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Science Forum Index » Physics - Electromagnetic Forum » Current and voltage
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| Guest |
 Posted: Mon Jun 04, 2007 6:58 pm |
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Hi,
I just read a text that said that a microwave oven
contains voltage at about 5kV - at high current, and
that CRTs have 35kV - at low current. Thus, the
microwave oven is more dangerous because of
the high current. But I don't get it. Isn't the current
directly proportional to the voltage, by Ohm's law?
High voltage should generate high current right?
I understand that a CRT may have a lot more
resistance and by that way have a lower current,
but how is that relevant to safety? A circuit may
have low or high current, but isn't it only the voltage
and the resistance in my body that decides the
current that will pass through my body? I don't
see why the current in the appliance have any
significance.
A similar question is about transformers. An ideal
transfomer has no power loss, right? That means
you could get high voltage with low current and
vice versa. But... How does that comply with
Ohm's law? Here also, I think the high voltage
should also have a high current. It seems so
strange that a high voltage gets a low current.
Grateful if someone could shed some light on this.
Thanks! |
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| Guest |
| Posted: Mon Jun 04, 2007 7:45 pm |
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banansol@hushmail.com wrote:
Quote: Hi,
I just read a text that said that a microwave oven
contains voltage at about 5kV - at high current, and
that CRTs have 35kV - at low current. Thus, the
microwave oven is more dangerous because of
the high current. But I don't get it. Isn't the current
directly proportional to the voltage, by Ohm's law?
High voltage should generate high current right?
Every real thing has a real internal resistance that is
determined by it's construction.
A CRT needs very little current to accelerate electrons
to the face, while a microwave oven needs power, and
hence more current, to do any good, so the power supplies
are designed and built accordingly.
As for which is more dangerous, you only have to get
close to the 35kV before it leaps out and, if you are
lucky and don't get stuck to it, all your muscles spasm
throwing you across the room into a dazed, quivering pile.
BTW, you won't find a modern CRT running at 35kV because
of all the X-rays that would put out.
Quote: I understand that a CRT may have a lot more
resistance and by that way have a lower current,
but how is that relevant to safety? A circuit may
have low or high current, but isn't it only the voltage
and the resistance in my body that decides the
current that will pass through my body? I don't
see why the current in the appliance have any
significance.
The current that will flow through your body is determined
by the applied voltage and the sum of the resistance of
your body plus the internal resistance of the voltage
source.
Quote: A similar question is about transformers. An ideal
transfomer has no power loss, right? That means
you could get high voltage with low current and
vice versa. But... How does that comply with
Ohm's law? Here also, I think the high voltage
should also have a high current. It seems so
strange that a high voltage gets a low current.
Real transformers are made with real wire which has
real resistance, and generally the smallest wire that
will carry the currents for which the transformer is
rated.
If you compare the power transformers for a microwave
oven and a CRT, you will find the CRT transformer is
made with very small wire for the secondary by comarison.
There is also the limitation that the power out is
equal to the power in (minus the real losses) so
Vout x Iout = Vin x Iin (ignoring the loss).
--
Jim Pennino
Remove .spam.sux to reply. |
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| Don Kelly |
| Posted: Mon Jun 04, 2007 9:55 pm |
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Guest
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<banansol@hushmail.com> wrote in message
news:1181001513.212500.285360@p77g2000hsh.googlegroups.com...
Quote: Hi,
I just read a text that said that a microwave oven
contains voltage at about 5kV - at high current, and
that CRTs have 35kV - at low current. Thus, the
microwave oven is more dangerous because of
the high current. But I don't get it. Isn't the current
directly proportional to the voltage, by Ohm's law?
High voltage should generate high current right?
I understand that a CRT may have a lot more
resistance and by that way have a lower current,
but how is that relevant to safety? A circuit may
have low or high current, but isn't it only the voltage
and the resistance in my body that decides the
current that will pass through my body? I don't
see why the current in the appliance have any
significance.
A similar question is about transformers. An ideal
transfomer has no power loss, right? That means
you could get high voltage with low current and
vice versa. But... How does that comply with
Ohm's law? Here also, I think the high voltage
should also have a high current. It seems so
strange that a high voltage gets a low current.
