:00:35.43,Default,,0000,0000,0000,,Now notice there is a flow,
Dialogue: 0,0:00:35.43,0:00:37.26,Default,,0000,0000,0000,,but there is no net flow (chuckles)
Dialogue: 0,0:00:37.26,0:00:39.99,Default,,0000,0000,0000,,and therefore here we say\Nthere is zero current.
Dialogue: 0,0:00:39.99,0:00:42.90,Default,,0000,0000,0000,,Another interesting example is\Nwhat if you have equal amount
Dialogue: 0,0:00:42.90,0:00:45.06,Default,,0000,0000,0000,,of positive and negative charges flowing
Dialogue: 0,0:00:45.06,0:00:47.43,Default,,0000,0000,0000,,in the same direction in\Nthe same time, let's say
Dialogue: 0,0:00:47.43,0:00:49.68,Default,,0000,0000,0000,,through again, a given\Ncross-sectional area.
Dialogue: 0,0:00:49.68,0:00:51.84,Default,,0000,0000,0000,,Again, notice there is a flow of charges,
Dialogue: 0,0:00:51.84,0:00:54.51,Default,,0000,0000,0000,,but the total flow over here, total charge
Dialogue: 0,0:00:54.51,0:00:55.98,Default,,0000,0000,0000,,that's flowing is zero. (chuckles)
Dialogue: 0,0:00:55.98,0:00:57.87,Default,,0000,0000,0000,,So net charge is still zero
Dialogue: 0,0:00:57.87,0:01:00.51,Default,,0000,0000,0000,,and therefore there is no\Nelectric current over here.
Dialogue: 0,0:01:00.51,0:01:05.07,Default,,0000,0000,0000,,Okay, what about now? Ooh, now\Nwe do have electric current.
Dialogue: 0,0:01:05.07,0:01:08.61,Default,,0000,0000,0000,,Now we have a net positive\Ncharge flowing to the right.
Dialogue: 0,0:01:08.61,0:01:10.17,Default,,0000,0000,0000,,Over here there is an electric current.
Dialogue: 0,0:01:10.17,0:01:13.80,Default,,0000,0000,0000,,Now we do have a net negative\Ncharges flowing to the right.
Dialogue: 0,0:01:13.80,0:01:16.56,Default,,0000,0000,0000,,We do have an electric current. Okay?
Dialogue: 0,0:01:16.56,0:01:20.94,Default,,0000,0000,0000,,So it's a flow of net charge,\Nbut how do you measure it?
Dialogue: 0,0:01:20.94,0:01:24.54,Default,,0000,0000,0000,,Well, we measure it as the\Namount of charges flowing
Dialogue: 0,0:01:24.54,0:01:27.90,Default,,0000,0000,0000,,through any given\Ncross-sectional area per second.
Dialogue: 0,0:01:27.90,0:01:31.17,Default,,0000,0000,0000,,So you can think of it\Nas coulomb per second.
Dialogue: 0,0:01:31.17,0:01:35.16,Default,,0000,0000,0000,,How many coulombs are flowing per second?
Dialogue: 0,0:01:35.16,0:01:37.59,Default,,0000,0000,0000,,And the coulombs per\Nsecond is also called,
Dialogue: 0,0:01:38.58,0:01:43.58,Default,,0000,0000,0000,,it's also called Amperes,\Nokay? Capital A, Amperes.
Dialogue: 0,0:01:43.62,0:01:45.39,Default,,0000,0000,0000,,And just to give you typical numbers,
Dialogue: 0,0:01:45.39,0:01:47.67,Default,,0000,0000,0000,,your air conditioners heaters,
Dialogue: 0,0:01:47.67,0:01:50.58,Default,,0000,0000,0000,,they drop out 10 to 15 Amperes of current.
Dialogue: 0,0:01:50.58,0:01:54.33,Default,,0000,0000,0000,,Your ceiling fan tube lights,\Ntelevision sets less than
Dialogue: 0,0:01:54.33,0:01:56.13,Default,,0000,0000,0000,,that, about one or two amps.
Dialogue: 0,0:01:56.13,0:01:59.25,Default,,0000,0000,0000,,And your smaller circuits\Nlike you know the toy circuits
Dialogue: 0,0:01:59.25,0:02:01.47,Default,,0000,0000,0000,,and stuff, they would be even lesser.
Dialogue: 0,0:02:01.47,0:02:04.40,Default,,0000,0000,0000,,It would be fraction of Amperes.\NBut what about lightning?
Dialogue: 0,0:02:04.40,0:02:05.46,Default,,0000,0000,0000,,Ooh. (chuckles)
Dialogue: 0,0:02:05.46,0:02:09.27,Default,,0000,0000,0000,,Lightning can have tens of\Nthousands of Amperes in them.
Dialogue: 0,0:02:09.27,0:02:11.10,Default,,0000,0000,0000,,Okay, how do we set up\Nan electric current?
Dialogue: 0,0:02:11.10,0:02:12.45,Default,,0000,0000,0000,,How do we get an electric current?
Dialogue: 0,0:02:12.45,0:02:16.38,Default,,0000,0000,0000,,Well, for an electric\Ncurrent we need a voltage.
Dialogue: 0,0:02:16.38,0:02:18.60,Default,,0000,0000,0000,,Just like how, if you need\Nto make a ball roll, you need
Dialogue: 0,0:02:18.60,0:02:20.91,Default,,0000,0000,0000,,to have a height difference,
Dialogue: 0,0:02:20.91,0:02:24.33,Default,,0000,0000,0000,,which produces a gravitational\Npotential difference
Dialogue: 0,0:02:24.33,0:02:26.10,Default,,0000,0000,0000,,across the end of say a plank.
