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Ohm's Law, Voltage and Current confusion

Ohm's Law, Voltage and Current confusion - Physics Forum

Ohm's Law, Voltage and Current confusion - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 01-06-2007, 05:58 PM
Buddy
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Default Ohm's Law, Voltage and Current confusion



My primary education is not the electrical/electronic field, but I do
have a science background. What I am trying to get my head around is
something I heard from a friend who said, "It is not the voltage that
kills/hurts but the amperage". I understand that statement to a
certain point, but not satifactorily enough. I get the idea of the
Coloumb -- it is the most straightforward to understand. The amount of
"charge carriers" (electrons). I find the concept of Amperage fairly
well straightforward also. A rate at which these charge carriers pass
an arbitrary point (or plane). It makes fairly well good sense that
difference of rate of these "passing/moving through" (a current) the
body will give differences of intensity of the "shock". Now, when I
think of the formula E=I/R, I imagine limits of the numerator and
denominator. You could have a very small current, but if the
resistance is nearing zero, the EMF/voltage will approach infinity or
at least be very high. So if you have, say, 3000 volts, and the
resistance is very low, so that the current is low, a very small
amperage would flow through the body when you touch the terminals of
the object of energy in question with that voltage ( a short?), so the
coloumbs (amount of charges) is low. Now 3000 volts should fry
somebody in normal circumstances. This, finally, leads me to the
question; if a battery/generator or whatever source has 3000 volts, is
their a minimum practical size it has to be and therefore a minimum
Coloumbs it must have?

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  #2  
Old 01-06-2007, 06:13 PM
andy-k
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Default Ohm's Law, Voltage and Current confusion

"Buddy" wrote:
<snip>
It's the heating effect that kills you, and this is given by the rate of
flow of electrical charge through your body -- i.e. the current. Given a
certain resistance to the pathway that the electricity takes through your
body, you can calculate the voltage required to create the lethal current,
using Ohm's law, but you must also take into account the internal resistance
of the source. If its internal resistance is high, then as current is drawn
the supply voltage will drop accordingly (once again, Ohm's law, but for the
source not the sink). So with a supply having very low internal resistance,
a fairly low voltage (i.e. a hundred volts -ish) is sufficient to kill, but
with a supply having very high internal resistance, thousands of volts under
no-load conditions will fall to a possibly sub-lethal voltage once a load is
applied.


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  #3  
Old 01-06-2007, 06:13 PM
andy-k
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Default Ohm's Law, Voltage and Current confusion

"Buddy" wrote:
<snip>
It's the heating effect that kills you, and this is given by the rate of
flow of electrical charge through your body -- i.e. the current. Given a
certain resistance to the pathway that the electricity takes through your
body, you can calculate the voltage required to create the lethal current,
using Ohm's law, but you must also take into account the internal resistance
of the source. If its internal resistance is high, then as current is drawn
the supply voltage will drop accordingly (once again, Ohm's law, but for the
source not the sink). So with a supply having very low internal resistance,
a fairly low voltage (i.e. a hundred volts -ish) is sufficient to kill, but
with a supply having very high internal resistance, thousands of volts under
no-load conditions will fall to a possibly sub-lethal voltage once a load is
applied.


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  #4  
Old 01-06-2007, 08:40 PM
N:dlzc D:aol T:com \(dlzc\)
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Default Ohm's Law, Voltage and Current confusion

Dear Buddy:

"Buddy" <[Only registered users see links. ]> wrote in message
news:1168106286.942215.165430@s80g2000cwa.googlegr oups.com...

Correct. It is not unusual to find a 4000v differential on a
human. Even without scuffing your feet on a carpet.


A useful but limited analogy is water flow (like electron
current) and pressure loss (or head loss, like voltage drop).


That is an incorrect formula.
[Only registered users see links. ]

....

I took 15,000 volts AC into my elbow, which took out a fast blow
5A fuse on a 120v primary. All it did is black me out a bit, and
turn me 90 deg.

I also took 230 volts AC into my hand, and my "heart" took more
than an hour to feel right again.

DC clamps your muscles, usually such that you cannot let go.

It isn't the voltage, it is the path the curernt takes, and how
much there is.


No.

There must be a minimum separation between the terminals, so that
corona does not deplete its charge too quickly. And since there
is no perfect insulator, the bleed current through a battery's
case will be pretty high as well. You might be able to assemble
such a bettery just about as fast as the cells would fully
discharge.

