Voltage, Current, Chicken, Egg

C

Cursitor Doom

Guest
Hi all,

Is the following statement beyond dispute: "You have to induce a
voltage before you will get a current flowing."

ISTR it's not quite that simple and in some instances the current
comes first and a potential difference arises from that. Can any
knowledgeable fellow here elaborate?
 
On Fri, 01 Aug 2014 22:38:37 +0200, Cursitor Doom wrote:

Hi all,

Is the following statement beyond dispute: "You have to induce a voltage
before you will get a current flowing."

ISTR it's not quite that simple and in some instances the current comes
first and a potential difference arises from that. Can any knowledgeable
fellow here elaborate?
Click to expand...

I don't think it makes much sense in physics to try to pin one property
of matter down as being more fundamental than another. You can't have
energy without mass, you can't have mass without energy, you can't have
force without motion, or motion without force, etc.

It makes it easier for us to _think_ about circuits if we make statements
like "currents depend on voltages", but I don't think it necessarily
reflects whatever the underlying reality is.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
In article <5gunt9lf7ugem3a7gg1r53v5k5qihn3568@4ax.com>,
cd@spamfreezone.net says...
Hi all,

Is the following statement beyond dispute: "You have to induce a
voltage before you will get a current flowing."

ISTR it's not quite that simple and in some instances the current
comes first and a potential difference arises from that. Can any
knowledgeable fellow here elaborate?
Click to expand...

Yes, ohms law

V = R * I.

Resistance of a component times the measured current will equal voltage
required to generate that current.

Simply put, you need voltage for current to exist.

Maybe you're looking for something else but that is how I interpret it.


Jamie
 
On 2014-08-01, Cursitor Doom <cd@spamfreezone.net> wrote:
Hi all,

Is the following statement beyond dispute: "You have to induce a
voltage before you will get a current flowing."
Click to expand...

This is only true in ordinary conductors,

ISTR it's not quite that simple and in some instances the current
comes first and a potential difference arises from that. Can any
knowledgeable fellow here elaborate?
Click to expand...

causality isn't really that important

--
umop apisdn


--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
 
Robert Roland <fake@ddress.no> wrote in
news:jfbpt99105amnmveclieglqlp57mjiomph@4ax.com:

On Fri, 01 Aug 2014 22:38:37 +0200, Cursitor Doom
cd@spamfreezone.net> wrote:

Is the following statement beyond dispute: "You have to induce a
voltage before you will get a current flowing."

In a purely resistive circuit, the current is driven by the voltage.

Sometimes, it's easier to think of current as water flow. If there is
no pressure (voltage) difference, then the water will not move
(current).

ISTR it's not quite that simple and in some instances the current
comes first and a potential difference arises from that.

In an inductive circuit, changing the current will create a voltage.

Look at the water analogy again. The water has mass. The mass
represents the inductance. Once the water is moving through a pipe, it
will continue to move even though there is no pressure difference. If
you force it to stop, a pressure will occur. The more abruptly the
flow is stopped, the higher the pressure will be.

In short: You can not get a current going without voltage, but once it
is going, it will continue to flow if there is inductance.

If you are familiar with Newtonian physics, you can use a similar
analogy where voltage is force and inductance is mass. (Also,
resistance is friction and capacitance is a spring).

You need force to get the mass moving, but once it is moving, it will
create a force if you try to stop it.
Click to expand...

Its not simple at all if you dont exclude the special case of
superconductivity.

Take a ring made of a superconductor. Start above its transition
temperature and place between the poles of a strong magnet parallel to
the pole piece faces. Chill the ring below the transition temperature
and it will trap all the flux through the ring. Remove it from between
the pole pieces and a permanent current is induced in the ring sufficient
to maintain the trapped flux which will persist till the ring is warmed
above the transition temperature or is broken.

As it is a superconductor, the voltage difference between any two points
on its surface is always zero.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL
 
On Fri, 01 Aug 2014 22:38:37 +0200, Cursitor Doom
<cd@spamfreezone.net> wrote:

Is the following statement beyond dispute: "You have to induce a
voltage before you will get a current flowing."
Click to expand...

In a purely resistive circuit, the current is driven by the voltage.

Sometimes, it's easier to think of current as water flow. If there is
no pressure (voltage) difference, then the water will not move
(current).

ISTR it's not quite that simple and in some instances the current
comes first and a potential difference arises from that.
Click to expand...

In an inductive circuit, changing the current will create a voltage.

Look at the water analogy again. The water has mass. The mass
represents the inductance. Once the water is moving through a pipe, it
will continue to move even though there is no pressure difference. If
you force it to stop, a pressure will occur. The more abruptly the
flow is stopped, the higher the pressure will be.