Dialogue: 0,0:02:26.10,0:02:30.03,Default,,0000,0000,0000,,Similarly, if you need to set\Nup a current through a wire,
Dialogue: 0,0:02:30.03,0:02:34.11,Default,,0000,0000,0000,,you need to have an electric\Npotential difference
Dialogue: 0,0:02:34.11,0:02:35.49,Default,,0000,0000,0000,,across the ends of it.
Dialogue: 0,0:02:35.49,0:02:37.17,Default,,0000,0000,0000,,When you have an electric\Npotential difference,
Dialogue: 0,0:02:37.17,0:02:39.72,Default,,0000,0000,0000,,you can get a current, but\Nyou also need to make sure
Dialogue: 0,0:02:39.72,0:02:41.70,Default,,0000,0000,0000,,that there are some charges.
Dialogue: 0,0:02:41.70,0:02:45.15,Default,,0000,0000,0000,,There are charges that are\Nfree to move in your material.
Dialogue: 0,0:02:45.15,0:02:49.05,Default,,0000,0000,0000,,Not all materials have that,\Nfor example, glass or plastic.
Dialogue: 0,0:02:49.05,0:02:50.67,Default,,0000,0000,0000,,Well, they don't have free charges because
Dialogue: 0,0:02:50.67,0:02:52.86,Default,,0000,0000,0000,,if you look inside them,\Nwell you can model them
Dialogue: 0,0:02:52.86,0:02:54.06,Default,,0000,0000,0000,,and say that you know what?
Dialogue: 0,0:02:54.06,0:02:57.33,Default,,0000,0000,0000,,The electrons inside these\Natoms are very tightly bound.
Dialogue: 0,0:02:57.33,0:02:59.16,Default,,0000,0000,0000,,So there are no free electrons to move.
Dialogue: 0,0:02:59.16,0:03:00.45,Default,,0000,0000,0000,,There are no charges to move.
Dialogue: 0,0:03:00.45,0:03:01.28,Default,,0000,0000,0000,,So if you put a voltage
Dialogue: 0,0:03:01.28,0:03:04.23,Default,,0000,0000,0000,,across them, you'll probably\Nget no current over here.
Dialogue: 0,0:03:04.23,0:03:07.02,Default,,0000,0000,0000,,We call such material insulators, glass,
Dialogue: 0,0:03:07.02,0:03:10.92,Default,,0000,0000,0000,,wood, plastic, these are\Nexamples of insulators.
Dialogue: 0,0:03:10.92,0:03:12.51,Default,,0000,0000,0000,,On the other hand, if you take metals
Dialogue: 0,0:03:12.51,0:03:14.85,Default,,0000,0000,0000,,of which wires are made\Nof, then you'll find
Dialogue: 0,0:03:14.85,0:03:18.12,Default,,0000,0000,0000,,that the outermost electrons\Nare not tightly bound.
Dialogue: 0,0:03:18.12,0:03:21.96,Default,,0000,0000,0000,,As a result, they are free\Nto move around the material.
Dialogue: 0,0:03:21.96,0:03:23.64,Default,,0000,0000,0000,,We call them free electrons.
Dialogue: 0,0:03:23.64,0:03:25.95,Default,,0000,0000,0000,,And since you have free charges available
Dialogue: 0,0:03:25.95,0:03:28.22,Default,,0000,0000,0000,,for motion, we call these\Nmaterials conductors
Dialogue: 0,0:03:28.22,0:03:30.54,Default,,0000,0000,0000,,because if you put a voltage across them,
Dialogue: 0,0:03:30.54,0:03:34.26,Default,,0000,0000,0000,,well these electrons can move\Nand contribute to current.
Dialogue: 0,0:03:34.26,0:03:37.35,Default,,0000,0000,0000,,So you need a voltage\Nacross a conducting medium
Dialogue: 0,0:03:37.35,0:03:39.93,Default,,0000,0000,0000,,for electric current.
Dialogue: 0,0:03:39.93,0:03:42.75,Default,,0000,0000,0000,,Okay, but how do you get a\Nvoltage in the first place?
Dialogue: 0,0:03:42.75,0:03:45.05,Default,,0000,0000,0000,,Well, in small circuits,\Nyou probably already know,
Dialogue: 0,0:03:45.05,0:03:47.31,Default,,0000,0000,0000,,voltage is given by a battery.
Dialogue: 0,0:03:47.31,0:03:49.44,Default,,0000,0000,0000,,One end of the battery\Nis at a higher potential,
Dialogue: 0,0:03:49.44,0:03:51.25,Default,,0000,0000,0000,,another end of the battery\Nis at lower potential.
Dialogue: 0,0:03:51.25,0:03:53.85,Default,,0000,0000,0000,,And when you connect it\Nto a circuit, it provides
Dialogue: 0,0:03:53.85,0:03:54.96,Default,,0000,0000,0000,,the potential difference.
Dialogue: 0,0:03:54.96,0:03:57.48,Default,,0000,0000,0000,,But in larger circuits for\Nlike for example, the circuits
Dialogue: 0,0:03:57.48,0:04:00.33,Default,,0000,0000,0000,,in our houses, well the\Npotential difference is provided
Dialogue: 0,0:04:00.33,0:04:03.87,Default,,0000,0000,0000,,by large electric generators\Nin our power stations.
