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in 1820, hans christian oersted
made hit great discovery


of how to turn electricity
into magnetism.


at that point,


every scientist in europe


knoew there was another
great discovery


waiting to be made,


and that was
how touse magnetism


to create electricity.


they also knew that
whoever made that discovery


would become immortal,


and so all of them scurried
back to their laboratories


and went to work.


then 12 years passed,
and nothing happened.


now, 12 years
is a long time.
for most of you,
12 years is 2/3 of your lives.


it's nearly 1/2
of my life


and yet, in all that time,
the discovery was not made.


now, in science,
once you understand something,


it's very difficult
to remember what it's like


not to have
understood it.


the fact is
that during that period,


many experiments
were done


that had the dicovery
in them


in cooplocated ways
that couldn't be understood,


but once the discovery was made
and reduced to the essence,


it's just as simple
as it can possibly be.


here i have
a simple loop of wire.


my job is to make
an electric current


run around in this loop.


the loop's connected
to a galvanometer whose output


is that point of light.


if i make the current
run through the loop,


that light will move.


how will i make it work?


using magnetism.


to use this magnet to make
a current run around this loop


is very easy.


i just move the loop
into the magnet


or, for that matter,
back out again.


that was it,
the great discovery.


one thung more--


if i move the loop
back and forth,


and keeps on going.


virtually all electric power,
one way or another,


is generated
by that mechanism.


in other words,


that discovery
is the very basis

of our electrical
civilization,


and that's our subject
for today.


without nergy,
civilizations go nowhere,


not even in circles,


but with a storong
and steady wind


and a comparatively simple
machine,


an endless supply of power
can flow just about anywhere,


and once it's been
converted to electricity,


accomplish what the ancients
probably never even imagined.


these days, it looks
easy enough on the surface,


but to understand
how wind makes electricity,


it's necessary
to dig a little deeper


and to come up with a bit
of historical perspective.


the dutch were masters
when it came to windmills.


holland had more water


and less cultivated land
than she needed,


so necessity became
the mother of invention,


and the dutch
became very good


at using the windmill
to turn the rhine river delta


into good, dry farmland.


but by the 1800s,


another kind of revolution
was in the air,


and it was the churning
power and glory of steam.


for all their majesty,


the future belonged
not to windmills,


but to steam engines


fired by the mineral solid
called coal.


coal was inexpensive,


as steady as a rock,


and as convenient
as the closest railroad track,


so for a while there,
coal was really cooking.


however, in commerce
and chemistry alike,


few things have ever
remained the same for long,


and even as coal
was gathering steam


in the industrialized
nations,


a new and different
entered the picture.


it was called electricity.


nowhere was it
more hotly pursued


than at the royal institution
in london,
 

and no one had pursued it
more closely


than the 18th century's
most farasay.


he probed
the mysteries of science


because, for him, 
the building blocks of nature


had been created
with a fundamental unity.


and mr. faraday believed


that the blueprint
was to be found


within the laws
of physics.


with that belife,


faraday was motivated
to unearth a new link


between two of mother nature's
fundamental forces--


magnetism and electricity.


his quest began in 1821


with hans christian oersted's
discovery


that the flow
of an electric current


creates a magnetic field.


first, araday
reproduced the experiment,


and then,
in a roundabout way,


he took
a major step forward.


he invented a device
that kept twisting


as it followed
the circular magnetic field


created by a current.


from this point on,


when people thought of things
turning to create power,


they no longer thought


of windmills
and steam engines alone.


with this amazing device,


an electric current
could create physical work.


in other words,


this was the word's
first electric motor.


but faraday's restless mind


saw more than circles
of magnetic force


in oersted's discovery.


he saw a crucial
scientific question.


if an electric current
could create a magnetic field,


could a magnetic field
create an electric current?


the answer
didn't come easily.


it took
lamp oil and paper,


brilliance and tenacity,


untold combinations
of coils, magnets, and wires.


it took
more than a decade,


which is why biographers
note hit "lengendary patience,"


but in 1831, michael faraday
had the answer.


as he connected a battery
to one coil,


current flowed briefly
in another coil.


he'd found the answer,


and in the process,
he'd discovered


the principle
of electromagnetic induction.


how dose electromagnetic
induction really work?


in other words,


how can
a magnetic field


drive an electric current
around a circuit?


magnetic fields apply forces
to electric charges,


but only if the charges
are in motion.


the direction of the force
is perpendicular


to the velocity
and the field


and depends on the sign
of the charge,


in this case, negative.


for example,


the charges might be electrons
in a piece of wire.


just moving a wire
through a magnetic field


makes a current
try to flow.


but the field
of a bar magnet


can be perpendicular
at every point


to a circular loop
of wire,


so moving the loop
creates a force


that drives a current
all the way around the loop.