In short: You can not get a current going without voltage, but once it
is going, it will continue to flow if there is inductance.

If you are familiar with Newtonian physics, you can use a similar
analogy where voltage is force and inductance is mass. (Also,
resistance is friction and capacitance is a spring).

You need force to get the mass moving, but once it is moving, it will
create a force if you try to stop it.
--
RoRo
 
I know causality is not normally important in the real world, but I'm
interested in circuit theory at a fundamental level. Let me give two
examples which I believe illustrate that you can generate a current
without first having to generate a potential difference, and you tell
me whether my assumptions are valid or not:

a) A controlled current source with an extremely high internal
resistance. IOW a real-world device which AFAICS generates, in the
first instance, a current which then, as a result of this current,
goes on to develop a potential difference across the load.

b) The practical case of dragging a magnet across a wire which is in
circuit with other components. When I drag the magenet, am I in the
process disturbing electrons in the wire causing a current to flow?
The voltage as in case (a) then develops across the "other components"
in the circuit.

Surely in both these examples, you are generating a current *first* -
the voltage then arises out of this initial current?

Thanks for any observations...
 
In article <grhpt99o9kab741btu3r1nl6hscsd59tv1@4ax.com>,
cd@spamfreezone.net says...
I know causality is not normally important in the real world, but I'm
interested in circuit theory at a fundamental level. Let me give two
examples which I believe illustrate that you can generate a current
without first having to generate a potential difference, and you tell
me whether my assumptions are valid or not:

a) A controlled current source with an extremely high internal
resistance. IOW a real-world device which AFAICS generates, in the
first instance, a current which then, as a result of this current,
goes on to develop a potential difference across the load.

b) The practical case of dragging a magnet across a wire which is in
circuit with other components. When I drag the magenet, am I in the
process disturbing electrons in the wire causing a current to flow?
The voltage as in case (a) then develops across the "other components"
in the circuit.

Surely in both these examples, you are generating a current *first* -
the voltage then arises out of this initial current?

Thanks for any observations...
Click to expand...

No, the only reason you are observing current is due to the generator
or coil passing through a magnetic field, is producing voltage, and
when you drain it or short it the current is created.

I am going to give you the water analogy, which I think is as accurate
as it gets.

You are standing beside a small stream and the water is moving lets say
1 ft/sec. The speed of which that is moving is the VOLTAGE. Now stick a
board in that stream to dam it and then take note of the pressure
against that board. The water appears to stop flowing at the board but,
there's still force being applied there. The pressure being created at
the board is CURRENT.

Now, stand beside a must wider stream with the water going at 1 ft/sec,
the VOLTAGE is still the same however, stick a board in it to dam it,
the board being much wider now will create a lot more pressure
(Current). The speed of the water is still the same but volume of water
hitting the board over all is much more (current).

Take a straw and have a stream of 1 ft/sec come out of it and then
block it, you're find not much current there.

Now take that straw size tube and make 100 FT/sec water come out of it,
the pressure will be much greater in a concentrated stream, something
you may not want to stick your hands in front of to stop.

But stop the water flow and let it sit there, you have no CURRENT>>>

Jamie
 
On Sat, 2 Aug 2014 09:59:13 -0400, "Maynard A. Philbrook Jr."
<jamie_ka1lpa@charter.net> wrote:

Now, stand beside a must wider stream with the water going at 1 >ft/sec,
the VOLTAGE is still the same however, stick a board in it to dam it,
the board being much wider now will create a lot more pressure
(Current).
Click to expand...

I've come across this analogy before, but never seen "pressure"
equated to *current.*

Anyway, I think we're going off the point here. My question related to
initial conditions and the first few moments thereafter. To cut a long
story short, what you're all saying is that a magnet passing through a
magnetic field generates a voltage (a potential difference) NOT a
current.

Glad we got that straight and thanks, all!
 
On 8/2/2014 7:23 AM, Cursitor Doom wrote:
> I know causality is not normally important in the real world,

You guys are kidding, right? You think it's _unimportant_ that effects
don't precede causes in real life? I mean, it's only one of the five or
six most important bases for all thought, along with the law of
noncontradiction, the law of the excluded middle, and so on.

What in the world _do_ you think is important?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
 
On Sat, 02 Aug 2014 14:44:17 -0400, Phil Hobbs <hobbs@electrooptical.net> wrote:

On 8/2/2014 7:23 AM, Cursitor Doom wrote:
I know causality is not normally important in the real world,

You guys are kidding, right? You think it's _unimportant_ that effects
don't precede causes in real life? I mean, it's only one of the five or
six most important bases for all thought, along with the law of
noncontradiction, the law of the excluded middle, and so on.