Dialogue: 0,0:04:03.87,0:04:06.87,Default,,0000,0000,0000,,And by the way, while drawing\Na battery in our circuit,
Dialogue: 0,0:04:06.87,0:04:09.12,Default,,0000,0000,0000,,well we use a circuit\Nsymbol that looks like this.
Dialogue: 0,0:04:09.12,0:04:12.48,Default,,0000,0000,0000,,The longer line represents\Nthe positive terminal
Dialogue: 0,0:04:12.48,0:04:15.36,Default,,0000,0000,0000,,and the shorter thick line\Nrepresents a negative terminal.
Dialogue: 0,0:04:15.36,0:04:16.92,Default,,0000,0000,0000,,So that if you just draw\Nthis, we don't have to draw
Dialogue: 0,0:04:16.92,0:04:18.87,Default,,0000,0000,0000,,like a big battery over here.
Dialogue: 0,0:04:18.87,0:04:20.46,Default,,0000,0000,0000,,Anyways, even though we have a battery
Dialogue: 0,0:04:20.46,0:04:23.10,Default,,0000,0000,0000,,in this circuit right now,\Nwe don't have a current,
Dialogue: 0,0:04:23.10,0:04:25.71,Default,,0000,0000,0000,,we don't have a potential\Ndifference across this bulb.
Dialogue: 0,0:04:25.71,0:04:26.58,Default,,0000,0000,0000,,Why?
Dialogue: 0,0:04:26.58,0:04:27.90,Default,,0000,0000,0000,,Well, you can see over here, that's
Dialogue: 0,0:04:27.90,0:04:30.39,Default,,0000,0000,0000,,because the circuit is not closed.
Dialogue: 0,0:04:30.39,0:04:32.97,Default,,0000,0000,0000,,We say because there\Nis some air in between.
Dialogue: 0,0:04:32.97,0:04:35.52,Default,,0000,0000,0000,,Air is an excellent insulator and
Dialogue: 0,0:04:35.52,0:04:38.13,Default,,0000,0000,0000,,therefore there's not going\Nto be any current over here.
Dialogue: 0,0:04:38.13,0:04:40.02,Default,,0000,0000,0000,,In order for there to\Nbe a current, we need
Dialogue: 0,0:04:40.02,0:04:43.41,Default,,0000,0000,0000,,to close the circuit, meaning\Nwe need to connect this gap
Dialogue: 0,0:04:43.41,0:04:45.93,Default,,0000,0000,0000,,and that's where the switch\Nis, this is a switch.
Dialogue: 0,0:04:45.93,0:04:48.63,Default,,0000,0000,0000,,So if I close the switch
Dialogue: 0,0:04:48.63,0:04:52.77,Default,,0000,0000,0000,,like this, now the circuit is complete.
Dialogue: 0,0:04:52.77,0:04:55.32,Default,,0000,0000,0000,,Now there'll be a potential\Ndifference across the ends
Dialogue: 0,0:04:55.32,0:04:58.50,Default,,0000,0000,0000,,of the ball when now there'll\Nbe a current over here.
Dialogue: 0,0:04:58.50,0:05:00.00,Default,,0000,0000,0000,,I'm gonna open the switch.
Dialogue: 0,0:05:00.00,0:05:02.25,Default,,0000,0000,0000,,There is no electric current,\Nthe circuit is broken.
Dialogue: 0,0:05:02.25,0:05:04.95,Default,,0000,0000,0000,,Close the switch, there's going\Nto be an electric current.
Dialogue: 0,0:05:04.95,0:05:06.69,Default,,0000,0000,0000,,Now because I compared charges moving
Dialogue: 0,0:05:06.69,0:05:08.88,Default,,0000,0000,0000,,through a ball rolling\Ndown, we might model it
Dialogue: 0,0:05:08.88,0:05:12.30,Default,,0000,0000,0000,,by thinking that hey,\Nwhen there is no voltage,
Dialogue: 0,0:05:12.30,0:05:14.10,Default,,0000,0000,0000,,all the charges are at\Nrest, say the electrons
Dialogue: 0,0:05:14.10,0:05:16.70,Default,,0000,0000,0000,,over here are at rest and when\NI do complete the circuit,
Dialogue: 0,0:05:16.70,0:05:18.78,Default,,0000,0000,0000,,the electrons are now nicely moving.
Dialogue: 0,0:05:18.78,0:05:20.91,Default,,0000,0000,0000,,But that's not a very accurate\Nway to think about it,
Dialogue: 0,0:05:20.91,0:05:22.05,Default,,0000,0000,0000,,that's not a good model.
Dialogue: 0,0:05:22.05,0:05:23.97,Default,,0000,0000,0000,,Instead, a better model\Nis if you were to peek
Dialogue: 0,0:05:23.97,0:05:26.61,Default,,0000,0000,0000,,inside the wire, we\Nfind that the electrons
Dialogue: 0,0:05:26.61,0:05:28.20,Default,,0000,0000,0000,,are randomly moving, bumping
Dialogue: 0,0:05:28.20,0:05:29.91,Default,,0000,0000,0000,,into stuff because they have a lot
Dialogue: 0,0:05:29.91,0:05:32.88,Default,,0000,0000,0000,,of energy even when there is no voltage.
Dialogue: 0,0:05:32.88,0:05:34.47,Default,,0000,0000,0000,,So they're not at rest, they're
Dialogue: 0,0:05:34.47,0:05:36.96,Default,,0000,0000,0000,,in fact moving at very high speeds.
Dialogue: 0,0:05:36.96,0:05:38.76,Default,,0000,0000,0000,,But what happens when we close the switch?