David A. Smith


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  #5  
Old 01-06-2007, 08:40 PM
N:dlzc D:aol T:com \(dlzc\)
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Posts: n/a
Default Ohm's Law, Voltage and Current confusion

Dear Buddy:

"Buddy" <[Only registered users see links. ]> wrote in message
news:1168106286.942215.165430@s80g2000cwa.googlegr oups.com...

Correct. It is not unusual to find a 4000v differential on a
human. Even without scuffing your feet on a carpet.


A useful but limited analogy is water flow (like electron
current) and pressure loss (or head loss, like voltage drop).


That is an incorrect formula.
[Only registered users see links. ]

....

I took 15,000 volts AC into my elbow, which took out a fast blow
5A fuse on a 120v primary. All it did is black me out a bit, and
turn me 90 deg.

I also took 230 volts AC into my hand, and my "heart" took more
than an hour to feel right again.

DC clamps your muscles, usually such that you cannot let go.

It isn't the voltage, it is the path the curernt takes, and how
much there is.


No.

There must be a minimum separation between the terminals, so that
corona does not deplete its charge too quickly. And since there
is no perfect insulator, the bleed current through a battery's
case will be pretty high as well. You might be able to assemble
such a bettery just about as fast as the cells would fully
discharge.

David A. Smith


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  #6  
Old 01-06-2007, 10:44 PM
Buddy
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Posts: n/a
Default Ohm's Law, Voltage and Current confusion

On Jan 6, 3:40 pm, "N:dlzc D:aol T:com \(dlzc\)" <[Only registered users see links. ]> wrote:





Thanks for the analogy. I know that electrons flow much more rapidly.
If you have a water hose, then it would make sense that high pressure
(a potential motivation for a lot of force) of a water source will push
water particles (analogue electrons) at a high rate (analogue amperage)
and once that hits you at the other end, depending on resistance
(thinness) of the hose you would feel a strong force hitting. And if
the hose is thin enough the current would cause a mechanical force
expanding the walls of the hose (analogue heat energy) given if the
work was great enough to keep the pressure of the water source
constant.




My bad. I confused myself with this and is the major reason why I
couldn't grasp voltage nor could I grasp resistance for that matter.
It makes sense now when I state in words that Voltage
(Electromotiveforce) is directly to proportional to Resistance and
Current, instead of stating that Voltage is inversely proptional to
Resistance. When resistance goes down there is not much "pressure",
and the two points chosen for pressure differential (voltage
differential) would give a low reading. The two points are equalizing
(shorting?). When you are "seeing" an increase in current through a
"hose" more pressure (voltage) is being applied to the cavity of the
hose (by opening the spighot more perhaps). The force you feel hitting
you would be stronger. Thanks for the weblink.






Thanks for the information. It is useful and interesting.




For now, I assume idealistic conditions to get the basic concepts down
tight first. However, this information is still very helpful. You
mention that there is no perfect insulator. I understand this fairly
well. I would assume there is no ideal conductor either -- even a
superconductor would have some resistance. Ah, it makes more sense to
me now to say that there would be no voltage if there were no
resistance rather than saying there would be "infinite" voltage with
resistance approaching zero (almost no resistance) based on the wrong
formula I was trying to wrap my mind around! In the ideal (semi-real)
world, if you applied a zero resistance medium (say a wire) between the
terminals, there would be an instantaneous short, right before a
voltage across the medium could be observed, according to the formula.


Buddy

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  #7  
Old 01-06-2007, 10:44 PM
Buddy
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Posts: n/a
Default Ohm's Law, Voltage and Current confusion

On Jan 6, 3:40 pm, "N:dlzc D:aol T:com \(dlzc\)" <[Only registered users see links. ]> wrote:





Thanks for the analogy. I know that electrons flow much more rapidly.
If you have a water hose, then it would make sense that high pressure
(a potential motivation for a lot of force) of a water source will push
water particles (analogue electrons) at a high rate (analogue amperage)
and once that hits you at the other end, depending on resistance
(thinness) of the hose you would feel a strong force hitting. And if
the hose is thin enough the current would cause a mechanical force
expanding the walls of the hose (analogue heat energy) given if the
work was great enough to keep the pressure of the water source
constant.




My bad. I confused myself with this and is the major reason why I
couldn't grasp voltage nor could I grasp resistance for that matter.
It makes sense now when I state in words that Voltage
(Electromotiveforce) is directly to proportional to Resistance and
Current, instead of stating that Voltage is inversely proptional to
Resistance. When resistance goes down there is not much "pressure",
and the two points chosen for pressure differential (voltage
differential) would give a low reading. The two points are equalizing
(shorting?). When you are "seeing" an increase in current through a
"hose" more pressure (voltage) is being applied to the cavity of the
hose (by opening the spighot more perhaps). The force you feel hitting
you would be stronger. Thanks for the weblink.






