# The Naked Scientists Forum

### Author Topic: Why wouldn't this work? (perpetuum motion)  (Read 18068 times)

#### AlmostHuman

• Jr. Member
• Posts: 23
##### Why wouldn't this work? (perpetuum motion)
« Reply #75 on: 04/07/2011 15:40:14 »
Ok, I ran a quick numerical simulation.  This plot is of the potential (vertical axis) over space for two plates that extend from -20 to 20 on the x-axis (the one pointing towards you) and places at -10 and 10 on the y-axis.  The plates themselves correspond to the peak and trough.  Since the potential energy of a particle placed near the plates is it's charge times this potential, if left to move freely with the plates held fixed, the particle will tend to roll either down or up this potential.  The kinetic energy gained is the difference between the particle's starting and ending positions.

You can see that as the particle escapes from the region between the plates, it will eventually reach the same potential energy with which it entered the plate region.  This means the field extending beyond the edges of the plates slows it down by pulling on it.

Thank you JP, Geezer, and the rest of you.
This is the best answer I got so far, but I can't be sure about all of this. One thing isn't so obvious...

Why were we told that electric field is significantly weaker outside capacitor plates, and compared to the magnitude of the field inside capacitor, it's close to zero function?
I mean, why were all those PhDs wasting their time on that if that wasn't true? But, if that is true, how come we have that nice simulation that JP have done?

One other thing... I have seen experiment where beam of electrons were shot between capacitor plates, and that beam was deflected. Angle of deflection was linear function of voltage applied to capacitor.

And last, but not the least...
How come Moon is still orbiting the Earth? I mean, Moon's gravity pull is reason for ocean tides, and Moon is working it's 4ss off to keep that tide rolling 'round the planet.

#### JP

• Neilep Level Member
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• Thanked: 2 times
##### Why wouldn't this work? (perpetuum motion)
« Reply #76 on: 04/07/2011 15:54:12 »
Thank you JP, Geezer, and the rest of you.
Why were we told that electric field is significantly weaker outside capacitor plates, and compared to the magnitude of the field inside capacitor, it's close to zero function?
I mean, why were all those PhDs wasting their time on that if that wasn't true? But, if that is true, how come we have that nice simulation that JP have done?
The region between the capacitor does have a higher field than outside.  It has to: the fields from the plates add up constructively there (they point in the same direction.)  Outside the plates, the fields generally don't add up fully constructively, so the total field is much less.  If you assume the plates are infinitely large (a classic, but purely theoretical problem), you only get a field between the plates and it is zero outside due to them adding purely destructively outside the plates.

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One other thing... I have seen experiment where beam of electrons were shot between capacitor plates, and that beam was deflected. Angle of deflection was linear function of voltage applied to capacitor.
Deflection is possible, I believe, if the particle starts far away and ends far away.  Gain of energy isn't.  I'm pretty certain a particle can change direction but not speed in my simulation.  (We're ignoring one effect--if a particle changes direction, it radiates away energy, so it actually will lose a slight amount of energy if it changes direction.)

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And last, but not the least...
How come Moon is still orbiting the Earth? I mean, Moon's gravity pull is reason for ocean tides, and Moon is working it's 4ss off to keep that tide rolling 'round the planet.
I don't see why the moon isn't losing energy due to this.  I can't find a mention of it in a Google search, though.  My guess is that it's not a very significant source of energy loss.

#### AlmostHuman

• Jr. Member
• Posts: 23
##### Why wouldn't this work? (perpetuum motion)
« Reply #77 on: 04/07/2011 18:32:57 »
Hm... if your simulation is correct, and I don't see why it wouldn't be, field just outside the capacitor should be extremly strong. Because electric field is negative gradient of potential.
All after all, my confusion levels have significantly dropped due to this discussion. Thank you!

#### JP

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##### Why wouldn't this work? (perpetuum motion)
« Reply #78 on: 05/07/2011 16:45:47 »
Hm... if your simulation is correct, and I don't see why it wouldn't be, field just outside the capacitor should be extremly strong. Because electric field is negative gradient of potential.
All after all, my confusion levels have significantly dropped due to this discussion. Thank you!

It should be strong, but not as strong as between the plates.  The reason for this is that the electric fields tend to fall off as you move away from the charged plates.  Just outside the capacitor, you're close to the plates, so you expect the field to be large.  You expect it to be largest in between the plates, since the fields from the two plates point in the same direction, so they add up.  Outside the plates, they may point in different directions, so one might be subtracted from the other.  But if you're closer to one than to the other, the closer plate's field isn't completely canceled out, so you still get a large(ish) field.
« Last Edit: 05/07/2011 17:48:13 by JP »

#### AlmostHuman

• Jr. Member
• Posts: 23
##### Why wouldn't this work? (perpetuum motion)
« Reply #79 on: 06/07/2011 14:58:16 »
I was talking just about gradient of that function in your simulation. I always get same plot if I run simulation. We can see that greatest change happens at max and min of that function (at capacitor edges), and gradient is just that - the amount of change. That is if you look at this just as a math problem. But this is off-topic...

Thank you!

#### JP

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##### Why wouldn't this work? (perpetuum motion)
« Reply #80 on: 06/07/2011 16:14:50 »
Yeah, it is just a math problem, but as Geezer always points out to me, trusting the math without physical intuition can cause all sorts of problems.  :)  The fact that the math is backed up by physical reasoning makes me trust that simulation a lot more.

#### The Naked Scientists Forum

##### Why wouldn't this work? (perpetuum motion)
« Reply #80 on: 06/07/2011 16:14:50 »