Dialogue: 0,0:05:38.76,0:05:40.74,Default,,0000,0000,0000,,When we close the circuit, look,
Dialogue: 0,0:05:40.74,0:05:42.27,Default,,0000,0000,0000,,there is a potential difference
Dialogue: 0,0:05:42.27,0:05:45.24,Default,,0000,0000,0000,,and therefore there is an\Nelectric field setup in the wire
Dialogue: 0,0:05:45.24,0:05:47.64,Default,,0000,0000,0000,,that electric field starts\Npushing on the electrons.
Dialogue: 0,0:05:47.64,0:05:49.08,Default,,0000,0000,0000,,And look, you can now see the electrons
Dialogue: 0,0:05:49.08,0:05:51.72,Default,,0000,0000,0000,,are slowly drifting to the left.
Dialogue: 0,0:05:51.72,0:05:56.25,Default,,0000,0000,0000,,It's that drifting motion\Nthat constitutes the current
Dialogue: 0,0:05:56.25,0:05:58.38,Default,,0000,0000,0000,,and what causes them to drift to the left?
Dialogue: 0,0:05:58.38,0:06:00.03,Default,,0000,0000,0000,,Again, there are some analogies which says
Dialogue: 0,0:06:00.03,0:06:02.82,Default,,0000,0000,0000,,that electrons push on each\Nother making them drift.
Dialogue: 0,0:06:02.82,0:06:05.13,Default,,0000,0000,0000,,But that's again not very accurate.
Dialogue: 0,0:06:05.13,0:06:06.66,Default,,0000,0000,0000,,A better way to think about it is
Dialogue: 0,0:06:06.66,0:06:08.79,Default,,0000,0000,0000,,that the battery produces\Nthe electric field.
Dialogue: 0,0:06:08.79,0:06:10.59,Default,,0000,0000,0000,,There's an electric field\Nset up inside the wire.
Dialogue: 0,0:06:10.59,0:06:14.16,Default,,0000,0000,0000,,It's that electric field that is causing,
Dialogue: 0,0:06:14.16,0:06:16.38,Default,,0000,0000,0000,,that's pushing the\Nelectrons, making them drift
Dialogue: 0,0:06:16.38,0:06:17.85,Default,,0000,0000,0000,,to the left over here.
Dialogue: 0,0:06:17.85,0:06:19.26,Default,,0000,0000,0000,,But wait a second, why did I show
Dialogue: 0,0:06:19.26,0:06:22.38,Default,,0000,0000,0000,,that the electrons are\Ndrifting to the left over here?
Dialogue: 0,0:06:22.38,0:06:23.76,Default,,0000,0000,0000,,Let's think about it.
Dialogue: 0,0:06:23.76,0:06:25.35,Default,,0000,0000,0000,,So one way to think\Nabout it's, you could say
Dialogue: 0,0:06:25.35,0:06:26.67,Default,,0000,0000,0000,,that hey, electrons are being attracted
Dialogue: 0,0:06:26.67,0:06:29.49,Default,,0000,0000,0000,,by the positive terminal of\Nthe battery being repelled
Dialogue: 0,0:06:29.49,0:06:31.47,Default,,0000,0000,0000,,by the negative terminal\Nof the battery, making
Dialogue: 0,0:06:31.47,0:06:33.15,Default,,0000,0000,0000,,the electrons go this way.
Dialogue: 0,0:06:33.15,0:06:34.71,Default,,0000,0000,0000,,But a question that could raise is,
Dialogue: 0,0:06:34.71,0:06:36.75,Default,,0000,0000,0000,,in the wire that means\Nthe electrons are going
Dialogue: 0,0:06:36.75,0:06:39.51,Default,,0000,0000,0000,,from a lower potential\Nto a higher potential
Dialogue: 0,0:06:39.51,0:06:40.53,Default,,0000,0000,0000,,like going uphill.
Dialogue: 0,0:06:40.53,0:06:41.82,Default,,0000,0000,0000,,How does that make any sense?
Dialogue: 0,0:06:41.82,0:06:43.92,Default,,0000,0000,0000,,That was a point of\Nconfusion for a long time.
Dialogue: 0,0:06:43.92,0:06:46.38,Default,,0000,0000,0000,,So let's talk about it a little bit. Okay?
Dialogue: 0,0:06:46.38,0:06:48.27,Default,,0000,0000,0000,,If I have a big positive charge
Dialogue: 0,0:06:48.27,0:06:52.05,Default,,0000,0000,0000,,and next to it I keep a\Nvery tiny positive charge
Dialogue: 0,0:06:52.05,0:06:53.64,Default,,0000,0000,0000,,and at rest, let's say,
Dialogue: 0,0:06:53.64,0:06:56.22,Default,,0000,0000,0000,,and I let go of it, then\Nwe know it gets repelled
Dialogue: 0,0:06:56.22,0:06:59.07,Default,,0000,0000,0000,,and it gains kinetic\Nenergy in this direction.
Dialogue: 0,0:06:59.07,0:07:01.14,Default,,0000,0000,0000,,Now because energy is conserved,\Nwe could ask where did
Dialogue: 0,0:07:01.14,0:07:02.49,Default,,0000,0000,0000,,that kinetic energy come from?
Dialogue: 0,0:07:02.49,0:07:06.30,Default,,0000,0000,0000,,We say, ah, there it must have\Ncome from potential energy.
