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- [Lecturer] Electricity that lights up

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above looks very different
than lightning strikes,

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but they're actually more
similar than one might think

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because they both have electric current.

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So let's understand what
electric current is,

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how they're produced,

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and also get to understand a
little bit about lightning.

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So what exactly is electric current?

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Well think of electric current

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as a flow of net charge
through any given area.

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Here's what I mean by flow of net charge.

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Well imagine you have a
tiny cross-sectional area

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through which you have equal amount

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of positive charges flowing

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to the right and left in any given time.

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Now notice there is a flow,

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but there is no net flow (chuckles)

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and therefore here we say
there is zero current.

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Another interesting example is
what if you have equal amount

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of positive and negative charges flowing

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in the same direction in
the same time, let's say

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through again, a given
cross-sectional area.

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Again, notice there is a flow of charges,

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but the total flow over here, total charge

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that's flowing is zero. (chuckles)

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So net charge is still zero

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and therefore there is no
electric current over here.

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Okay, what about now? Ooh, now
we do have electric current.

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Now we have a net positive
charge flowing to the right.

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Over here there is an electric current.

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Now we do have a net negative
charges flowing to the right.

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We do have an electric current. Okay?

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So it's a flow of net charge,
but how do you measure it?

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Well, we measure it as the
amount of charges flowing

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through any given
cross-sectional area per second.

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So you can think of it
as coulomb per second.

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How many coulombs are flowing per second?

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And the coulombs per
second is also called,

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it's also called Amperes,
okay? Capital A, Amperes.

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And just to give you typical numbers,

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your air conditioners heaters,

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they drop out 10 to 15 Amperes of current.

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Your ceiling fan tube lights,
television sets less than

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that, about one or two amps.

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And your smaller circuits
like you know the toy circuits

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and stuff, they would be even lesser.

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It would be fraction of Amperes.
But what about lightning?

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Ooh. (chuckles)

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Lightning can have tens of
thousands of Amperes in them.

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Okay, how do we set up
an electric current?

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How do we get an electric current?

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Well, for an electric
current we need a voltage.

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Just like how, if you need
to make a ball roll, you need

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to have a height difference,

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which produces a gravitational
potential difference

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across the end of say a plank.

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Similarly, if you need to set
up a current through a wire,

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you need to have an electric
potential difference

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across the ends of it.

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When you have an electric
potential difference,

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you can get a current, but
you also need to make sure

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that there are some charges.

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There are charges that are
free to move in your material.

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Not all materials have that,
for example, glass or plastic.

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Well, they don't have free charges because

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if you look inside them,
well you can model them

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and say that you know what?

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The electrons inside these
atoms are very tightly bound.

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So there are no free electrons to move.

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There are no charges to move.

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So if you put a voltage

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across them, you'll probably
get no current over here.

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We call such material insulators, glass,

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wood, plastic, these are
examples of insulators.

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On the other hand, if you take metals

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of which wires are made
of, then you'll find

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that the outermost electrons
are not tightly bound.

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As a result, they are free
to move around the material.

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We call them free electrons.

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And since you have free charges available

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for motion, we call these
materials conductors

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because if you put a voltage across them,

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well these electrons can move
and contribute to current.

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So you need a voltage
across a conducting medium

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for electric current.

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Okay, but how do you get a
voltage in the first place?

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Well, in small circuits,
you probably already know,

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voltage is given by a battery.

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One end of the battery
is at a higher potential,

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another end of the battery
is at lower potential.

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And when you connect it
to a circuit, it provides

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the potential difference.

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But in larger circuits for
like for example, the circuits

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in our houses, well the
potential difference is provided

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by large electric generators
in our power stations.

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And by the way, while drawing
a battery in our circuit,

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well we use a circuit
symbol that looks like this.

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The longer line represents
the positive terminal

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and the shorter thick line
represents a negative terminal.

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So that if you just draw
this, we don't have to draw

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like a big battery over here.

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Anyways, even though we have a battery

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in this circuit right now,
we don't have a current,

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we don't have a potential
difference across this bulb.

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Why?

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Well, you can see over here, that's

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because the circuit is not closed.

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We say because there
is some air in between.

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Air is an excellent insulator and

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therefore there's not going
to be any current over here.

