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The Big Misconception About Electricity - YouTube
Channel: Veritasium
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This video was sponsored
by Cas茅ta by Lutron.
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Imagine you have a giant circuit
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consisting of a battery,
a switch, a light bulb,
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and two wires which are each
300,000 kilometers long.
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That is the distance light
travels in one second.
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So, they would reach
out half way to the moon
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and then come back to be
connected to the light bulb,
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which is one meter away.
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Now, the question is,
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after I close this switch,
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how long would it take
for the bulb to light up.
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Is it half a second,
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one second,
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two seconds,
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1/c seconds,
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or none of the above.
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You have to make some
simplifying assumptions
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about this circuit,
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like the wires have to have no resistance,
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otherwise this wouldn't work
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and the light bulb has
to turn on immediately
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when current passes through it.
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But I want you to commit to an answer
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and put it down in the comments
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so you can't say,
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oh yeah I knew that was the answer,
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when I tell you the answer later on.
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This question actually relates
to how electrical energy
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get from a power plant to your home.
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Unlike a battery,
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the electricity in the grid
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comes in the form of
alternating current, or AC,
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which means electrons in the power lines
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are just wiggling back and forth.
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They never actually go anywhere.
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So, if the charges don't
come from the power plant
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to your home,
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how does the electrical
energy actually reach you?
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When I used to teach this subject,
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I would say that power lines
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are like this flexible plastic tubing
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and the electrons inside
are like this chain.
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So, what a power station does,
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is it pushes and pulls the
electrons back and forth
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60 times a second.
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Now, at your house,
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you can plug in a device, like a toaster,
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which essentially means
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allowing the electrons to run through it.
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So when the power station
pushes and pulls the electrons,
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well, they encounter resistance
in the toaster element,
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and they dissipate their energy as heat,
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and so you can toast your bread.
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Now, this is a great story,
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I think it's easy to visualize,
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and I think my students understood it.
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The only problem is, it's wrong.
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For one thing,
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there is no continuous conducting wire
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that runs all the way from a
power station to your house.
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No, there are physical gaps,
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there are breaks in the line,
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like in transformers
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where one coil of wire
is wrapped on one side,
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a different coil of wire is
wrapped on the other side.
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So, electrons cannot possibly flow
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from one the other.
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Plus, if it's the electrons
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that are carrying the energy
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from the power station to your device,
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then when those same electrons
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flow back to the power station,
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why are they not also carrying energy
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back from your house to the power station?
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If the flow of current is two ways,
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then why does energy only
flow in one direction?
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These are the lies you were
taught about electricity,
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that electrons themselves
have potential energy,
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that they are pushed or pulled
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through a continuous conducting loop
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and that they dissipate
their energy in the device.
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My claim in this video
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is that all of that is false.
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So, how does it actually work?
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In the 1860's and 70's,
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there was a huge breakthrough
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in our understanding of the universe
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when Scottish physicist,
James Clerk Maxwell,
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realized that light is made up
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of oscillating electric
and magnetic fields.
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The fields are oscillating
perpendicular to each other
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and they are in phase,
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meaning when one is at its maximum,
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so is the other wave.
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Now, he works out the equations
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that govern the behavior of
electric and magnetic fields
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and hence, these waves,
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those are now called Maxwell's equations.
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But in 1883,
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one of Maxwell's former
students, John Henry Poynting,
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is thinking about conversation of energy.
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If energy is conserved locally
in every tiny bit of space,
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well, then you should be
able to trace the path
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that energy flows from
one place to another.
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So, think about the energy
that comes to us from the sun,
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during those eight minutes
when the light is traveling,
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the energy is stored and being transmitted
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in the electric and magnetic
fields of the light.
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Now, Poynting works out an equation
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to describe energy flux,
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that is, how much electromagnetic energy
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is passing through an area per second.
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This is known as the Poynting vector
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and it's given the symbol S.
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And the formula is really pretty simple,
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it's just a constant one over mu naught,
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which is the permeability of free space
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times E X B.
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Now, E X B,
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is the cross product
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of the electric and magnetic fields.