Dialogue: 0,0:07:06.30,0:07:07.86,Default,,0000,0000,0000,,So as it goes from here to here,
Dialogue: 0,0:07:07.86,0:07:10.41,Default,,0000,0000,0000,,the system must lose potential energy
Dialogue: 0,0:07:10.41,0:07:12.06,Default,,0000,0000,0000,,and therefore we can now say that hey,
Dialogue: 0,0:07:12.06,0:07:14.94,Default,,0000,0000,0000,,this point represents\Nhigh potential region.
Dialogue: 0,0:07:14.94,0:07:17.49,Default,,0000,0000,0000,,This point represents low potential region
Dialogue: 0,0:07:17.49,0:07:21.51,Default,,0000,0000,0000,,and this represents the downhill\Ndirection for the charges.
Dialogue: 0,0:07:21.51,0:07:23.28,Default,,0000,0000,0000,,As you go from here to\Nhere, it's potential energy
Dialogue: 0,0:07:23.28,0:07:24.93,Default,,0000,0000,0000,,starts getting converted\Ninto kinetic energy.
Dialogue: 0,0:07:24.93,0:07:27.63,Default,,0000,0000,0000,,Kind of like what happens\Nto this ball rolling down.
Dialogue: 0,0:07:27.63,0:07:30.00,Default,,0000,0000,0000,,But what about negative charges?
Dialogue: 0,0:07:30.00,0:07:32.04,Default,,0000,0000,0000,,Well, negative charges will\Nbe exactly the opposite.
Dialogue: 0,0:07:32.04,0:07:35.04,Default,,0000,0000,0000,,They will get attracted\Nby this positive charge.
Dialogue: 0,0:07:35.04,0:07:38.64,Default,,0000,0000,0000,,So they will gain kinetic energy this way.
Dialogue: 0,0:07:38.64,0:07:41.04,Default,,0000,0000,0000,,And for negative charges,\Nit's the exact opposite
Dialogue: 0,0:07:41.04,0:07:43.50,Default,,0000,0000,0000,,as they go from here to\Nhere, this is a direction
Dialogue: 0,0:07:43.50,0:07:45.87,Default,,0000,0000,0000,,in which they are losing potential energy
Dialogue: 0,0:07:45.87,0:07:47.46,Default,,0000,0000,0000,,and gaining kinetic energy.
Dialogue: 0,0:07:47.46,0:07:49.14,Default,,0000,0000,0000,,So this must be high, this must be low,
Dialogue: 0,0:07:49.14,0:07:51.42,Default,,0000,0000,0000,,this should represent the\Ndirection of the downhill.
Dialogue: 0,0:07:51.42,0:07:53.88,Default,,0000,0000,0000,,But now the problem is which\Ndirection should we say
Dialogue: 0,0:07:53.88,0:07:56.34,Default,,0000,0000,0000,,is down for the charges?
Dialogue: 0,0:07:56.34,0:07:58.68,Default,,0000,0000,0000,,Well, we could say, hey, for\Npositive charge, this is down
Dialogue: 0,0:07:58.68,0:08:00.87,Default,,0000,0000,0000,,and say negative charges,\Nthis is the down,
Dialogue: 0,0:08:00.87,0:08:03.21,Default,,0000,0000,0000,,but we decided no, no, no,\Nlet's just use one of these
Dialogue: 0,0:08:03.21,0:08:06.12,Default,,0000,0000,0000,,as our reference and we'll\Njust consider one direction
Dialogue: 0,0:08:06.12,0:08:07.59,Default,,0000,0000,0000,,as our actual down.
Dialogue: 0,0:08:07.59,0:08:09.51,Default,,0000,0000,0000,,So we decided, hey, whatever happens
Dialogue: 0,0:08:09.51,0:08:11.58,Default,,0000,0000,0000,,for a positive charge,\Nlet's use positive charge
Dialogue: 0,0:08:11.58,0:08:12.45,Default,,0000,0000,0000,,as our reference,
Dialogue: 0,0:08:12.45,0:08:14.85,Default,,0000,0000,0000,,and whichever direction\Npositive charge natural tends
Dialogue: 0,0:08:14.85,0:08:17.46,Default,,0000,0000,0000,,to go, we'll call that\Ndirection as our down
Dialogue: 0,0:08:17.46,0:08:19.86,Default,,0000,0000,0000,,for charges, right, down in potential.
Dialogue: 0,0:08:19.86,0:08:21.69,Default,,0000,0000,0000,,Because of that reference,
Dialogue: 0,0:08:21.69,0:08:24.12,Default,,0000,0000,0000,,by definition, positive charges go
Dialogue: 0,0:08:24.12,0:08:25.95,Default,,0000,0000,0000,,down the electric potential.
Dialogue: 0,0:08:25.95,0:08:27.99,Default,,0000,0000,0000,,Negative charges look end up going
Dialogue: 0,0:08:27.99,0:08:29.50,Default,,0000,0000,0000,,up the electric potential, not
Dialogue: 0,0:08:29.50,0:08:31.14,Default,,0000,0000,0000,,because they're literally going
Dialogue: 0,0:08:31.14,0:08:32.85,Default,,0000,0000,0000,,to a higher potential energy region.
Dialogue: 0,0:08:32.85,0:08:34.11,Default,,0000,0000,0000,,No, no, they're also going towards
Dialogue: 0,0:08:34.11,0:08:35.34,Default,,0000,0000,0000,,lower potential energy region.
Dialogue: 0,0:08:35.34,0:08:37.98,Default,,0000,0000,0000,,It's just a reference because\Nour reference point for high
Dialogue: 0,0:08:37.98,0:08:40.35,Default,,0000,0000,0000,,and low is chosen, you\Nknow, from the perspective
Dialogue: 0,0:08:40.35,0:08:41.85,Default,,0000,0000,0000,,of a positive charge.
