Dow does this work?

I'm not an expert at Physics - I have a B grade 'O' level. I teach
basic electrics to primary children, so my knowledge is somewhere
above (I hope) the level of complete imbecile.

I have a SON hub dynamo on my bike. From the generator to the front
light are two wires - no problems so far. The turning of a magnet
inside a coil (or the turning of a coil around a magent) gets
electrons moving - exactly how it happens is beyond my understanding,
but that's not the point. When the electrons encounter the resistance
of the front bulb they glow, making light - a simplistic explaination,
I know. The electrons, delpeted of some energy, then return to the
generator - a very basic explaination, perhaps, but understandable so
far.

My rear mudguard light had not been attached to the circuit. I
ordered a 2.4w front bulb, downgraded from a 3w bulb, and a 2m cable
to link the 0.6w rear light to the generator. I plugged the cable
into a socket on the B&M front light case. I ran the cable, with the
Rholoff gear cables to the rear mudguard, and through a hole to the
underside of the mudguard where it is taped to the underside (I may
superglue it later) and is run to the end of the mudguard where it
makes indirect contact with the bulb.

All would be fine and perfectly understandable if there was another
cable - but there is only one. So what happens to the electrons after
they pass through the rear bulb?

I always tell my primary school pupils that for a light to work a
circuit must be complete, and we perform endless investigations to see
if we can light a bulb with an incomplete circuit. To date, our
investigations have all been fruitless. My only thought is that
perhaps the bike's frame works as part of the circuit, but surely this
is not possible. The mudguard is plastic with metal supports,
attached to the rear rack, attached to the frame, with the front lamp
also attached to the frame. Surely the whole bike hasn't been
designed to allow it to be used as a circuit? But I can think of no
other explaination.

Can anyone enlighten me?
--
Let us have a moment of silence for all Americans who
are now stuck in traffic on their way to a health club
to ride a stationary bicycle. -
Congressman Earl Blumenauer (Oregon)

Tom Crispin wrote:

I'm not an expert at Physics - I have a B grade 'O' level. I teach
basic electrics to primary children, so my knowledge is somewhere
above (I hope) the level of complete imbecile.

I have a SON hub dynamo on my bike. From the generator to the front
light are two wires - no problems so far. The turning of a magnet
inside a coil (or the turning of a coil around a magent) gets
electrons moving - exactly how it happens is beyond my understanding,
but that's not the point. When the electrons encounter the resistance
of the front bulb they glow, making light - a simplistic explaination,
I know. The electrons, delpeted of some energy, then return to the
generator - a very basic explaination, perhaps, but understandable so
far.

My rear mudguard light had not been attached to the circuit. I
ordered a 2.4w front bulb, downgraded from a 3w bulb, and a 2m cable
to link the 0.6w rear light to the generator. I plugged the cable
into a socket on the B&M front light case. I ran the cable, with the
Rholoff gear cables to the rear mudguard, and through a hole to the
underside of the mudguard where it is taped to the underside (I may
superglue it later) and is run to the end of the mudguard where it
makes indirect contact with the bulb.

All would be fine and perfectly understandable if there was another
cable - but there is only one. So what happens to the electrons after
they pass through the rear bulb?

I always tell my primary school pupils that for a light to work a
circuit must be complete, and we perform endless investigations to see
if we can light a bulb with an incomplete circuit. To date, our
investigations have all been fruitless. My only thought is that
perhaps the bike's frame works as part of the circuit, but surely this
is not possible. The mudguard is plastic with metal supports,
attached to the rear rack, attached to the frame, with the front lamp
also attached to the frame. Surely the whole bike hasn't been
designed to allow it to be used as a circuit? But I can think of no
other explaination.

Can anyone enlighten me?

The mudguard is chromoplastic. This means it has a conductive foil
laminated into it (look really carefully at the cut edges). The lamp
earths itself via the mudguard, then the frame, to the front lamp which
has a metal body. Simple!

Faraday's law states that: V=-Ndphi/dt

In english, that means that a voltage is generated across a coil of N
turns that is proportional
to the rate of change of phi with time, where phi are the lines of
magnetic flux - as are seen when you put iron filings
on a piece of paper on top of a bar magnet.

So the faster you pedal, the higher the V generated.