Grateful if someone could shed some light on this.
Thanks!
There is more to consider than Ohm's Law. You hav to look at the whole
circuit.
You are correct in using Ohm's Law to determine what current passes through
your body for a given voltage. However, the resistance of the source does
have an effect so the open circuit voltage and the body resistance aren't
the only factors -simply what often is a simplistic worst case situation (In
fact, for some situations, body resistance is ignored for a more realistic
worst case scenario).
As for the transformer, you are dealing with a device that you cannot
handle with Ohm's Law only.
For a 2:1 ideal transformer, a voltage of 100V and a current of 1A on the
secondary "looks like"
200V and 0.5A on the primary (which really doesn't care about anything
more than looks). This has nothing to do with Ohm's Law but is due to the
fundamentals of 'electromagnetic energy conversion" in which primary and
secondary are related by magnetic factors. Essentially a transformer is an
electrical equivalent to a mechanical gear.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
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| Autymn D. C. |
| Posted: Thu Jun 07, 2007 5:02 am |
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Guest
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On Jun 4, 7:55 pm, "Don Kelly" <d...@shaw.ca> wrote:
Quote: secondary are related by magnetic factors. Essentially a transformer is an
electrical equivalent to a mechanical gear.
It is not. A gear does not wiggel, as by my DC-DC-DC transformer
thread. |
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| Don Kelly |
| Posted: Thu Jun 07, 2007 11:23 pm |
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Guest
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"Autymn D. C." <lysdexia@sbcglobal.net> wrote in message
news:1181210557.320573.134090@z28g2000prd.googlegroups.com...
Quote: On Jun 4, 7:55 pm, "Don Kelly" <d...@shaw.ca> wrote:
secondary are related by magnetic factors. Essentially a transformer is
an
electrical equivalent to a mechanical gear.
It is not. A gear does not wiggel, as by my DC-DC-DC transformer
thread.
--------------
Who said anything about "wiggel"?
The analogy is there, is conceptionally useful in many situations , and as
with all analogies there are limitations.
If you don't see the analogy, it's usefulness and it's limits, then that is
your problem.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
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| Autymn D. C. |
| Posted: Fri Jun 08, 2007 4:46 am |
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Guest
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On Jun 7, 9:23 pm, "Don Kelly" <d...@shaw.ca> wrote:
Quote: "Autymn D. C." <lysde...@sbcglobal.net> wrote in messagenews:1181210557.320573.134090@z28g2000prd.googlegroups.com...> On Jun 4, 7:55 pm, "Don Kelly" <d...@shaw.ca> wrote:
secondary are related by magnetic factors. Essentially a transformer is
an
electrical equivalent to a mechanical gear.
It is not. A gear does not wiggel, as by my DC-DC-DC transformer
thread.
Who said anything about "wiggel"?
The analogy is there, is conceptionally useful in many situations , and as
with all analogies there are limitations.
No, AC is not DC. Gears work as DC. Find a mekanic AC anaḷg.
Quote: If you don't see the analogy, it's usefulness and it's limits, then that is
your problem.
"it's" = you're illiterate |
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| brian a m stuckless |
| Posted: Fri Jun 08, 2007 8:05 am |
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Guest
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$$ Koine & current-say.
Quote: "Autymn D. C." <lysde...@sbcglobal.net> wrote > > "Don Kelly":
secondary are related by magnetic factors. Essentially a
transformer is an electrical equivalent to a mechanical gear.
It is not. A gear does not wiggel, as by my DC-DC-DC
transformer thread.
Who said anything about "wiggel"?
The analogy is there, is conceptionally useful in many
situations , and as with all analogies there are limitations.
No, AC is not DC. Gears work as DC. Find a mekanic AC anaḷg.
$$ AC a simply "alternating" DC. AC "reciprocating" gear meCANic.
$$ A jUDEAO-dominant "Hellene" banished "hamlet and isles KOiNE".
$$ [The "hamlet and isles KOiNE" of the "KOiNE and CURRENT SAY"].
$$ [The KOiNE is the very MOST ANCiENT rooted Atlantic language].
$$ [The KOiNE was persued "into hiding" on to "iREland" & Newfy].
$$ The iRE-ish COMPLETELY assimilated, and Beotucks exterminated.
$$
$$ But the language survived mostly and is most common one today.