Dialogue: 0,0:08:41.85,0:08:43.20,Default,,0000,0000,0000,,Because of that reference,
Dialogue: 0,0:08:43.20,0:08:46.20,Default,,0000,0000,0000,,negative charges end up\Ngoing up the potential,
Dialogue: 0,0:08:46.20,0:08:48.33,Default,,0000,0000,0000,,they have a natural tendency\Nto go up the potential.
Dialogue: 0,0:08:48.33,0:08:49.17,Default,,0000,0000,0000,,Does that make sense?
Dialogue: 0,0:08:49.17,0:08:51.15,Default,,0000,0000,0000,,And therefore, electrons,\Nwhich are negative charges,
Dialogue: 0,0:08:51.15,0:08:54.99,Default,,0000,0000,0000,,have a natural tendency to\Ngo up the electric potential.
Dialogue: 0,0:08:54.99,0:08:56.58,Default,,0000,0000,0000,,Now, the final question we could have is
Dialogue: 0,0:08:56.58,0:08:57.66,Default,,0000,0000,0000,,the direction of the current.
Dialogue: 0,0:08:57.66,0:08:59.79,Default,,0000,0000,0000,,What is the direction of\Nthe current over here?
Dialogue: 0,0:08:59.79,0:09:01.23,Default,,0000,0000,0000,,Well, we could say, hey,
Dialogue: 0,0:09:01.23,0:09:02.94,Default,,0000,0000,0000,,whichever direction the\Ncharges are drifting, well
Dialogue: 0,0:09:02.94,0:09:04.92,Default,,0000,0000,0000,,that itself could be the\Ndirection of the current.
Dialogue: 0,0:09:04.92,0:09:06.63,Default,,0000,0000,0000,,That's the most natural way\Nto think about it, right?
Dialogue: 0,0:09:06.63,0:09:07.92,Default,,0000,0000,0000,,So electrons are drifting this way.
Dialogue: 0,0:09:07.92,0:09:10.11,Default,,0000,0000,0000,,So let's say that that is the current,
Dialogue: 0,0:09:10.11,0:09:11.04,Default,,0000,0000,0000,,but again, there's a problem
Dialogue: 0,0:09:11.04,0:09:12.84,Default,,0000,0000,0000,,because we have positive\Nand negative charges.
Dialogue: 0,0:09:12.84,0:09:16.08,Default,,0000,0000,0000,,Remember that example\Nwhere we had both positive
Dialogue: 0,0:09:16.08,0:09:17.19,Default,,0000,0000,0000,,and negative charge, equal positive
Dialogue: 0,0:09:17.19,0:09:18.02,Default,,0000,0000,0000,,and negative charges flowing
Dialogue: 0,0:09:18.02,0:09:19.65,Default,,0000,0000,0000,,through an area giving me zero current
Dialogue: 0,0:09:19.65,0:09:21.90,Default,,0000,0000,0000,,because a net charge over here is zero.
Dialogue: 0,0:09:21.90,0:09:23.76,Default,,0000,0000,0000,,Well, if I said that, hey, you know,
Dialogue: 0,0:09:23.76,0:09:25.62,Default,,0000,0000,0000,,whichever direction charges\Nare moving, let's just call
Dialogue: 0,0:09:25.62,0:09:27.99,Default,,0000,0000,0000,,that direction as the current,\Nthen I have a problem.
Dialogue: 0,0:09:27.99,0:09:29.75,Default,,0000,0000,0000,,Because I could say that hey,\Npositive charges is giving me
Dialogue: 0,0:09:29.75,0:09:32.13,Default,,0000,0000,0000,,a current this way, negative charges
Dialogue: 0,0:09:32.13,0:09:34.05,Default,,0000,0000,0000,,also giving me a current this way,
Dialogue: 0,0:09:34.05,0:09:36.48,Default,,0000,0000,0000,,but I know the total current must be zero.
Dialogue: 0,0:09:36.48,0:09:38.79,Default,,0000,0000,0000,,So that doesn't work\Nbecause you know these two,
Dialogue: 0,0:09:38.79,0:09:41.25,Default,,0000,0000,0000,,if I add up, I don't get zero,\NI should get a net current
Dialogue: 0,0:09:41.25,0:09:42.39,Default,,0000,0000,0000,,to the right, but that's not true.
Dialogue: 0,0:09:42.39,0:09:44.43,Default,,0000,0000,0000,,I know that the current should be zero.
Dialogue: 0,0:09:44.43,0:09:47.43,Default,,0000,0000,0000,,Again, to solve for that, we\Ndecided, hey, you know what?
Dialogue: 0,0:09:47.43,0:09:50.10,Default,,0000,0000,0000,,Whichever direction, positive\Ncharges are moving, we'll say
Dialogue: 0,0:09:50.10,0:09:52.08,Default,,0000,0000,0000,,that is the direction of the current.
Dialogue: 0,0:09:52.08,0:09:53.91,Default,,0000,0000,0000,,And for the negative charges,
Dialogue: 0,0:09:53.91,0:09:56.04,Default,,0000,0000,0000,,we'll say the opposite is\Nthe direction of the current.
Dialogue: 0,0:09:56.04,0:09:59.13,Default,,0000,0000,0000,,So we said if the negative\Ncharges are moving to the right,
Dialogue: 0,0:09:59.13,0:10:02.64,Default,,0000,0000,0000,,we will say the direction of\Nthe current is to the left.