the current
flows one way


if the loop
moves upward


and the other
way


if the loop
moves downwaed.


of course, a current also flows
if the loop stans still


and the magnet moves...


one way if the magnet
moves downward...


the other if the magnet
moves upward.


in fact, faraday found
that any method


of changing the magnetic field
through a circuit


would make a current flow.


for example,
he could induce a current


by moving a bar magnet
relative to a coil.


when faraday connected a coil
to a battery,


the sudden increase
in the magnetic field


induced a surge of current
in another coil.


both these ideas
can be expressed


in terms
of magnetic flux.


 the total magnetic flux
 through a surface


 is the integral of
 the component of magnetic field


 perpendicular
 to each bit of area


 that makes up
 the surface.


 as a loop of wire


 moves through the field
 of a sar magnet,


 the flux
 through the loop
 keeps changing.


 the changing flux
 causes a current to flow


 as if it were driven
 by a voltage.


 that voltage, calld
 the electromotive
 force, e,


 is equal to minus
 the rate of change


 of magnetic flux
 through the loop.


 that's called faraday's law
 of electromagnetic induction.


a generation later
and a continent away,


other pioneering qeniuses


would make the most
of faraday's discovery.


their names
were thomas edison


and nikola tesla.


unlike faeaday, however,


their primary ambition
was to make machines work


and to make them
pay for themselves.


edison saw
a lot of profit


in the business
of electricity,


but if he were
to realize it,


he'd need to light up
the city of new york.


that would require a network
of very powerful generators.


no matter the engineering shape
they'd take,


the principle
of every dynamo


would come from faraday'slaw
of electromagnetic induction.


 rotating a loop of wire


 in a constant magnetic field

 continually changes the flux


 through the loop.





the resulting voltage

and curret are sinusoidal.

going first one way.

then the other

using elaborate
sliding contacts

attached
to the rotating coils.

edison could compensate
for the oscillating voltage.

drawing current
that always ran

in the same direction
from his generators.

tesla thought edison's system
of direct current

was costly and inefficient

and that alternating current
would do the job better.

but ac or dc.
one question remains--

which way does
the induced current flow?

current flows
tarough a coil of wire

whenever the magnetic flux:tarough it is changing.

the current flows one way
if the flux decreses

and the other way
when the flux increases.

but the current
induced in the wire

also creates
a flux of its own.

whose direction depends on
the direction of the current

the flux created
by the induced current

always opposes the change
in the external flux.

this rule is known
as lenz's lam.

the net result of both fields
added together

is to slow down the rate

at which the magnetic flux
tarough any circuit

is able to change.

as electric power
grew more important,

lenz's law didn't slow down
the rate of change

of technological progress,

and steam ensines were't
out of of the picture,

not by a long shot.

engines field
by both goal and oil

continued to turn generetors

and create the power
of electricity.

however, the more
that goal and oil

ware taken away
and used up,

the more obvious it became

that the earth began
with a limited amount,

and for some,

when it came
to fossil fuels,

the end was already
in sight,

sothe search
for alternative continued.

for a time,

hydroelectric power
flowed with promise.

but in the search 

for a clean, efficient,
and abudant source of energy,

all the world's rivers
together

would never provide enough,

and by itself,
neither would the wind.

but along with geothermal
and solar power,

the wind provides
cheap, decentralized energy
that can share the load

with traditional, cenyralized
generating plants. 

taking advantage
of current technology,

these windmills
are much more efficient  

than their dutch conterparts
of a few centuries ago.

in any case,
whether electrik power 

comes from the wind 
or the water,

from nuclear power
or from coal,

it's generated

using the principle
of electromagnetic induction,

and the same principle 
plays a role

in any circuit
where electricity is used.

an electric current 
in any circuit

creates a magnetic field which ,
whenever it changes

induces a current 
in the same circuit 

that opposes the change.

this phenomenon is called
self-induction.

the effect
of self-induction 

is larger in a coil 
of many turns,

precisely because
it loops many times

around the changing flux.

that's why a solenoid
is used

as a symbol of self-inductance
in an electoric circuit,

where an inductor 
may be an element

like a resistor 
or capasitor.

current increasing 
in an inductor

causes an opposing 
electromotive force

accoding
to faraday's law

and since the flux
is proportional

to the current
that creates it

that electrorotive force
is equal to minus a constant l

times the rate
of change of current

l is called the inductance
of the circuit element

an inductor opposes
and slows down

the build-up of current 
in a circuit

while a resistor may cause 
the current to decay

by disspating energy.

but bayond
such practical concerns,

what's the value
of michael faraday's theory?

for starters,
unlike a motor

it's not simple
and obvious,

and unlike any machine

it's not something
one can put a price on

infact, the value
of electromagnetic induction

is beyond the economics
of technology.