A nice way of thinking about it is this:

Electrons are charged particles, and experience a force in a magnetic
field if there is relative motion between
the field and the charge. If you pedal with nothing connected, electrons
pile up at one end of your
conductor, making it more negative than the other end of the conductor.
Opposite charges
attract, so in this situation, any self respecting
electron would fall back to the positive end, but for every one that
does, it is replaced by another forced to the
negative end by the magnetic field.But like charges repel each other, so
eventually the force due to the magnetic
field is balanced out by the repulsion of the elctrons. No more
electrons move. In reality this build up of voltage
happens almost instantaneously, so is not noticed.


Because the electrons are somewhere, but would rather be somewhere else,
they have potential energy - like a child at the top of a slide.
This energy can be used to do things - like move, or generate heat.
If the circuit is now connected to something, then electrons can flow
from the negtaive to the positive end of the
generator, just as children flow down a slide. On the way, they will
hit things (ok, so the slide analogy has just fallen
apart). They loose some energy, and the things they hit vibrate, and
generate heat.

The amount of heat generated is a function of the amount of current
flowing (electrons per second), and the resiatance of the wire - how
many things there are to bump into. As long as the flow of electrons is
less than that the magnetic field can "pump" back up, then the voltage
stays constant. As more current flows, more heat is generated. If the
resistance of the circuit is too low however, the voltage will fall, as,
to reuse the analogy, children are sliding down faster than they can
climb back up.

Bit of a dodgy explanation, but reasonably right. As for the "1 wire"
circuit, it is very common to use the bike frame as part of the dynamo
circuit -
unless you have a bike made of something very hi-tech, it is almost
certainly going to be steel or aluminum. Both of these are good
conductors- in fact the resistance od the return path through the frame
is likely to
be lower than that along the wire, since the effective surface area of
the wire is larger -
the bigger the wire, the lower it's resistance per m. This makes sense
when you thing about an electron trying to get from A to B hitting as
few things
as possible -easier the larger the cross sectional area of the conductor.

Hope this helps,

Ben




Tom Crispin wrote:
I'm not an expert at Physics - I have a B grade 'O' level. I teach
basic electrics to primary children, so my knowledge is somewhere
above (I hope) the level of complete imbecile.

I have a SON hub dynamo on my bike. From the generator to the front
light are two wires - no problems so far. The turning of a magnet
inside a coil (or the turning of a coil around a magent) gets
electrons moving - exactly how it happens is beyond my understanding,
but that's not the point. When the electrons encounter the resistance
of the front bulb they glow, making light - a simplistic explaination,
I know. The electrons, delpeted of some energy, then return to the
generator - a very basic explaination, perhaps, but understandable so
far.

My rear mudguard light had not been attached to the circuit. I
ordered a 2.4w front bulb, downgraded from a 3w bulb, and a 2m cable
to link the 0.6w rear light to the generator. I plugged the cable
into a socket on the B&M front light case. I ran the cable, with the
Rholoff gear cables to the rear mudguard, and through a hole to the
underside of the mudguard where it is taped to the underside (I may
superglue it later) and is run to the end of the mudguard where it
makes indirect contact with the bulb.

All would be fine and perfectly understandable if there was another
cable - but there is only one. So what happens to the electrons after
they pass through the rear bulb?

I always tell my primary school pupils that for a light to work a
circuit must be complete, and we perform endless investigations to see
if we can light a bulb with an incomplete circuit. To date, our
investigations have all been fruitless. My only thought is that
perhaps the bike's frame works as part of the circuit, but surely this
is not possible. The mudguard is plastic with metal supports,
attached to the rear rack, attached to the frame, with the front lamp
also attached to the frame. Surely the whole bike hasn't been
designed to allow it to be used as a circuit? But I can think of no
other explaination.

Can anyone enlighten me?
--
Let us have a moment of silence for all Americans who
are now stuck in traffic on their way to a health club
to ride a stationary bicycle. -
Congressman Earl Blumenauer (Oregon)

Ben Barker wrote:

Tom Crispin wrote:

My rear mudguard light had not been attached to the circuit. I
ordered a 2.4w front bulb, downgraded from a 3w bulb, and a 2m cable
to link the 0.6w rear light to the generator. I plugged the cable
into a socket on the B&M front light case. I ran the cable, with the
Rholoff gear cables to the rear mudguard, and through a hole to the
underside of the mudguard where it is taped to the underside (I may
superglue it later) and is run to the end of the mudguard where it
makes indirect contact with the bulb.