What in the world _do_ you think is important?

Cheers

Phil Hobbs
Click to expand...

Bacon.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
 
On 8/2/2014 3:46 PM, John Larkin wrote:
On Sat, 02 Aug 2014 14:44:17 -0400, Phil Hobbs <hobbs@electrooptical.net> wrote:

On 8/2/2014 7:23 AM, Cursitor Doom wrote:
I know causality is not normally important in the real world,

You guys are kidding, right? You think it's _unimportant_ that effects
don't precede causes in real life? I mean, it's only one of the five or
six most important bases for all thought, along with the law of
noncontradiction, the law of the excluded middle, and so on.

What in the world _do_ you think is important?

Cheers

Phil Hobbs

Bacon.


I see your point, but still. ;)
Click to expand...

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
 
On Sat, 2 Aug 2014 09:59:13 -0400, "Maynard A. Philbrook Jr."
<jamie_ka1lpa@charter.net> wrote:

I am going to give you the water analogy, which I think is as accurate
as it gets.
Click to expand...

Your water flow analogy is completely wrong.

Voltage represents pressure.
Current represents quantity of water per time.
Resistance represents the flow restriction.
--
RoRo
 
On Sat, 02 Aug 2014 14:44:17 -0400, Phil Hobbs
<hobbs@electrooptical.net> wrote:

On 8/2/2014 7:23 AM, Cursitor Doom wrote:
I know causality is not normally important in the real world,

You guys are kidding, right?
Click to expand...

Define important.

99.99% of the planet's population get by just fine without any
interest in the subject of causality.

Tragigally, this includes journalists. When a journalist reads a
scientific document that concludes that people who read horoscopes
have, on average, smaller shoe sizes (fact), he will publish an
article that says: "Reading horoscopes shrinks your feet".
--
RoRo
 
On Sat, 02 Aug 2014 13:23:58 +0200, Cursitor Doom
<cd@spamfreezone.net> wrote:

a) A controlled current source with an extremely high internal
resistance. IOW a real-world device which AFAICS generates, in the
first instance, a current which then, as a result of this current,
goes on to develop a potential difference across the load.
Click to expand...

Measure the voltage across this current source when it is in open
circuit (when not supplying current). It will be very high. The device
regulates current by applying varying voltage.

b) The practical case of dragging a magnet across a wire which is in
circuit with other components. When I drag the magenet, am I in the
process disturbing electrons in the wire causing a current to flow?
Click to expand...

It creates a voltage that is dependent on the strength of the magnetic
field and the speed of the conductor in the field.
--
RoRo
 
On 8/2/2014 7:58 AM, Ian Malcolm wrote:
Robert Roland <fake@ddress.no> wrote in
news:jfbpt99105amnmveclieglqlp57mjiomph@4ax.com:

On Fri, 01 Aug 2014 22:38:37 +0200, Cursitor Doom
cd@spamfreezone.net> wrote:

Is the following statement beyond dispute: "You have to induce a
voltage before you will get a current flowing."

In a purely resistive circuit, the current is driven by the voltage.

Sometimes, it's easier to think of current as water flow. If there is
no pressure (voltage) difference, then the water will not move
(current).

ISTR it's not quite that simple and in some instances the current
comes first and a potential difference arises from that.

In an inductive circuit, changing the current will create a voltage.

Look at the water analogy again. The water has mass. The mass
represents the inductance. Once the water is moving through a pipe, it
will continue to move even though there is no pressure difference. If
you force it to stop, a pressure will occur. The more abruptly the
flow is stopped, the higher the pressure will be.

In short: You can not get a current going without voltage, but once it
is going, it will continue to flow if there is inductance.

If you are familiar with Newtonian physics, you can use a similar
analogy where voltage is force and inductance is mass. (Also,
resistance is friction and capacitance is a spring).

You need force to get the mass moving, but once it is moving, it will
create a force if you try to stop it.

Its not simple at all if you dont exclude the special case of
superconductivity.

Take a ring made of a superconductor. Start above its transition
temperature and place between the poles of a strong magnet parallel to
the pole piece faces. Chill the ring below the transition temperature
and it will trap all the flux through the ring. Remove it from between
the pole pieces and a permanent current is induced in the ring sufficient
to maintain the trapped flux which will persist till the ring is warmed
above the transition temperature or is broken.

As it is a superconductor, the voltage difference between any two points
on its surface is always zero.
Click to expand...

