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Home > Archive > Electrical Engineering > March 2006 > AC Question
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| Herbert Lambert 2006-03-12, 11:21 am |
| Please don't laugh, but I was wondering if anyone can tells me what =
happens when AC voltage changes polarity and goes "negative" with =
respect to ground. Does the electrons now goes from hot to neutral or =
neutral to hot. What happens in a simple circuit like a light bulb, is =
the bulb actually lit when the voltage goes negative, or is it off but =
because the frequency is 60hz, is that too fast for the human eye to =
see. Thanks for your help.
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| It's pretty much like if you were able to switch the leads on a battery
very quickly while lighting a bulb. It would light, there would be a
flicker (in the US because we 'switch batteries' 60 times a second it's
at a frequency of 120 flickers a second. Think about that for a moment
and you'll see that it lights up when + is at one end, dims a little
when you're switching leads, and lights up again when + is at the other
end.
That's a straight answer, I hope in response to a straight question.
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| It's pretty much like if you were able to switch the leads on a battery
very quickly while lighting a bulb. It would light, there would be a
flicker (in the US because we 'switch batteries' 60 times a second it's
at a frequency of 120 flickers a second. Think about that for a moment
and you'll see that it lights up when + is at one end, dims a little
when you're switching leads, and lights up again when + is at the other
end.
That's a straight answer, I hope in response to a straight question.
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| TimPerry 2006-03-12, 3:21 pm |
|
>"Herbert Lambert" <hdlambert@sbcglobal.net> wrote in message
news:xSSQf.46083$F_3.44102@newssvr29.news.prodigy.net...
>Please don't laugh, but I was wondering if anyone can tells me what happens
when AC voltage changes polarity and goes "negative" with respect to ground.
Does the electrons now goes from hot to neutral or neutral to hot.
Ben Franklin was a pretty smart guy but back then it hadent yes been
determines theat electrons were negitivly charged particles. We dont really
hold it aginst him and call his version "conventional current" or "hole
flow". this is the flow of "holes" (the places that the electrons used to
be). its particularly useful in describing some solid state circuits.
with "electron flow" it's negitive to positive so on the negitive half
cycle... you guessed it, the little critters are running to ground.
> What happens in a simple circuit like a light bulb, is the bulb actually
lit when the voltage goes negative, or is it off but because the frequency
is 60hz, is that too fast for the human eye to see. Thanks for your help.
in an incandescent lamp the light is emitted because the filament is hot. it
takes a certain amount of time for the filament to cool down enough so it
stops glowing.
remember that the voltage crosses the zero point twice each full cycle and
the time duration that the voltage (and current) is zero is zero seconds.
(come to think of it, it might be a little longer then that but this would
be a matter for someone a lot more versed in theoretical physics then me) in
any event its probably a number smaller then the inverse of the Democratic
budget.
sorry i can be more comprehensive as this time but you have me wondering
about how long it takes for electrons to reverse at 60 Hz... i don't even
know where to look that up. maybe that Daestrom person can fill us in 
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| Don Kelly 2006-03-13, 1:21 am |
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--
Don Kelly @shawcross.ca
remove the X to answer
----------------------------
"Herbert Lambert" <hdlambert@sbcglobal.net> wrote in message
news:xSSQf.46083$F_3.44102@newssvr29.news.prodigy.net...
Please don't laugh, but I was wondering if anyone can tells me what happens
when AC voltage changes polarity and goes "negative" with respect to ground.
Does the electrons now goes from hot to neutral or neutral to hot. What
happens in a simple circuit like a light bulb, is the bulb actually lit when
the voltage goes negative, or is it off but because the frequency is 60hz,
is that too fast for the human eye to see. Thanks for your help.
-----------
Actually, even at DC, the electrons simply drift along slowly. The water out
of a hose is not the water in at the same instant- crudely speaking, push in
at one end and like a freeway accident- the "push" propagates. The "push"
propagates at near light speed but the electrons are ambling along, looking
at the scenery.
For AC the electrons bumble, or wiggle, back and forth but the "push" is
still near light speed.
As Tim Perry says, the choice of "positive current flow' is independent of
the charge carriers. Physics and EE university courses use "conventional
current" as it;
a) works
b)gets rid of a lot of "do I or don't I add a minus sign" by, when, in the
rare cases it is necessary, consideration of the actual charged particles
which exist.
Many Tech schools (hopefully fewer and fewer) still use electron flow
because it seemed to them that it was a better convention than the
conventional current for the original "tube" devices. Many school texts
stick to this, adding to the confusion.
In any case, note that the power in a resistor such as a light bulb, is
dependent on I squared. (-I)squared =(+I)squared. The power then varies from
0 to a maximum and the average is +. In fact, using an rms voltage of 120V
60 Hz, the average power in a resistor is the same as for DC 120V. Google
"root mean square"
--
Don Kelly @shawcross.ca
remove the X to answer
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| Salmon Egg 2006-03-13, 3:21 am |
| On 3/12/06 2:38 AM, in article
xSSQf.46083$F_3.44102@newssvr29.news.prodigy.net, "Herbert Lambert"
<hdlambert@sbcglobal.net> wrote:
> Please don't laugh, but I was wondering if anyone can tells me what happens
> when AC voltage changes polarity and goes "negative" with respect to ground.
> Does the electrons now goes from hot to neutral or neutral to hot. What
> happens in a simple circuit like a light bulb, is the bulb actually lit when
> the voltage goes negative, or is it off but because the frequency is 60hz, is
> that too fast for the human eye to see. Thanks for your help.
If the explanation involves electrons, holes, or any microscopic properties,
it is not a good explanation.
Bill
-- Ferme le Bush
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| sir
i didn't get the conventional current flow concept what u said in the
answer
plz help me in this regard
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| Salmon Egg 2006-03-14, 2:21 pm |
| On 3/14/06 9:21 AM, in article
1142356910.587272.233800@p10g2000cwp.googlegroups.com, "kittu"
<gvramkrishna@gmail.com> wrote:
> sir
> i didn't get the conventional current flow concept what u said in the
> answer
> plz help me in this regard
>
Think of electrical charge without fregard to its microscopic explanation or
rationale. Current is the flow of charge or the rate of change of charge.
Mathematically i = dq/dt where i is current, q is charge and t is time.
If this simple concept of calculus is too complicated for you, you have
additional evidence of why our country is in a state of technical hurt.
Bill
-- Ferme le Bush
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