- [Lecturer] Electricity that lights up
above looks very different than lightning strikes,
but they're actually more similar than one might think
because they both have electric current.
So let's understand what electric current is,
how they're produced,
and also get to understand a little bit about lightning.
So what exactly is electric current?
Well think of electric current
as a flow of net charge through any given area.
Here's what I mean by flow of net charge.
Well imagine you have a tiny cross-sectional area
through which you have equal amount
of positive charges flowing
to the right and left in any given time.
Now notice there is a flow,
but there is no net flow (chuckles)
and therefore here we say there is zero current.
Another interesting example is what if you have equal amount
of positive and negative charges flowing
in the same direction in the same time, let's say
through again, a given cross-sectional area.
Again, notice there is a flow of charges,
but the total flow over here, total charge
that's flowing is zero. (chuckles)
So net charge is still zero
and therefore there is no electric current over here.
Okay, what about now? Ooh, now we do have electric current.
Now we have a net positive charge flowing to the right.
Over here there is an electric current.
Now we do have a net negative charges flowing to the right.
We do have an electric current. Okay?
So it's a flow of net charge, but how do you measure it?
Well, we measure it as the amount of charges flowing
through any given cross-sectional area per second.
So you can think of it as coulomb per second.
How many coulombs are flowing per second?
And the coulombs per second is also called,
it's also called Amperes, okay? Capital A, Amperes.
And just to give you typical numbers,
your air conditioners heaters,
they drop out 10 to 15 Amperes of current.
Your ceiling fan tube lights, television sets less than
that, about one or two amps.
And your smaller circuits like you know the toy circuits
and stuff, they would be even lesser.
It would be fraction of Amperes. But what about lightning?
Ooh. (chuckles)
Lightning can have tens of thousands of Amperes in them.
Okay, how do we set up an electric current?
How do we get an electric current?
Well, for an electric current we need a voltage.
Just like how, if you need to make a ball roll, you need
to have a height difference,
which produces a gravitational potential difference
across the end of say a plank.
Similarly, if you need to set up a current through a wire,
you need to have an electric potential difference
across the ends of it.
When you have an electric potential difference,
you can get a current, but you also need to make sure
that there are some charges.
There are charges that are free to move in your material.
Not all materials have that, for example, glass or plastic.
Well, they don't have free charges because
if you look inside them, well you can model them
and say that you know what?
The electrons inside these atoms are very tightly bound.
So there are no free electrons to move.
There are no charges to move.
So if you put a voltage
across them, you'll probably get no current over here.
We call such material insulators, glass,
wood, plastic, these are examples of insulators.
On the other hand, if you take metals
of which wires are made of, then you'll find
that the outermost electrons are not tightly bound.
As a result, they are free to move around the material.
We call them free electrons.
And since you have free charges available
for motion, we call these materials conductors
because if you put a voltage across them,
well these electrons can move and contribute to current.
So you need a voltage across a conducting medium
for electric current.
Okay, but how do you get a voltage in the first place?
Well, in small circuits, you probably already know,
voltage is given by a battery.
One end of the battery is at a higher potential,
another end of the battery is at lower potential.
And when you connect it to a circuit, it provides
the potential difference.
But in larger circuits for like for example, the circuits
in our houses, well the potential difference is provided
by large electric generators in our power stations.
And by the way, while drawing a battery in our circuit,
well we use a circuit symbol that looks like this.
The longer line represents the positive terminal
and the shorter thick line represents a negative terminal.
So that if you just draw this, we don't have to draw
like a big battery over here.
Anyways, even though we have a battery
in this circuit right now, we don't have a current,
we don't have a potential difference across this bulb.
Why?
Well, you can see over here, that's
because the circuit is not closed.
We say because there is some air in between.
Air is an excellent insulator and
therefore there's not going to be any current over here.
In order for there to be a current, we need
to close the circuit, meaning we need to connect this gap
and that's where the switch is, this is a switch.
So if I close the switch
like this, now the circuit is complete.
Now there'll be a potential difference across the ends
of the ball when now there'll be a current over here.
I'm gonna open the switch.
There is no electric current, the circuit is broken.
Close the switch, there's going to be an electric current.
Now because I compared charges moving
through a ball rolling down, we might model it
by thinking that hey, when there is no voltage,
all the charges are at rest, say the electrons
over here are at rest and when I do complete the circuit,
the electrons are now nicely moving.
But that's not a very accurate way to think about it,
that's not a good model.
Instead, a better model is if you were to peek
inside the wire, we find that the electrons
are randomly moving, bumping
into stuff because they have a lot
of energy even when there is no voltage.
So they're not at rest, they're
in fact moving at very high speeds.
But what happens when we close the switch?
When we close the circuit, look,
there is a potential difference
and therefore there is an electric field setup in the wire
that electric field starts pushing on the electrons.
And look, you can now see the electrons
are slowly drifting to the left.
It's that drifting motion that constitutes the current
and what causes them to drift to the left?
Again, there are some analogies which says
that electrons push on each other making them drift.
But that's again not very accurate.
A better way to think about it is
that the battery produces the electric field.
There's an electric field set up inside the wire.
It's that electric field that is causing,
that's pushing the electrons, making them drift
to the left over here.
But wait a second, why did I show
that the electrons are drifting to the left over here?
Let's think about it.
So one way to think about it's, you could say
that hey, electrons are being attracted
by the positive terminal of the battery being repelled
by the negative terminal of the battery, making
the electrons go this way.
But a question that could raise is,
in the wire that means the electrons are going
from a lower potential to a higher potential
like going uphill.
How does that make any sense?
That was a point of confusion for a long time.
So let's talk about it a little bit. Okay?