Dialogue: 0,0:10:02.64,0:10:06.36,Default,,0000,0000,0000,,And now look, now the\Ntotal current becomes zero
Dialogue: 0,0:10:06.36,0:10:09.57,Default,,0000,0000,0000,,because your right and\Nleft current cancels out.
Dialogue: 0,0:10:09.57,0:10:10.83,Default,,0000,0000,0000,,Now it makes sense.
Dialogue: 0,0:10:10.83,0:10:12.33,Default,,0000,0000,0000,,So the convention
Dialogue: 0,0:10:12.33,0:10:14.52,Default,,0000,0000,0000,,for choosing the direction of the current
Dialogue: 0,0:10:14.52,0:10:17.07,Default,,0000,0000,0000,,is whichever direction\Npositive charges are going,
Dialogue: 0,0:10:17.07,0:10:19.05,Default,,0000,0000,0000,,that is the direction of the current.
Dialogue: 0,0:10:19.05,0:10:21.30,Default,,0000,0000,0000,,If you have negative charges, opposite.
Dialogue: 0,0:10:21.30,0:10:23.49,Default,,0000,0000,0000,,Whichever direction negative\Ncharges are going, opposite to
Dialogue: 0,0:10:23.49,0:10:25.53,Default,,0000,0000,0000,,that, that will be the\Ndirection of the current.
Dialogue: 0,0:10:25.53,0:10:26.52,Default,,0000,0000,0000,,Okay? (chuckles)
Dialogue: 0,0:10:26.52,0:10:29.67,Default,,0000,0000,0000,,Now, because in wires, it's the electrons
Dialogue: 0,0:10:29.67,0:10:32.16,Default,,0000,0000,0000,,that are always drifting,\Nthat's those are the one
Dialogue: 0,0:10:32.16,0:10:33.42,Default,,0000,0000,0000,,that constitutes the current
Dialogue: 0,0:10:33.42,0:10:36.81,Default,,0000,0000,0000,,and the electrons are\Nnegatively charged particles.
Dialogue: 0,0:10:36.81,0:10:39.93,Default,,0000,0000,0000,,Our convention for the current\Nwould be not the direction
Dialogue: 0,0:10:39.93,0:10:42.03,Default,,0000,0000,0000,,of the electron flow, but\Nin the opposite direction
Dialogue: 0,0:10:42.03,0:10:44.76,Default,,0000,0000,0000,,of the electron flow,\Nit would be this way.
Dialogue: 0,0:10:44.76,0:10:47.97,Default,,0000,0000,0000,,So the conventional direction\Nof the current, notice, is
Dialogue: 0,0:10:47.97,0:10:51.12,Default,,0000,0000,0000,,in the opposite direction\Nof the electron flow.
Dialogue: 0,0:10:51.12,0:10:53.34,Default,,0000,0000,0000,,And I'll tell you what can be frustrating
Dialogue: 0,0:10:53.34,0:10:57.15,Default,,0000,0000,0000,,because in most cases we'll be\Ndealing with electron flows.
Dialogue: 0,0:10:57.15,0:10:58.23,Default,,0000,0000,0000,,This will be frustrating because
Dialogue: 0,0:10:58.23,0:11:00.48,Default,,0000,0000,0000,,in most cases our\Nconventional current will be
Dialogue: 0,0:11:00.48,0:11:03.09,Default,,0000,0000,0000,,in the opposite direction\Nof the actual motion
Dialogue: 0,0:11:03.09,0:11:05.76,Default,,0000,0000,0000,,of the charges, actual\Ndrifting motion of the charges.
Dialogue: 0,0:11:05.76,0:11:07.93,Default,,0000,0000,0000,,But it's unfortunate that electrons,
Dialogue: 0,0:11:07.93,0:11:09.39,Default,,0000,0000,0000,,which are the major charge carriers
Dialogue: 0,0:11:09.39,0:11:11.73,Default,,0000,0000,0000,,in most of the circuits,\Nend up being (chuckles)
Dialogue: 0,0:11:11.73,0:11:13.50,Default,,0000,0000,0000,,a negatively charged particle.
Dialogue: 0,0:11:13.50,0:11:15.63,Default,,0000,0000,0000,,And our positive charges\Nare reference for us.
Dialogue: 0,0:11:15.63,0:11:18.39,Default,,0000,0000,0000,,And so it might slightly\Nfeel awkward initially,
Dialogue: 0,0:11:18.39,0:11:20.85,Default,,0000,0000,0000,,but you'll get used to\Nit, don't worry too much.
Dialogue: 0,0:11:20.85,0:11:22.62,Default,,0000,0000,0000,,This now finally brings us to lightning.
Dialogue: 0,0:11:22.62,0:11:24.18,Default,,0000,0000,0000,,What exactly is lightning?
Dialogue: 0,0:11:24.18,0:11:26.19,Default,,0000,0000,0000,,Well, lightning is also\Nan electric current,
Dialogue: 0,0:11:26.19,0:11:28.50,Default,,0000,0000,0000,,meaning flow of charges.
Dialogue: 0,0:11:28.50,0:11:30.21,Default,,0000,0000,0000,,But how does it happen?
Dialogue: 0,0:11:30.21,0:11:33.03,Default,,0000,0000,0000,,And more importantly, lightning is a flow
Dialogue: 0,0:11:33.03,0:11:35.73,Default,,0000,0000,0000,,of charges through air,\Nbut air is an insulator.
Dialogue: 0,0:11:35.73,0:11:38.16,Default,,0000,0000,0000,,And we saw that insulators\Ndo not conduct electricity.
Dialogue: 0,0:11:38.16,0:11:39.57,Default,,0000,0000,0000,,So what's going on over here?