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Now, the cross product
is just a particular way
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of multiplying two vectors together,
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where you multiply their
perpendicular magnitudes
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and to find the direction,
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you put your fingers in the
direction of the first vector,
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which in this case is the electric field,
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and curl them in the direction
of the second vector,
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the magnetic fields,
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then your thumb points
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in the direction of the resulting vector,
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the energy flux.
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So, what this shows us about light
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is that the energy is
flowing perpendicular
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to both the electronic
an the magnetic fields.
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And it's in the same direction
as the light is traveling,
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so this makes a lot of sense.
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Light carries energy from its source
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out to its destination.
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But the kicker is this,
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Poynting's equation doesn't
just work for light,
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it works anytime there are electric
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and magnetic fields coinciding.
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Anytime you have electric
and magnetic fields together,
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there is a flow of energy
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and you can calculate
using Poynting's vector.
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To illustrate this,
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let's consider a simple circuit
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with a battery and a light bulb.
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The battery by itself
has an electric field
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but since no charges are moving,
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there is no magnetic field
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so the battery doesn't lose energy.
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When the battery is
connected into the circuit,
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its electric field extends
through the circuit
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at the speed of light.
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This electric field
pushes electrons around
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so they accumulate on some of
the surfaces of the conductors
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making them negatively charged,
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and are depleted elsewhere
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leaving their surfaces positively charged.
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These surface charges
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create a small electric
field inside the wires,
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causing electrons to drift
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preferentially in one direction.
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Note that this drift
velocity is extremely slow
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around a tenth of a millimeter per second.
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But this is current,
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well, conventional current
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is defined to flow opposite
the motion of electrons,
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but this is what's making it happen.
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The charge on the
surfaces of the conductors
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also creates an eclectic
field outside the wires
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and the current inside the wires
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creates a magnetic
field outside the wires.
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So, now there is a combination
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of electric and magnetic fields
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in this space around the circuit.
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So, according to Poynting's theory,
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energy should be flowing
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and we can work out the
direction of this energy flow
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using the right hand rule.
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Around the battery for example,
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the electric field is down
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and the magnetic field is into the screen.
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So, you find the energy
flux is to the right
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away from the battery.
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In fact, all around the battery,
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you'll find the energy
is radially outwards.
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Energy is going out through
the sides of the battery
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into the fields.
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Along the wires, again,
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you can use the right hand rule
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to find the energy is
flowing to the right.
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This is true for the
fields along the top wire
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and the bottom wire.
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But at the filament,
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the Poynting vector is directed
in toward the light bulb.
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So, the light bulb is getting
energy from the field.
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If you do the cross product,
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you find the energy is coming
in from all around the bulb.
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It takes many paths from
the battery to the bulb,
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but in all cases,
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the energy is transmitted
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by the electric and magnetic fields.
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- People seem to think that
you're pumping electrons
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and that you're buying
electrons or something,
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which is just so wrong. (laughs)
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For most people,
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and I think to this day,
it's quite counterintuitive
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to think that the energy is
flowing through the space
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around the conductor,
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but the energy is,
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which is traveling through the field,
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yeah, is going quite fast.
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- So, there are a few
things to notice here.
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Even though the electrons go two ways
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away from the battery and towards it,
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by using the Poynting vector,
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you find that the energy
flux only goes one way
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from the battery to the bulb.
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This also shows it's the fields
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and not the electrons
that carry the energy.
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- How far do the electrons go
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in this little thing you're talking about,
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they barely move,
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they probably don't move at all.
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- Now, what happens if
in place of a battery,
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we use an alternating current source?
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Well then, the direction of current
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reverses every half cycle.
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But this means that both the
electric and magnetic fields
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flip at the same time,
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so at any instant,
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the Poynting vector still
points in the same direction,
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from the source to the bulb.
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So the exact same analysis we used for DC
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still works for AC.
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And this explains how
energy is able to flow
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from power plants to home in power lines.
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Inside the wires,
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electrons just oscillate back and forth.
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Their motion is greatly exaggerated here.
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But they do not carry the energy.
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Outside the wires,
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oscillating eclectic and magnetic fields
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travel from the power
station to your home.