All would be fine and perfectly understandable if there was another
cable - but there is only one. So what happens to the electrons after
they pass through the rear bulb?
I always tell my primary school pupils that for a light to work a
circuit must be complete, and we perform endless investigations to see
if we can light a bulb with an incomplete circuit. To date, our
investigations have all been fruitless. My only thought is that
perhaps the bike's frame works as part of the circuit, but surely this
is not possible. The mudguard is plastic with metal supports,
attached to the rear rack, attached to the frame, with the front lamp
also attached to the frame. Surely the whole bike hasn't been
designed to allow it to be used as a circuit? But I can think of no
other explaination.

Can anyone enlighten me?

groan

Faraday's law states that: V=-Ndphi/dt

snip rest as irrelevant

Bit of a dodgy explanation, but reasonably right. As for the "1 wire"
circuit, it is very common to use the bike frame as part of the dynamo
circuit -

Hope this helps,

Well it would but you forgot the one wire bit is hanging on a plastic
mudguard, and it is that which requires explanation. The mudguard
isn't plastic but a cunning sandwich of metal and plastic. The fixings
for the light and the stays go through the metal and thus in
conjunction with the bike frame complete the circuit from bulb to
dyno.
--
Phil Cook looking north over the park to the "Westminster Gasworks"

Tom Crispin wrote:
I'm not an expert at Physics - I have a B grade 'O' level. I teach
basic electrics to primary children, so my knowledge is somewhere
above (I hope) the level of complete imbecile.

I have a SON hub dynamo on my bike. From the generator to the front
light are two wires - no problems so far. The turning of a magnet
inside a coil (or the turning of a coil around a magent) gets
electrons moving - exactly how it happens is beyond my understanding,
but that's not the point. When the electrons encounter the resistance
of the front bulb they glow, making light - a simplistic explaination,
I know. The electrons, delpeted of some energy, then return to the
generator - a very basic explaination, perhaps, but understandable so
far.

My rear mudguard light had not been attached to the circuit. I
ordered a 2.4w front bulb, downgraded from a 3w bulb, and a 2m cable
to link the 0.6w rear light to the generator. I plugged the cable
into a socket on the B&M front light case. I ran the cable, with the
Rholoff gear cables to the rear mudguard, and through a hole to the
underside of the mudguard where it is taped to the underside (I may
superglue it later) and is run to the end of the mudguard where it
makes indirect contact with the bulb.

All would be fine and perfectly understandable if there was another
cable - but there is only one. So what happens to the electrons after
they pass through the rear bulb?

I always tell my primary school pupils that for a light to work a
circuit must be complete, and we perform endless investigations to see
if we can light a bulb with an incomplete circuit. To date, our
investigations have all been fruitless. My only thought is that
perhaps the bike's frame works as part of the circuit, but surely this
is not possible. The mudguard is plastic with metal supports,
attached to the rear rack, attached to the frame, with the front lamp
also attached to the frame. Surely the whole bike hasn't been
designed to allow it to be used as a circuit? But I can think of no
other explaination.

Can anyone enlighten me?
--


The SON dynamo does not have a connection to ground, but bicyclelights
usually do. The headlight is grounded through the mounting bolt, as is
the rearlight. Brakecables and incomplete lubrication in the headset are
usually sufficient to establish a return path for the small current.

Fitting a separate returnwire between headlight and rearlight is
recommended though for added reliability, most lights have connectors
(look for the earthsymbol) for this purpose
--
---
Marten Gerritsen

INFOapestaartjeM-GINEERINGpuntNL
www.m-gineering.nl

On Wed, 22 Feb 2006 19:31:40 +0000, Ben Barker
wrote:

Faraday's law states that: V=-Ndphi/dt

In english, that means that a voltage is generated across a coil of N
turns that is proportional
to the rate of change of phi with time, where phi are the lines of
magnetic flux - as are seen when you put iron filings
on a piece of paper on top of a bar magnet.

So the faster you pedal, the higher the V generated.