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In order for there to
be a current, we need

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to close the circuit, meaning
we need to connect this gap

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and that's where the switch
is, this is a switch.

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So if I close the switch

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like this, now the circuit is complete.

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Now there'll be a potential
difference across the ends

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of the ball when now there'll
be a current over here.

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I'm gonna open the switch.

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There is no electric current,
the circuit is broken.

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Close the switch, there's going
to be an electric current.

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Now because I compared charges moving

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through a ball rolling
down, we might model it

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by thinking that hey,
when there is no voltage,

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all the charges are at
rest, say the electrons

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over here are at rest and when
I do complete the circuit,

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the electrons are now nicely moving.

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But that's not a very accurate
way to think about it,

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that's not a good model.

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Instead, a better model
is if you were to peek

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inside the wire, we
find that the electrons

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are randomly moving, bumping

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into stuff because they have a lot

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of energy even when there is no voltage.

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So they're not at rest, they're

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in fact moving at very high speeds.

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But what happens when we close the switch?

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When we close the circuit, look,

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there is a potential difference

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and therefore there is an
electric field setup in the wire

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that electric field starts
pushing on the electrons.

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And look, you can now see the electrons

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are slowly drifting to the left.

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It's that drifting motion
that constitutes the current

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and what causes them to drift to the left?

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Again, there are some analogies which says

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that electrons push on each
other making them drift.

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But that's again not very accurate.

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A better way to think about it is

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that the battery produces
the electric field.

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There's an electric field
set up inside the wire.

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It's that electric field that is causing,

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that's pushing the
electrons, making them drift

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to the left over here.

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But wait a second, why did I show

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that the electrons are
drifting to the left over here?

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Let's think about it.

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So one way to think
about it's, you could say

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that hey, electrons are being attracted

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by the positive terminal of
the battery being repelled

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by the negative terminal
of the battery, making

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the electrons go this way.

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But a question that could raise is,

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in the wire that means
the electrons are going

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from a lower potential
to a higher potential

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like going uphill.

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How does that make any sense?

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That was a point of
confusion for a long time.

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So let's talk about it a little bit. Okay?

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If I have a big positive charge

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and next to it I keep a
very tiny positive charge

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and at rest, let's say,

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and I let go of it, then
we know it gets repelled

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and it gains kinetic
energy in this direction.

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Now because energy is conserved,
we could ask where did

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that kinetic energy come from?

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We say, ah, there it must have
come from potential energy.

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So as it goes from here to here,

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the system must lose potential energy

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and therefore we can now say that hey,

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this point represents
high potential region.

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This point represents low potential region

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and this represents the downhill
direction for the charges.

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As you go from here to
here, it's potential energy

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starts getting converted
into kinetic energy.

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Kind of like what happens
to this ball rolling down.

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But what about negative charges?

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Well, negative charges will
be exactly the opposite.

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They will get attracted
by this positive charge.

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So they will gain kinetic energy this way.

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And for negative charges,
it's the exact opposite

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as they go from here to
here, this is a direction

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in which they are losing potential energy

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and gaining kinetic energy.

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So this must be high, this must be low,

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this should represent the
direction of the downhill.

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But now the problem is which
direction should we say

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is down for the charges?

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Well, we could say, hey, for
positive charge, this is down

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and say negative charges,
this is the down,

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but we decided no, no, no,
let's just use one of these

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as our reference and we'll
just consider one direction

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as our actual down.

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So we decided, hey, whatever happens

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for a positive charge,
let's use positive charge

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as our reference,

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and whichever direction
positive charge natural tends

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to go, we'll call that
direction as our down

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for charges, right, down in potential.

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Because of that reference,

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by definition, positive charges go

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down the electric potential.

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Negative charges look end up going

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up the electric potential, not

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because they're literally going

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to a higher potential energy region.

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No, no, they're also going towards

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lower potential energy region.

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It's just a reference because
our reference point for high

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and low is chosen, you
know, from the perspective

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of a positive charge.

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Because of that reference,

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negative charges end up
going up the potential,

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they have a natural tendency
to go up the potential.

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Does that make sense?

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And therefore, electrons,
which are negative charges,

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have a natural tendency to
go up the electric potential.