Dialogue: 0,0:11:39.57,0:11:40.98,Default,,0000,0000,0000,,Well, we'll not give you too much details,
Dialogue: 0,0:11:40.98,0:11:45.00,Default,,0000,0000,0000,,but it turns out that clouds\Nusually have charges separated.
Dialogue: 0,0:11:45.00,0:11:47.43,Default,,0000,0000,0000,,The top of it is usually\Npositively charged
Dialogue: 0,0:11:47.43,0:11:50.25,Default,,0000,0000,0000,,and the bottom is negatively charged.
Dialogue: 0,0:11:50.25,0:11:52.13,Default,,0000,0000,0000,,Now because the bottom is closer
Dialogue: 0,0:11:52.13,0:11:55.44,Default,,0000,0000,0000,,to the earth, the negative\Ncharges push electrons
Dialogue: 0,0:11:55.44,0:11:58.41,Default,,0000,0000,0000,,of the earth away from\Nit 'cause negative repel.
Dialogue: 0,0:11:58.41,0:12:02.10,Default,,0000,0000,0000,,And as the electrons get\Nrepelled away, the surface
Dialogue: 0,0:12:02.10,0:12:06.54,Default,,0000,0000,0000,,of the ground will be\Nmostly positively charged.
Dialogue: 0,0:12:06.54,0:12:09.84,Default,,0000,0000,0000,,Now during a thunderstorm,\Nthe charges builds up
Dialogue: 0,0:12:09.84,0:12:11.61,Default,,0000,0000,0000,,because the air is an insulator,
Dialogue: 0,0:12:11.61,0:12:12.93,Default,,0000,0000,0000,,because there's no corona over here,
Dialogue: 0,0:12:12.93,0:12:14.85,Default,,0000,0000,0000,,the charges can build up, and as a result,
Dialogue: 0,0:12:14.85,0:12:17.79,Default,,0000,0000,0000,,the potential difference\Nbecome incredibly high.
Dialogue: 0,0:12:17.79,0:12:20.37,Default,,0000,0000,0000,,It can reach millions of moist.
Dialogue: 0,0:12:20.37,0:12:24.78,Default,,0000,0000,0000,,Now, eventually what happens\Nis that the electrons
Dialogue: 0,0:12:24.78,0:12:28.50,Default,,0000,0000,0000,,from the atoms of the air\Nmolecules, like oxygen, nitrogen,
Dialogue: 0,0:12:28.50,0:12:32.01,Default,,0000,0000,0000,,and all of those stuff can\Nactually get ripped apart.
Dialogue: 0,0:12:32.01,0:12:34.05,Default,,0000,0000,0000,,And we'll not get into\Nagain the details of how
Dialogue: 0,0:12:34.05,0:12:36.06,Default,,0000,0000,0000,,that happens, but you can now imagine,
Dialogue: 0,0:12:36.06,0:12:38.40,Default,,0000,0000,0000,,if electrons start getting ripped apart.
Dialogue: 0,0:12:38.40,0:12:41.37,Default,,0000,0000,0000,,Now we start having charges.
Dialogue: 0,0:12:41.37,0:12:42.81,Default,,0000,0000,0000,,Once we have charged particles
Dialogue: 0,0:12:42.81,0:12:45.78,Default,,0000,0000,0000,,in between, we have a conducting channel.
Dialogue: 0,0:12:45.78,0:12:47.73,Default,,0000,0000,0000,,And once we have that conducting channel,
Dialogue: 0,0:12:47.73,0:12:51.03,Default,,0000,0000,0000,,the charges can sort of\Nget dumped into the earth.
Dialogue: 0,0:12:51.03,0:12:53.73,Default,,0000,0000,0000,,And that's basically\Nwhat we call a lightning.
Dialogue: 0,0:12:53.73,0:12:55.80,Default,,0000,0000,0000,,Now this lightning produces a lot of heat.
Dialogue: 0,0:12:55.80,0:12:58.74,Default,,0000,0000,0000,,That's one of the reason it\Nglows and you can see it.
Dialogue: 0,0:12:58.74,0:13:01.02,Default,,0000,0000,0000,,But that heat also causes rapid expansions
Dialogue: 0,0:13:01.02,0:13:03.81,Default,,0000,0000,0000,,in the air, making the air vibrate.
Dialogue: 0,0:13:03.81,0:13:06.39,Default,,0000,0000,0000,,And these vibrations\Neventually reach our ears
Dialogue: 0,0:13:06.39,0:13:10.80,Default,,0000,0000,0000,,after some time, and we\Ncall that as thunder.
Dialogue: 0,0:13:10.80,0:13:14.55,Default,,0000,0000,0000,,So look, lightning is an\Nelectric current, and guess what?
Dialogue: 0,0:13:14.55,0:13:17.10,Default,,0000,0000,0000,,Sparking that happens\Nsometimes, those annoying sparks
Dialogue: 0,0:13:17.10,0:13:18.63,Default,,0000,0000,0000,,we get whenever we get charged up
Dialogue: 0,0:13:18.63,0:13:20.85,Default,,0000,0000,0000,,and we're trying to reach out\Nto a doorknob, for example.
Dialogue: 0,0:13:20.85,0:13:21.68,Default,,0000,0000,0000,,(laughs)
Dialogue: 0,0:13:21.68,0:13:23.43,Default,,0000,0000,0000,,It's very similar to what\Nhappens in a lightning.
Dialogue: 0,0:13:23.43,0:13:26.31,Default,,0000,0000,0000,,It's a miniature version of lightning.