At DC, that is. Superconductors also exhibit an astounding amount of
inductance from the inertia of the electron pairs.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
 
On Sat, 02 Aug 2014 14:44:17 -0400, Phil Hobbs
<hobbs@electrooptical.net> wrote:

On 8/2/2014 7:23 AM, Cursitor Doom wrote:
I know causality is not normally important in the real world,

You guys are kidding, right? You think it's _unimportant_ that effects
don't precede causes in real life? I mean, it's only one of the five or
six most important bases for all thought, along with the law of
noncontradiction, the law of the excluded middle, and so on.

What in the world _do_ you think is important?
Click to expand...

I was referring to causality only in terms of the narrow confines of
this thread. I wasn't expecting anyone to apply it to (for example)
analysing the Palestine/Israel conflict!
 
In article <q91st99pdum1bb9k4p721aitlmg8q4bkle@4ax.com>, fake@ddress.no
says...
On Sat, 2 Aug 2014 09:59:13 -0400, "Maynard A. Philbrook Jr."
jamie_ka1lpa@charter.net> wrote:

I am going to give you the water analogy, which I think is as accurate
as it gets.

Your water flow analogy is completely wrong.

Voltage represents pressure.
Current represents quantity of water per time.
Resistance represents the flow restriction.
Click to expand...

What ever you say... I've gone by that for the last 45 years
and it has not failed me yet. AS I have also past that on
to many and it has worked for them, too!

Think as you may, it works for me.


Jamie
 
On Sat, 2 Aug 2014 09:59:13 -0400, "Maynard A. Philbrook Jr."
<jamie_ka1lpa@charter.net> wrote:


I am going to give you the water analogy, which I think is as accurate
as it gets.

You are standing beside a small stream and the water is moving lets say
1 ft/sec. The speed of which that is moving is the VOLTAGE.
Click to expand...

---
Well, first of all, if you're talking about the flow of water down a
stream, isn't the flow referred to as a 'current'? , as in: "Be
careful crossing that stream, it has a dangerously fast current."

Knowing that, it's not much of a leap to realize that if you want to
use the water analogy and you liken the water molecules to
electrons, then since the stream's current will indicate how many
water molecules move past a given point in a certain time, electric
current will indicate how many electrons move past a given point in
a certain time.

In the electric world, the unit of current is the ampere, and one
ampere is 6.241×10e18 electrons moving past a fixed point in one
second. Twice that many moving past a fixed point in one second
would be two amperes, and so on.

6.241×10e18 electrons is called a 'coulomb' and, for the purpose of
the analogy, can be likened to, say, a gallon's worth of water
molecules.

Now we can equate flow in both systems by saying that a flow of one
coulomb per second through a wire is like a flow of one gallon per
second through a pipe.

So now let's double the pressure in the hose and what'll happen?

The flow will increase to two gallons per minute, and if we double
the voltage on the wire what'll happen?

Bingo! The current will increase to two coulombs per second, which
is two amperes.

So now we have pressure analogous to voltage and flow analogous to
current.

All that's left now is resistance, and if we say that if, at a given
pressure, we can pump one gallon per second through a pipe with a
given length and cross-sectional area, we can also say that if we
increase the length or decrease the cross sectional area by a factor
of two, the flow will drop to one half-gallon per second.

Likewise, if we have a length of wire with a given voltage across it
and a current of one ampere through it and we increase the length of
the wire or decrease the cross sectional area by a factor of two,
the current will drop to one half of an ampere.

In both cases the resistance was increased, resulting in a decrease
of flow, so there you have it:

Voltage is analogous to pressure,
Current is analogous to flow, and
Resistance is analogous to resistance.
 
On Sunday, August 3, 2014 10:16:37 AM UTC-7, John Larkin wrote:
On Sun, 03 Aug 2014 08:54:51 -0400, Phil Hobbs

pcdhSpamMeSenseless@electrooptical.net> wrote:

As it is a superconductor, the voltage difference between any two points
on its surface is always zero.

At DC, that is. Superconductors also exhibit an astounding amount of
inductance from the inertia of the electron pairs.
Click to expand...

Inductance is an inertia-like effect, but the real mass and charge of electron
pairs is unimportant; it's the magnetic field that causes inductance.

> Is the inductance of a superconducting ring or coil very high?

Same as that geometry would have in regular-old-wire conductors; the
real difference is in the L/R ratio. That inductive decay time is REAL long.

> I really want surface-mount room-temp superconducting inductors!

Unless your other elements are superconducting, too, you lose the L/R
benefit... what's left, is small size for high current, with no self-heating
limit.
 
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