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Now, the final question we could have is

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the direction of the current.

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What is the direction of
the current over here?

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Well, we could say, hey,

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whichever direction the
charges are drifting, well

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that itself could be the
direction of the current.

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That's the most natural way
to think about it, right?

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So electrons are drifting this way.

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So let's say that that is the current,

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but again, there's a problem

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because we have positive
and negative charges.

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Remember that example
where we had both positive

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and negative charge, equal positive

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and negative charges flowing

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through an area giving me zero current

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because a net charge over here is zero.

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Well, if I said that, hey, you know,

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whichever direction charges
are moving, let's just call

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that direction as the current,
then I have a problem.

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Because I could say that hey,
positive charges is giving me

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a current this way, negative charges

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also giving me a current this way,

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but I know the total current must be zero.

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So that doesn't work
because you know these two,

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if I add up, I don't get zero,
I should get a net current

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to the right, but that's not true.

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I know that the current should be zero.

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00:09:44,430 --> 00:09:47,430
Again, to solve for that, we
decided, hey, you know what?

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00:09:47,430 --> 00:09:50,100
Whichever direction, positive
charges are moving, we'll say

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00:09:50,100 --> 00:09:52,080
that is the direction of the current.

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00:09:52,080 --> 00:09:53,910
And for the negative charges,

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00:09:53,910 --> 00:09:56,040
we'll say the opposite is
the direction of the current.

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00:09:56,040 --> 00:09:59,130
So we said if the negative
charges are moving to the right,

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00:09:59,130 --> 00:10:02,640
we will say the direction of
the current is to the left.

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00:10:02,640 --> 00:10:06,360
And now look, now the
total current becomes zero

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because your right and
left current cancels out.

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00:10:09,570 --> 00:10:10,830
Now it makes sense.

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00:10:10,830 --> 00:10:12,330
So the convention

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00:10:12,330 --> 00:10:14,520
for choosing the direction of the current

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is whichever direction
positive charges are going,

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that is the direction of the current.

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00:10:19,050 --> 00:10:21,300
If you have negative charges, opposite.

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00:10:21,300 --> 00:10:23,490
Whichever direction negative
charges are going, opposite to

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00:10:23,490 --> 00:10:25,530
that, that will be the
direction of the current.

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00:10:25,530 --> 00:10:26,520
Okay? (chuckles)

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00:10:26,520 --> 00:10:29,670
Now, because in wires, it's the electrons

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00:10:29,670 --> 00:10:32,160
that are always drifting,
that's those are the one

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00:10:32,160 --> 00:10:33,420
that constitutes the current

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00:10:33,420 --> 00:10:36,810
and the electrons are
negatively charged particles.

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Our convention for the current
would be not the direction

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00:10:39,930 --> 00:10:42,030
of the electron flow, but
in the opposite direction

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00:10:42,030 --> 00:10:44,760
of the electron flow,
it would be this way.

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00:10:44,760 --> 00:10:47,970
So the conventional direction
of the current, notice, is

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00:10:47,970 --> 00:10:51,120
in the opposite direction
of the electron flow.

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00:10:51,120 --> 00:10:53,340
And I'll tell you what can be frustrating

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because in most cases we'll be
dealing with electron flows.

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00:10:57,150 --> 00:10:58,230
This will be frustrating because

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00:10:58,230 --> 00:11:00,480
in most cases our
conventional current will be

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00:11:00,480 --> 00:11:03,090
in the opposite direction
of the actual motion

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00:11:03,090 --> 00:11:05,760
of the charges, actual
drifting motion of the charges.

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00:11:05,760 --> 00:11:07,933
But it's unfortunate that electrons,

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which are the major charge carriers

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00:11:09,391 --> 00:11:11,730
in most of the circuits,
end up being (chuckles)

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00:11:11,730 --> 00:11:13,500
a negatively charged particle.

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00:11:13,500 --> 00:11:15,630
And our positive charges
are reference for us.

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00:11:15,630 --> 00:11:18,390
And so it might slightly
feel awkward initially,

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00:11:18,390 --> 00:11:20,850
but you'll get used to
it, don't worry too much.

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00:11:20,850 --> 00:11:22,620
This now finally brings us to lightning.