Quote: If you don't see the analogy, it's usefulness and it's limits,
then that is your problem.
"it's" = you're illiterate [End of POST]. |
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| Benj |
| Posted: Fri Jun 08, 2007 1:17 pm |
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Guest
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Autymn D. C. wrote:
Quote: On Jun 7, 9:23 pm, "Don Kelly" <d...@shaw.ca> wrote:
"Autymn D. C." <lysde...@sbcglobal.net> wrote in messagenews:1181210557..320573.134090@z28g2000prd.googlegroups.com...> On Jun 4, 7:55 pm, "Don Kelly" <d...@shaw.ca> wrote:
secondary are related by magnetic factors. Essentially a transformer is
an
electrical equivalent to a mechanical gear.
It is not. A gear does not wiggel, as by my DC-DC-DC transformer
thread.
Who said anything about "wiggel"?
The analogy is there, is conceptionally useful in many situations , and as
with all analogies there are limitations.
No, AC is not DC. Gears work as DC. Find a mekanic AC anaḷg.
If you don't see the analogy, it's usefulness and it's limits, then that is
your problem.
"it's" = you're illiterate |
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| Benj |
| Posted: Fri Jun 08, 2007 1:20 pm |
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Guest
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Autymn D. C. wrote:
Quote: "it's" = you're illiterate
"mekanic"? You are illiterate and crazy, Deborah!
We simply use our OWN version of language. We KNOW you can understand
that! |
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| Don Kelly |
| Posted: Fri Jun 08, 2007 10:17 pm |
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Guest
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----------------------------
"Autymn D. C." <lysdexia@sbcglobal.net> wrote in message
news:1181295974.193851.29590@n15g2000prd.googlegroups.com...
On Jun 7, 9:23 pm, "Don Kelly" <d...@shaw.ca> wrote:
Quote: "Autymn D. C." <lysde...@sbcglobal.net> wrote in
messagenews:1181210557.320573.134090@z28g2000prd.googlegroups.com...> On
Jun 4, 7:55 pm, "Don Kelly" <d...@shaw.ca> wrote:
secondary are related by magnetic factors. Essentially a transformer
is
an
electrical equivalent to a mechanical gear.
It is not. A gear does not wiggel, as by my DC-DC-DC transformer
thread.
Who said anything about "wiggel"?
The analogy is there, is conceptionally useful in many situations , and
as
with all analogies there are limitations.
No, AC is not DC. Gears work as DC. Find a mekanic AC anaḷg.
Quote: If you don't see the analogy, it's usefulness and it's limits, then that
is
your problem.
"it's" = you're illiterate
Yes you caught me when I used it's instead of its. I admit the proofreading
error.
10 lashes with a page from the dictionary.
Grow up!
--
Don Kelly dhky@shawcross.ca
remove the X to answer |
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| aasigma |
| Posted: Mon Jun 11, 2007 3:41 am |
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Guest
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On Jun 9, 8:17 am, "Don Kelly" <d...@shaw.ca> wrote:
Quote: ----------------------------
"Autymn D. C." <lysde...@sbcglobal.net> wrote in messagenews:1181295974.193851.29590@n15g2000prd.googlegroups.com...
On Jun 7, 9:23 pm, "Don Kelly" <d...@shaw.ca> wrote:
"Autymn D. C." <lysde...@sbcglobal.net> wrote in
messagenews:1181210557.320573.134090@z28g2000prd.googlegroups.com...> On
Jun 4, 7:55 pm, "Don Kelly" <d...@shaw.ca> wrote:
secondary are related by magnetic factors. Essentially a transformer
is
an
electrical equivalent to a mechanical gear.
It is not. A gear does not wiggel, as by my DC-DC-DC transformer
thread.
Who said anything about "wiggel"?