If I have a big positive charge
and next to it I keep a very tiny positive charge
and at rest, let's say,
and I let go of it, then we know it gets repelled
and it gains kinetic energy in this direction.
Now because energy is conserved, we could ask where did
that kinetic energy come from?
We say, ah, there it must have come from potential energy.
So as it goes from here to here,
the system must lose potential energy
and therefore we can now say that hey,
this point represents high potential region.
This point represents low potential region
and this represents the downhill direction for the charges.
As you go from here to here, it's potential energy
starts getting converted into kinetic energy.
Kind of like what happens to this ball rolling down.
But what about negative charges?
Well, negative charges will be exactly the opposite.
They will get attracted by this positive charge.
So they will gain kinetic energy this way.
And for negative charges, it's the exact opposite
as they go from here to here, this is a direction
in which they are losing potential energy
and gaining kinetic energy.
So this must be high, this must be low,
this should represent the direction of the downhill.
But now the problem is which direction should we say
is down for the charges?
Well, we could say, hey, for positive charge, this is down
and say negative charges, this is the down,
but we decided no, no, no, let's just use one of these
as our reference and we'll just consider one direction
as our actual down.
So we decided, hey, whatever happens
for a positive charge, let's use positive charge
as our reference,
and whichever direction positive charge natural tends
to go, we'll call that direction as our down
for charges, right, down in potential.
Because of that reference,
by definition, positive charges go
down the electric potential.
Negative charges look end up going
up the electric potential, not
because they're literally going
to a higher potential energy region.
No, no, they're also going towards
lower potential energy region.
It's just a reference because our reference point for high
and low is chosen, you know, from the perspective
of a positive charge.
Because of that reference,
negative charges end up going up the potential,
they have a natural tendency to go up the potential.
Does that make sense?
And therefore, electrons, which are negative charges,
have a natural tendency to go up the electric potential.
Now, the final question we could have is
the direction of the current.
What is the direction of the current over here?
Well, we could say, hey,
whichever direction the charges are drifting, well
that itself could be the direction of the current.
That's the most natural way to think about it, right?
So electrons are drifting this way.
So let's say that that is the current,
but again, there's a problem
because we have positive and negative charges.
Remember that example where we had both positive
and negative charge, equal positive
and negative charges flowing
through an area giving me zero current
because a net charge over here is zero.
Well, if I said that, hey, you know,
whichever direction charges are moving, let's just call
that direction as the current, then I have a problem.
Because I could say that hey, positive charges is giving me
a current this way, negative charges
also giving me a current this way,
but I know the total current must be zero.
So that doesn't work because you know these two,
if I add up, I don't get zero, I should get a net current
to the right, but that's not true.
I know that the current should be zero.
Again, to solve for that, we decided, hey, you know what?
Whichever direction, positive charges are moving, we'll say
that is the direction of the current.
And for the negative charges,
we'll say the opposite is the direction of the current.
So we said if the negative charges are moving to the right,
we will say the direction of the current is to the left.
And now look, now the total current becomes zero
because your right and left current cancels out.
Now it makes sense.
So the convention
for choosing the direction of the current
is whichever direction positive charges are going,
that is the direction of the current.
If you have negative charges, opposite.
Whichever direction negative charges are going, opposite to
that, that will be the direction of the current.
Okay? (chuckles)
Now, because in wires, it's the electrons
that are always drifting, that's those are the one
that constitutes the current
and the electrons are negatively charged particles.
Our convention for the current would be not the direction
of the electron flow, but in the opposite direction
of the electron flow, it would be this way.
So the conventional direction of the current, notice, is
in the opposite direction of the electron flow.
And I'll tell you what can be frustrating
because in most cases we'll be dealing with electron flows.
This will be frustrating because
in most cases our conventional current will be
in the opposite direction of the actual motion
of the charges, actual drifting motion of the charges.
But it's unfortunate that electrons,
which are the major charge carriers
in most of the circuits, end up being (chuckles)
a negatively charged particle.
And our positive charges are reference for us.
And so it might slightly feel awkward initially,
but you'll get used to it, don't worry too much.
This now finally brings us to lightning.
What exactly is lightning?
Well, lightning is also an electric current,
meaning flow of charges.
But how does it happen?
And more importantly, lightning is a flow
of charges through air, but air is an insulator.
And we saw that insulators do not conduct electricity.
So what's going on over here?
Well, we'll not give you too much details,
but it turns out that clouds usually have charges separated.
The top of it is usually positively charged
and the bottom is negatively charged.
Now because the bottom is closer
to the earth, the negative charges push electrons
of the earth away from it 'cause negative repel.
And as the electrons get repelled away, the surface
of the ground will be mostly positively charged.
Now during a thunderstorm, the charges builds up
because the air is an insulator,
because there's no corona over here,
the charges can build up, and as a result,
the potential difference become incredibly high.
It can reach millions of moist.
Now, eventually what happens is that the electrons
from the atoms of the air molecules, like oxygen, nitrogen,
and all of those stuff can actually get ripped apart.
And we'll not get into again the details of how
that happens, but you can now imagine,
if electrons start getting ripped apart.
Now we start having charges.
Once we have charged particles
in between, we have a conducting channel.
And once we have that conducting channel,
the charges can sort of get dumped into the earth.
And that's basically what we call a lightning.
Now this lightning produces a lot of heat.
That's one of the reason it glows and you can see it.
But that heat also causes rapid expansions
in the air, making the air vibrate.
And these vibrations eventually reach our ears
after some time, and we call that as thunder.
So look, lightning is an electric current, and guess what?
Sparking that happens sometimes, those annoying sparks
we get whenever we get charged up
and we're trying to reach out to a doorknob, for example.
(laughs)
It's very similar to what happens in a lightning.
It's a miniature version of lightning.