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00:11:22,620 --> 00:11:24,180
What exactly is lightning?

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00:11:24,180 --> 00:11:26,190
Well, lightning is also
an electric current,

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00:11:26,190 --> 00:11:28,500
meaning flow of charges.

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00:11:28,500 --> 00:11:30,210
But how does it happen?

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00:11:30,210 --> 00:11:33,030
And more importantly, lightning is a flow

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00:11:33,030 --> 00:11:35,730
of charges through air,
but air is an insulator.

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00:11:35,730 --> 00:11:38,160
And we saw that insulators
do not conduct electricity.

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00:11:38,160 --> 00:11:39,570
So what's going on over here?

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00:11:39,570 --> 00:11:40,980
Well, we'll not give you too much details,

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00:11:40,980 --> 00:11:45,000
but it turns out that clouds
usually have charges separated.

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00:11:45,000 --> 00:11:47,430
The top of it is usually
positively charged

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00:11:47,430 --> 00:11:50,250
and the bottom is negatively charged.

307
00:11:50,250 --> 00:11:52,130
Now because the bottom is closer

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00:11:52,130 --> 00:11:55,440
to the earth, the negative
charges push electrons

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00:11:55,440 --> 00:11:58,410
of the earth away from
it 'cause negative repel.

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00:11:58,410 --> 00:12:02,100
And as the electrons get
repelled away, the surface

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00:12:02,100 --> 00:12:06,540
of the ground will be
mostly positively charged.

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00:12:06,540 --> 00:12:09,840
Now during a thunderstorm,
the charges builds up

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00:12:09,840 --> 00:12:11,610
because the air is an insulator,

314
00:12:11,610 --> 00:12:12,930
because there's no corona over here,

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00:12:12,930 --> 00:12:14,850
the charges can build up, and as a result,

316
00:12:14,850 --> 00:12:17,790
the potential difference
become incredibly high.

317
00:12:17,790 --> 00:12:20,370
It can reach millions of moist.

318
00:12:20,370 --> 00:12:24,780
Now, eventually what happens
is that the electrons

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00:12:24,780 --> 00:12:28,500
from the atoms of the air
molecules, like oxygen, nitrogen,

320
00:12:28,500 --> 00:12:32,010
and all of those stuff can
actually get ripped apart.

321
00:12:32,010 --> 00:12:34,050
And we'll not get into
again the details of how

322
00:12:34,050 --> 00:12:36,060
that happens, but you can now imagine,

323
00:12:36,060 --> 00:12:38,400
if electrons start getting ripped apart.

324
00:12:38,400 --> 00:12:41,370
Now we start having charges.

325
00:12:41,370 --> 00:12:42,811
Once we have charged particles

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00:12:42,811 --> 00:12:45,780
in between, we have a conducting channel.

327
00:12:45,780 --> 00:12:47,730
And once we have that conducting channel,

328
00:12:47,730 --> 00:12:51,030
the charges can sort of
get dumped into the earth.

329
00:12:51,030 --> 00:12:53,730
And that's basically
what we call a lightning.

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00:12:53,730 --> 00:12:55,800
Now this lightning produces a lot of heat.

331
00:12:55,800 --> 00:12:58,740
That's one of the reason it
glows and you can see it.

332
00:12:58,740 --> 00:13:01,020
But that heat also causes rapid expansions

333
00:13:01,020 --> 00:13:03,810
in the air, making the air vibrate.

334
00:13:03,810 --> 00:13:06,390
And these vibrations
eventually reach our ears

335
00:13:06,390 --> 00:13:10,800
after some time, and we
call that as thunder.

336
00:13:10,800 --> 00:13:14,550
So look, lightning is an
electric current, and guess what?

337
00:13:14,550 --> 00:13:17,100
Sparking that happens
sometimes, those annoying sparks

338
00:13:17,100 --> 00:13:18,630
we get whenever we get charged up

339
00:13:18,630 --> 00:13:20,850
and we're trying to reach out
to a doorknob, for example.

340
00:13:20,850 --> 00:13:21,683
(laughs)

341
00:13:21,683 --> 00:13:23,430
It's very similar to what
happens in a lightning.

342
00:13:23,430 --> 00:13:26,313
It's a miniature version of lightning.