The analogy is there, is conceptionally useful in many situations , and
as
with all analogies there are limitations.
No, AC is not DC. Gears work as DC. Find a mekanic AC anaḷg.
If you don't see the analogy, it's usefulness and it's limits, then that
is
your problem.
"it's" = you're illiterate
Yes you caught me when I used it's instead of its. I admit the proofreading
error.
10 lashes with a page from the dictionary.
Grow up!
--
Don Kelly d...@shawcross.ca
remove the X to answer
Can you get current even when there is no voltage ?Give examples. |
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| Don Kelly |
| Posted: Mon Jun 11, 2007 10:48 pm |
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Guest
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----------------------------
"aasigma" <acharya.anjana@gmail.com> wrote in message
news:1181551308.882012.85080@m36g2000hse.googlegroups.com...
On Jun 9, 8:17 am, "Don Kelly" <d...@shaw.ca> wrote:
Quote: ----------------------------
"Autymn D. C." <lysde...@sbcglobal.net> wrote in
messagenews:1181295974.193851.29590@n15g2000prd.googlegroups.com...
On Jun 7, 9:23 pm, "Don Kelly" <d...@shaw.ca> wrote:
"Autymn D. C." <lysde...@sbcglobal.net> wrote in
messagenews:1181210557.320573.134090@z28g2000prd.googlegroups.com...> On
Jun 4, 7:55 pm, "Don Kelly" <d...@shaw.ca> wrote:
secondary are related by magnetic factors. Essentially a transformer
is
an
electrical equivalent to a mechanical gear.
It is not. A gear does not wiggel, as by my DC-DC-DC transformer
thread.
Who said anything about "wiggel"?
The analogy is there, is conceptionally useful in many situations , and
as
with all analogies there are limitations.
No, AC is not DC. Gears work as DC. Find a mekanic AC anaḷg.
If you don't see the analogy, it's usefulness and it's limits, then
that
is
your problem.
"it's" = you're illiterate
Yes you caught me when I used it's instead of its. I admit the
proofreading
error.
10 lashes with a page from the dictionary.
Grow up!
--
Don Kelly d...@shawcross.ca
remove the X to answer
Can you get current even when there is no voltage ?Give examples.
----
I can't. but...
when you use an ideal transformer and a T or pi network to represent an
actual transformer, the ideal transformer is a ficticious beast independent
of frequency and essentially is such that e1/e2 =N1/N2 =i2/i1 (satisfying
conservation of energy) No more, no less. The attached network is frequency
dependent. The "ideal transformer" is a ratio changer independent of
frequency. Sure it doesn't exist but it is simply the part of the <model>
of an actual transformer that accounts for turns ratios- a non existent item
that is convenient. In fact the overall model simply is a mathematical
entity that satisfies terminal measured conditions (a bit more tractable
than a black box holding an old man with some meters and a hand cranked
generator- but with the same result) but still a <model>that allows us, with
some reasonable success, to predict behavior.
How is the fact that output voltage and current are 0 when DC is applied to
the model?- the external T or pi handles that part as well as any other
inductive or resistive effects.
Now consider a mechanical system with masses and springs etc (frequency
dependent) and including a set of gears. One can use one of two electrical
analogs. In either case you can use a network and an ideal gear such that
w1/w2=N1/N2=T1/T2 which is the mechanical equivalent of an ideal
transformer (I prefer the one where angular velocity w=voltage e and torque
T=current i).
In other words, for "linear" mechanical systems with gears, one can use an
electrical analog, including the ideal transformer and then make use of any
and all of the well developed electrical circuit theorems to solve the
mechanical problem.
It is true that the gears actually work for DC and a real electrical
transformer doesn't but the ideal transformer concept as part of the either
model is valid. As with any analogy, it is important to know enough to know
where it fails.
Write equations of motion for a mechanical system with gears, masses,
springs and dampers and then look at them in terms of electrical
equivalents. The parallels between electrical circuits and mechanical
systems show up in many texts dealing with electromechanical systems. These
parallels include gears-transformers.
--
Don Kelly dhky@shawcross.ca
remove the X to answer |
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