- All right, let's turn this bulb off.
What?
Huh?
A friend of mine called Mark Warner showed me
the really strange behavior of some bulbs that he bought.
They're not your typical screw-in Edison type bulb,
but they are fairly common.
What's strange about these bulbs is,
you can't turn them off.
He's in his house,
and he flicks the switch and it stays on.
You see, it's still glowing.
My first feeling was that well it's,
there's just a capacitor in there or something.
And you know, if you give it time,
it'll eventually discharge and just, it'll stop.
Or maybe it's no, that's not what's happening.
This will glow indefinitely.
How can that be?
Well, obviously I've managed to
recreate the effect here in the lab,
so I thought I'd run you through what's happening.
Like what's happening between this wire
and this wire and ends
and why you might find that kind of scenario
in someone's house.
Step one, direct connection.
Turns on there's a connection.
Turns off again, there's no glow.
So here's what that looks like as a circuit diagram.
You've got this red wire coming out from live,
it's going through the bulb into the black wire
which goes to neutral, very straightforward.
I can make it all a bit safer by adding
a fitting for the bulb
and using these terminal block thingys.
Look they go, the same thing happens.
And if I unplug it, it goes off completely.
Now, just for fun,
let's add loads and loads of wires.
This is 50 meters of wire
and I'm just gonna add it to the circuit.
I'm not gonna change anything about it.
I'm just =gonna make the wiring longer.
Let's follow the wiring.
The live wire comes out of here.
It goes round and round around, around
around, around, around, around, around, around
till it comes out the other end.
It comes around here.
It's stupid. (chuckles)
It goes straight back into there.
And that's the black wire, it's going round and round
around, around, around, around, around, around
it comes all the way back.
And now it's out here.
It goes through the bulb.
And then out through this black wire and back to neutral.
Remember I haven't changed the circuit diagram at all.
I've just added loads and loads of wire to it
that loops back on itself.
And it just so happens that the loop of wire
is kind of housed in this single casing.
Predictably, the bulb turns on there it is.
And when we unplug it, it turns off.
The thing hasn't happened yet.
The next step is to break the circuit at this point.
By the way, this is very, very dangerous.
Do not do this.
Don't do this.
Just don't do it, it's very dangerous.
We can just check that again.
Red wire comes out of here
around, around, around, around, around, around, stops.
There's a gap, blank wire around, around around, around
comes out here.
It goes to the bulb.
It goes to the neutral wire,
the black one comes back to the plug.
Plug it in.
And there it is.
There's that glow.
Look at that.
But the circuit's broken.
So how is it doing it?
Well actually, this is the type of wiring you end up with
when you want to add a switch to your bulbs.
Why didn't a wall switch and all this cabling here,
you can imagine that being in the walls
and the ceiling of your house,
connecting the light switch to the light.
And of course, when you turn it off, it breaks the circuit.
But in this case, the bulb stays on.
But the question is why?
Well, it turns out that all this cabling here,
is acting like a capacitor.
This capacitor isn't just storing charge
from when the bulb was turned on
and then slowly releasing it once it's turned off.
It's actually enabling the bulb to stay on permanently,
drawing power from the mains and costing you money.
A capacitor is just a component in a second,
that you can push charge into, by applying a voltage.
It can store charge.
And it typically looks a little bit like that.
How can a coil of wire act like a capacitor?
Because when you think
about electricity flowing through a wire
you're thinking about the flow of electrons, right?
And it's a bit like water flowing through a pipe.
That's a useful metaphor.
And when you apply a voltage to the end of a wire,
it's pushing on the electrons.
It's a bit like when you've got water in a pipe like this,
you can push on the electrons at one end
and electrons will pop out at the other end like that.
So long as the wire is connected to something.
But when the wire is not connected to something like this,
when the circuit is broken,
it's a bit like me holding my finger
over the end of this syringe.
It doesn't matter how hard I press.
It doesn't matter what voltage I apply.
I can't get the water to move
because water is essentially incompressible.
And actually that's where this metaphor breaks down.
A better analogy in this scenario
would be gas flowing through pipes.
Look, I can stop the end with my finger,
which is like this end is being stopped here.
I can still compress the gas because gas is compressible.
And so when I put this thing in,
the voltage here at the live wire,
this red wire is pushing electrons into the wire.
It's increasing the density of electrons.
And now the electrons
outnumber the metal ions.
There's a net negative charge built up
inside this red wire all the way through this coil.
And you probably know this already,
electrons repel each other, because like charges repel.
All the electrons inside this black wire
that's sat next to the red wire
all the way through this coil
are being repelled by the negative charge
inside this red wire.
To summarize, the voltage here
pushes electrons into the red wire.
You've got a build up of negative charge,
that negative charge repels the electrons in the black wire,
they're pushed out of the wire, eventually out of here,
through the bulb which lights up,
into the neutral wire and then into the
neutral line of the mains.
Now, if Edison had won, this wouldn't work.
If we were all using a DC power supply for our homes,
direct cone, this wouldn't happen because think about it.
If you've got a permanent positive voltage here
it will push electrons into the wire
and the electrons will build up and build up in density
until the repulsion of those electrons matches the voltage
and you reach equilibrium no more current flows.
But Edison didn't win.
Tesla won, and we all have AC power supplies in our homes,
alternating current.
So the voltage here at the live wire
is going up and down positive, negative, positive, negative.
It's pushing the electrons in
and then pulling them back out again,
repeatedly in the U.K. 50 times a second,
in the U.S. 60 times a second.
That's how you can have a permanently glowing bulb
even when it's switched off.
Here's what the diagram really looks like.
That big coil of wire is actually a capacitor.
And I've only just thought about this.
The symbol for capacitor is really appropriate
because it's a break in the circuit.
This is an accidental capacitor
but it's the same working principle
in deliberate capacitors,
except that instead of having two wires,
you have two metal sheets, maybe made of aluminum.
Minumum.
Interesting.
So basically like aluminium foil,
and they're separated, not by this rubber
but by maybe wax paper.
And then the whole assembly is rolled up into a cylinder
which is how you get the classic shape of a capacitor.
One thing that I really don't understand
is why this only seems to work with these cheap LED bulbs
from China and not the bog standard LED bulbs
that you might buy from a supermarket or a hardware store.
I've taken this one apart
and the LED bit looks really similar
but there's a lot more going on
with these components than there is in this one.
All this stuff somehow prevents the glow,
maybe deliberately, maybe by accident.
I'm not really qualified to figure that out
but don't worry.
I know someone who is.
To get the answer to this final part of the mystery,
you need to head over to a YouTube channel called
ElectroBOOM.
If you haven't heard of ElectroBOOM already, it's amazing.
You should subscribe to that channel.
But anyway, for now, I'll leave a link
to the companion video over on his channel
where he explains the difference between those things.
I'll put it in the card there, up there in the corner.
I'll also put a link
in the description and on the end screen as well.
But for now, that is the story of the capacitor
hidden inside the walls of your home.
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I hope you enjoyed this video.
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