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You can use the Poynting vector to check
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that the energy flux is
going in one direction.
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You might think this is
just an academic discussion
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that you could see the
energy as transmitted
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either by fields or by
the current in the wire.
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But that is not the case,
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and people learned this the hard way
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when they started laying
undersea telegraph cables.
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The first Trans Atlantic
cable was laid in 1858.
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- It only worked for about a month,
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it never worked properly.
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- There are all kinds of distortions
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when they try to send signals.
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- Enormous amounts of distortion.
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They could work it at
a few words per minute.
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- What they found was sending signals
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over such a long distance under the sea,
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the pulses became
distorted and lengthened.
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It was hard to differentiate
dots from dashes.
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To account for the failure,
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there was a debate among scientists.
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William Thomson, the future Lord Kelvin,
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thought electrical signals
moved through submarine cables
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like water flowing through a rubber tube.
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But others like Heaviside and Fitzgerald,
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argued it was the fields around the wires
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that carried the energy and information.
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And ultimately,
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this view proved correct.
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To insulate and protect
the submarine cable,
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the central copper conductor
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had been coated in an insulator
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and then encased in an iron sheath.
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The iron was only meant
to strengthen the cable,
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but as a good conductor,
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it interfered with a propagation
of electromagnetic fields
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because it increased the
capacitance of the line.
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This is why today, most power
lines are suspended high up.
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Even the damp earth acts as a conductor,
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so you want a large insulating gap of air
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to separate the wires from the ground.
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So, what is the answer
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to our giant circuit light bulb question?
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Well, after I close the switch,
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the light bulb will turn
on almost instantaneously,
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in roughly 1/C seconds.
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So, the correct answer is D.
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I think a lot of people imagine
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that the electric field needs to travel
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from the battery,
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all the way down the wire
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which is a light second long,
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so it should take a second
for the bulb to light up.
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But what we've learned in this video
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is it's not really what's
happening in the wires
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that matters,
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it's what happens around the wires.
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And the electric and magnetic fields
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can propagate out through space
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to this light bulb,
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which is only one meter
away in a few nanoseconds.
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And so, that is the limiting factor
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for the light bulb turning on.
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Now, the bulb won't receive
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the entire voltage of
the battery immediately,
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it'll be some fraction,
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which depends on the
impedance of these lines
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and the impedance of the bulb.
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Now, I asked several
experts about this question,
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and got kind of different answers,
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but we all agreed on these main points.
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So, I'm gonna put their
analysis in the description
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in case you want to learn more
about this particular setup.
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If I get called out on it
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and people don't think it's real,
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we can definitely invest the resources
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and string up some lines,
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and make our own power
lines in the desert.
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- I think you're gonna
get called out on it.
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- I agree, I think you're
gonna get called out.
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(laughing)
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I think that's right.
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- I think it's just kinda wild
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that this is one of those things
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that we use everyday,
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that almost nobody thinks about
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or knows the right answer to.
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These traveling electromagnetic
waves around power lines
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are really what's delivering your power.
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Hey, now that you understand
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how electrical energy actually flows,
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you can think about that
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every time you flick on a light switch.
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And if you want to take your
switches to the next level,
[819]
the sponsor of this
video, Caseta by Lutron,
[821]
provides premium smart lighting control,
[823]
including switches, remotes,
and plug-in smart dimmers.
[827]
And since one switch can
control many regular bulbs,
[830]
you can effectively make
all those bulbs smart
[832]
just by replacing the switch.
[834]
Then, you can turn your lights on and off
[836]
using your phone,
[838]
or you can use another device
[840]
like Alexa or Google Assistant.
[842]
Caseta works with more
leading smart home brands
[844]
than any other smart
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[847]
One of the things I
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[849]
The lights in my office for example,
[850]
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[852]
And this feature gives you peace of mind
[854]
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[855]
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And once you're already in bed,
[860]
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[862]
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[863]
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[865]
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[867]
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Learn more about Caseda
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[877]
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[879]
I will put that link
down in the description.
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So, I want to thank Lutron Electronics
[884]
for sponsoring this video,
[885]
and I want to thank you for watching.
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