that's becauce faraday's technorogy
reveals something new

about the nature of electric 
and magnetic fields,

and, inthe process,
changes the rules 

by which nature's game
is played.

magnetic fields circulate
but never converge to a point.

electric fields
converge to a point

but never circulate.

in mathematical language,

that means 
the line integral

of an electrostatic field
around any closed path

is always equal 0.

but when the magnetic flux
tarough a loop

changes with time,

it creates 
an electromotive force

around the loop,

and that electromotive force
is due to an electric field

whose line integral
is not equal to 0.

in other words,
there is a way

to make electric fields
circulate after all,

and it's accomplished
by changing the magnetic flux.

when faraday discovered
that a changing magnetic flux

could make
an electric field circulate,

he added something
extraordinary

to the laws
of electricity and magnetism.

so while it's often said

that there's nothing new
under the sun,

the saying dosen't apply
to electromagnetic induction.

i'd like to show you
something truly extraordinary,

but first, i'll explane
what you will see.

when we first started discussing
electromagnetic induction,

this is how i explaned
how it would work.

i said the wire moves
through the magnetic field

and because of the motion
of the wire,

the charges inside
have a force on them.

that sets the current
into motion,

but when we finary
formulated faraday's law,

we were describing it
in quite a different way.

then we said
the electromotive force

is proportional
to the rate of change

of the total magnetic flux
through the circuit.  

that suggests the experiment
could be done diffrentry

what we could do
is have a solenoid

that's a helical winding,
like this,

in which the wire
goes around and around.

if we pass a current
through that,

it creates a magnetic field
in this direction.

if i have a loop of wire
around this solenoid

and connected
to a galvanometer,

when i turn
the solenoid on,

the flex through that loop 
will increase 

even through there's never
any magnetic field

at the position 
of the wire.

according to faraday's law,
the galvanometer should jump.

yon'll say the magnetic field
at the position of the wire

is not really 0

because the magnetic flux
that goes through the solenoid

has to return outside,

so there's some
magnetic field outside.

i can cure that problem
by taking the solenoid

and making it band
around onto itself

into a doughnut
or toroid.

here's what
the toloid looks like.

it has windings
or metal wire around it,

and when a is passed
through those windings,

lines of magnetic flux

go around
through the toroid

and close on themselves 
without ever going outside,

so the magnetic flux
outside of a toroid

is really equal to 0.

now, if i took
a loop of wire

and ran it through
the middle of the doughnut

and then back around

and closed it
on the galvanometer,


when i turn on
the toroid,

the galvanometer
should jump

even though
the magnetic field

at the position
of the wire

is exactry 0.

that's what
i will show you.

i have here a toroid.

when i turn the toroid on
by means of switch,

there will be
magnetic flux inside,

but thre won't be any
magnetic field at all outside.

i still have my loop of wire
connected to the galvanometer.

you know if there's
a magnetic field.

turned on at this wire,
the galvanometer will jump.

i can put the wire
on top of the toroid.

i turn it on,

and the galvanometer
dosen't budge.

when i turn the switch,
absolutely nothing happenes.

if i take this wire
and put it through the doughnut,

the result
is completely different.

i open the wire

and thread it
through the middle

and close it again.

even though
there's no magnetic field

at the position
of the wire,

when i turn the toroid on,
the galvanometer will jump.

watch.

there it is.

when i turn it off,
the same thing happens.

that is the essence
of electromagnetic induction.

there's something else 
over here i wanted show you.

it's this pendulum.

this is 
an ordinary pendulum,

but it has a piece
of copper at the end.

the copper swings
between the poles

of an electromagnet.

the magnet's
not turned on right now.

if the magnet
were turned on,

the copper would swing
into the magnetic fields.

then by electromagnetic
induction,

electric currents
would be induced

to run around
in the copper.

whenever currents run around
in a piece of metal,

that causes heating.

energy would be lost
from somewhere.

where dose the energy 
come from?

the only place possible

is the swinging
of the pendulum.

the pendulum
should slow down

if i turn on
the magnet,

which i can do
with the switch.

i'll get the pendulum
swinging ahain,

and you get the rhythm
of the swinging,

and when i close
the switch,

you can detect
the pendulum slowing down.

here we go
i'll get it swinging again.

i'm going to close
the switch right now.

watch this. there.

did you notice that?

one more thing
i'd like to show you.

not only can
electromagnetic induction

stop a piece of metal,

it can get one
going again.

metal dosen't like a strong,
changing magnetic field,

which i hava right here.

if i put ring
of aluminum

in the field
and turn it on,

the alminum jump

when it's turned on.

there.

i can't do better
than that,


so i'll see you next time.

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