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I made a video about a gigantic circuit
with light-second long wires
that connect up to a light bulb,
which is just one meter away from the battery and switch,
and I asked you, after I closed the switch,
how long will it take for us to get light
from that light bulb?
And my answer was 1/c seconds.
- And his answer is wrong.
- We would be able to communicate
faster than the speed of light.
- That violates causality and common sense.
- This is actually a bit misleading.
- Misleading.
- Misleading in a way.
- Extremely unconvinced.
- Naughty Mr. Veritasium has stirred up a right hornet's nest.
- Clearly I did not do a good job of explaining
what was really going on in the last video.
So I wanna clear up any confusion that I created.
So behind me, we have a scaled down model of this circuit.
It is only 10 meters in length on either side.
Obviously that's a lot shorter than one light-second,
but for the first 30 nanoseconds,
this model should be identical to the big circuit,
and Caltech has very fast scopes,
so we'll be able to see what's going on in this time.
I got a ton of help on this from Richard Abbott,
who works on LIGO, the gravitational wave detector.
Over here, we are going to put a little resistor,
which is gonna be the stand in for our light bulb,
and we're going to measure it with a scope and see essentially,
what is the time delay between applying a pulse
on the other side, basically flicking the switch,
for us to get a voltage across our resistor.
And the magnitude of that voltage is really important.
A lot of people thought it would be negligible.
- The amount of energy supplied by this is so minuscule.
- A tiny, tiny effect, right?
- The amount of power you're getting
to the lamp over here, it's nuff-all
- He meant the light turns on
at any current level immediately.
- That is not what I meant.
- Well, actually, with that assumption,
Derek's answer is wrong.
The light never turns off
no matter the state of the switch.
Some electrons will jump the gap and result in an extremely
small continuous leakage current.
- Let me be clear about what I am claiming.
Okay, it is my claim that we will see voltage
and current through the load that is many orders
of magnitude greater than leakage current,
an amount of power
that would actually produce visible light
if you put it through an appropriate device,
and we will see that power there
in roughly the time it takes the light
to cross the one meter gap,
but to understand why this happens,
we first have to clear up some misconceptions
that I saw in responses.
Misconception number one is thinking that electrons
carry the energy from the battery to the bulb.
Let's say we have a simple circuit with a battery and a bulb
operating at steady state.
If you zoom in on the light bulb filament,
you'd see a lattice of positively charged cores of atoms,
the nucleus and lowest shells of electrons,
surrounded by a sea of negative electrons,
which are free to move around the lattice.
The actual speed of these electrons is very fast,
around a million meters per second,
but all in random directions.
The average drift velocity of an electron
is less than 0.1 millimeters per second.
Now frequently, an electron will bump into a metal ion,
and transfer some or all of its kinetic energy
to the lattice.
The electron slows down and the metal lattice
starts wiggling more.
It heats up.
And ultimately this is what causes the filament
to glow and emit light.
So a lot of people will look at this and conclude
the electron carried the energy from the battery to the bulb
where it dissipated its kinetic energy as heat,
but consider, where did the electron get its kinetic energy
from before the collision?
It didn't carry that energy from the battery.
In fact, if the circuit has only been on for a short time,
that electron hasn't been anywhere near the battery.
So how was it accelerated before the collision?
The answer is, it was by an electric field in the wire.
The electron repeatedly collides with the lattice,
and loses energy.
And after each collision,
it is again accelerated by the electric field.
So although it is the electron that transfers energy
to the lattice, the energy came from the electric field.
So where does that electric field come from?
Well, a lot of animations make it look like the electrons
push each other through the circuit
via their mutual repulsion.
So you might think the electric field
comes from the electron behind it.
There is the analogy of water flowing through a hose,
or marbles in a tube.
This is misconception two, thinking that mobile electrons
push each other through the circuit.
That is not how electrons flow in circuits.
The truth is if you average over a few atoms,
you find the charge density
everywhere inside a conductor is zero.
The negative charge of electrons and the positive cores
of atoms perfectly cancel out.
So for each repulsive force between electrons,
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