Thanks for the information. It is useful and interesting.




For now, I assume idealistic conditions to get the basic concepts down
tight first. However, this information is still very helpful. You
mention that there is no perfect insulator. I understand this fairly
well. I would assume there is no ideal conductor either -- even a
superconductor would have some resistance. Ah, it makes more sense to
me now to say that there would be no voltage if there were no
resistance rather than saying there would be "infinite" voltage with
resistance approaching zero (almost no resistance) based on the wrong
formula I was trying to wrap my mind around! In the ideal (semi-real)
world, if you applied a zero resistance medium (say a wire) between the
terminals, there would be an instantaneous short, right before a
voltage across the medium could be observed, according to the formula.


Buddy

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  #8  
Old 01-06-2007, 11:32 PM
Buddy
Guest
 
Posts: n/a
Default Ohm's Law, Voltage and Current confusion

Thanks very much for your speedy reply.

On Jan 6, 1:13 pm, "andy-k" <spam.free@last> wrote:

Yes. The energy of the charge flow is being transformed into heat
energy due to the resistance of the body.


This is interesting. Would the internal resistance limit the magnitude
of the current through the sink then? Would the voltage drop slower if
the internal resistance of the source is higher?


So resistance is the key here. I assume you could have some supply
with a low voltage but with a very large size. Afterall, voltage is a
concept of differences between a couple of points. The points could
have a large amount of Coulombs but a small difference between the
amounts would give a low voltage -- am I correct? Now, if there is not
much "motivation" for a current (low voltage), then a low amperage
through the human body would result once it completed the circuit.
However, a large amount of Coulombs may exist in the supply source, so
if you knocked resistance very low, a proportional increase in the
amount of current would result increasing likelihood of danger to the
body as opposed to a very small supply with a small amount of Coulombs
within it that may have a comparable voltage but due to the small
amount of Coulombs it cannot yield a high magnitude current even with
low resistance -- am I way off here? I guess I should really have to
know what question I am asking.

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  #9  
Old 01-06-2007, 11:32 PM
Buddy
Guest
 
Posts: n/a
Default Ohm's Law, Voltage and Current confusion

Thanks very much for your speedy reply.

On Jan 6, 1:13 pm, "andy-k" <spam.free@last> wrote:

Yes. The energy of the charge flow is being transformed into heat
energy due to the resistance of the body.


This is interesting. Would the internal resistance limit the magnitude
of the current through the sink then? Would the voltage drop slower if
the internal resistance of the source is higher?


So resistance is the key here. I assume you could have some supply
with a low voltage but with a very large size. Afterall, voltage is a
concept of differences between a couple of points. The points could
have a large amount of Coulombs but a small difference between the
amounts would give a low voltage -- am I correct? Now, if there is not
much "motivation" for a current (low voltage), then a low amperage
through the human body would result once it completed the circuit.
However, a large amount of Coulombs may exist in the supply source, so
if you knocked resistance very low, a proportional increase in the
amount of current would result increasing likelihood of danger to the
body as opposed to a very small supply with a small amount of Coulombs
within it that may have a comparable voltage but due to the small
amount of Coulombs it cannot yield a high magnitude current even with
low resistance -- am I way off here? I guess I should really have to
know what question I am asking.

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  #10  
Old 01-07-2007, 01:17 AM
N:dlzc D:aol T:com \(dlzc\)
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Posts: n/a
Default Ohm's Law, Voltage and Current confusion

Dear Buddy:

"Buddy" <[Only registered users see links. ]> wrote in message
news:1168123492.569079.204930@38g2000cwa.googlegro ups.com...


Actually for reasonable sized conductors, and reasonable
currents, the electron drift velocity is a few to a few tens of
meters per second. There are just *a lot* of electrons in a
cross section...


A given exit velocity, times the mass is all you care about. You
don't really care about the hose, since a nozzle can achieve the
same effect.


The pressure does this, not the velocity.


.... *required* ...


"accelerating charges" or at least replacing the energy lost in
conduction.


I don't believe a superconductor does have resistance. They do
have a nasty habit of quenching, and destroying equipment,
however.


Sort of. The "spacing between charges" is suddenly changed as
each of the terminal is connected, and this propagates pretty
fast.

David A. Smith


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