Electric current | Physics | Khan Academy

- [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.