A nice way of thinking about it is this:

Electrons are charged particles, and experience a force in a magnetic
field if there is relative motion between
the field and the charge. If you pedal with nothing connected, electrons
pile up at one end of your
conductor, making it more negative than the other end of the conductor.
Opposite charges
attract, so in this situation, any self respecting
electron would fall back to the positive end, but for every one that
does, it is replaced by another forced to the
negative end by the magnetic field.But like charges repel each other, so
eventually the force due to the magnetic
field is balanced out by the repulsion of the elctrons. No more
electrons move. In reality this build up of voltage
happens almost instantaneously, so is not noticed.


Because the electrons are somewhere, but would rather be somewhere else,
they have potential energy - like a child at the top of a slide.
This energy can be used to do things - like move, or generate heat.
If the circuit is now connected to something, then electrons can flow
from the negtaive to the positive end of the
generator, just as children flow down a slide. On the way, they will
hit things (ok, so the slide analogy has just fallen
apart). They loose some energy, and the things they hit vibrate, and
generate heat.

The amount of heat generated is a function of the amount of current
flowing (electrons per second), and the resiatance of the wire - how
many things there are to bump into. As long as the flow of electrons is
less than that the magnetic field can "pump" back up, then the voltage
stays constant. As more current flows, more heat is generated. If the
resistance of the circuit is too low however, the voltage will fall, as,
to reuse the analogy, children are sliding down faster than they can
climb back up.

Bit of a dodgy explanation, but reasonably right. As for the "1 wire"
circuit, it is very common to use the bike frame as part of the dynamo
circuit -
unless you have a bike made of something very hi-tech, it is almost
certainly going to be steel or aluminum. Both of these are good
conductors- in fact the resistance od the return path through the frame
is likely to
be lower than that along the wire, since the effective surface area of
the wire is larger -
the bigger the wire, the lower it's resistance per m. This makes sense
when you thing about an electron trying to get from A to B hitting as
few things
as possible -easier the larger the cross sectional area of the conductor.

Hope this helps,

Thanks Ben. It's much clearer. I'm a bit surprised that the bike
frame is part of the dynamo circuit - but it all seems to work very
well.
--
Let us have a moment of silence for all Americans who
are now stuck in traffic on their way to a health club
to ride a stationary bicycle. -
Congressman Earl Blumenauer (Oregon)

Tom Crispin wrote:
but that's not the point. When the electrons encounter the resistance
of the front bulb they glow, making light - a simplistic explaination,
I know.

More accurate to say that they heat the wire filament (a lot). As a
byproduct of the heat, you get light (in the same way that an electric
cooker filament glows visibly, or a poker left in the fire glows visibly).

investigations have all been fruitless. My only thought is that
perhaps the bike's frame works as part of the circuit, but surely this
is not possible. The mudguard is plastic with metal supports,
attached to the rear rack, attached to the frame, with the front lamp
also attached to the frame. Surely the whole bike hasn't been
designed to allow it to be used as a circuit? But I can think of no
other explaination.

I suspect that you'll find that either the mudguard has a conductive
strip which is plasticised over, which (together with the stays & frame)
completes the circuit, or that the single wire is in fact double (like
earphone speakers), although that is less likely since it'd be obvious
from the connector.

R.

It would be worth getting a multimeter (digital or analogue, it doesn't
matter) and measuring the resistance between the negative terminal of your
bulb and the dynamo. As you have figured out, there must be a return path
somewhere via the frame of the bicycle.

Ben Barker wrote:
Faraday's law states that: V=-Ndphi/dt

snip

Because the electrons are somewhere, but would rather be somewhere else

Always the same with bloody electrons. One moment you know where they
are, the next they're gone! Mostly back up the metal strip in the
mudguard, which in my experience never quite attaches to the go home
path :-(

Hmmmm...SON Dynamo checks list Nope, I don't have one of those...yet...

Richard wrote:

I suspect that you'll find that either the mudguard has a conductive
strip which is plasticised over, which (together with the stays & frame)
completes the circuit, or that the single wire is in fact double (like
earphone speakers), although that is less likely since it'd be obvious
from the connector.


Usually the rearlight bolts through the metal bracket which connects to
the metal stays.

--
---
Marten Gerritsen

INFOapestaartjeM-GINEERINGpuntNL
www.m-gineering.nl