"Lutz-Peter Hooge" <lpv### [at] gmxde> wrote in message
news:3d96e745$1@news.povray.org...
> In article <3d96d83d@news.povray.org>, orp### [at] btinternetcom says...
>
> > Isn't that the same as F = -r^2 / (4 * Pi * Epsilon_0 * q * Q)?
>
> Hu? No (beware: y/x*z = y*z/x and not =y/(x*z)).

Are you beginning to see why my math doesn't work? <sob!>

> > > Epsilon_0 is a physical constant (8.854 * 10^-12 kg*m^3/(s^2*C^2))
> >
> > So... 8.854e-12 is the number... kg*m^3 is the unit... what's the s^2 *
C^2
> > bit about?
>
> s^2 * C^2 also belongs to the unit (s: seconds, C: Coloumb).

Right... So if I let q and Q be the charge of my ball and magnet (what's the
correct unit for charge?), and I measure r in meters, then F will come out
in Netwons? While we're on the subject, what would be a suitable range of
magnitude for q and Q? (The ball is 80g in mass.)

Thanks!
Andrew.

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Andrew Coppin <orp### [at] btinternetcom> wrote:
> This is turning out to be *much* harder than I thought... Currently, the
> ball approaches on of the magnets, accelerates to implausibly heigh speed,
> and then ends up so far away from the magnets that is just demonstrates
> Newton's 1st - it travels in a straight line forever. Bum!

  The closer you get to a physically correct model, the more
real-life problems you will encounter.
  Orbits caused by gravitational or magnetic forces are very very
unstable. Even the slightest difference in a stable orbit can make it
unstable and the orbiting object will most probably be ejected away 
(unless you have modelled object collision as well, and the orbiting object
happens to fall into the other object :) ).

  (So how come the planets and moons in our solar system are in so nice
stable orbits? Because from the millions and millions of objects very long
time ago floating around the forming Sun, these particular objects happened
to be, by chance, in the right places at the right times and survived. All
the other objects either collided with these or were ejected from the solar
system.)

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

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In article <3d9710d0@news.povray.org>, war### [at] tagpovrayorg says...

>   Orbits caused by gravitational or magnetic forces are very very
> unstable.

This is only true for systems with three or more bodies.
A system of two bodies will always be stable I think (in real life, a 
simulation of it may be unstable, especially if computed using the Euler 
algorithm).

Lutz-Peter

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Lutz-Peter Hooge <lpv### [at] gmxde> wrote:
> This is only true for systems with three or more bodies.
> A system of two bodies will always be stable I think (in real life, a 
> simulation of it may be unstable, especially if computed using the Euler 
> algorithm).

  It may depend on how "stable" is defined.
  I think that an orbit is defined to be stable when the orbiting body
has a permanent and well-defined orbit around the other object
(that is, it will never collide with the other object nor it will be
ejected to infinity).
  In this sense it's perfectly possible to have an unstable orbit in
a two-body system (eg. simply by having them in collision course; it's
also possible that they will escape to infinity with respect to each other
if the minimum escaping speed is reached).

  Perhaps you confused this with the fact that a two-body
system can be modelled analytically while a three-(and higher) body
system cannot (but must be approximated numerically)?

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

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In article <3d97179b@news.povray.org>, war### [at] tagpovrayorg says...

>   In this sense it's perfectly possible to have an unstable orbit in
> a two-body system (eg. simply by having them in collision course; it's
> also possible that they will escape to infinity with respect to each other
> if the minimum escaping speed is reached).

But then it is no orbit at all. Of course it can collide, or escape to 
infty, but if it orbits at all, it will orbit forever (that is what I 
mean with stable).

Lutz-Peter

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>
> Right... So if I let q and Q be the charge of my ball and magnet (what's
the
> correct unit for charge?),

Coulombs (C)

> and I measure r in meters, then F will come out
> in Netwons?

Newtons. Yup.

>While we're on the subject, what would be a suitable range of
> magnitude for q and Q? (The ball is 80g in mass.)

Quite small. The electrostatic forces are very strong

eg. If you give one ball a charge of 5*10^-5 C and the other a charge
or -5*10^-5C and put them a metre apart the force between them is 22.5 N
That's equivalent to the gravitational force exerted by the earth on an
object of
mass 2.2kg.

If you charge a plastic rod by rubbing it with fur you can typically get a
charge
of 10^-9 C

Gail
--
#macro G(H,S)disc{0z.4pigment{onion color_map{[0rgb<sin(H/pi)cos(S/pi)*(H<6)
cos(S/pi)*(H>6)>*18][.4rgb 0]}}translate<H-5S-3,9>}#end G(3,5)G(2,5.5)G(1,5)
G(.6,4)G(.5,3)G(.6,2)G(1,1)G(2,.5)G(3,.7)G(3.2,1.6)G(3.1,2.5)G(2.2,2.5)G(9,5
)G(8,5.5)G(7,5)G(7,4)G(7.7,3.3)G(8.3,2.7)G(9,2)G(9,1)G(8,.5)G(7,1)///GS

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> Right... So if I let q and Q be the charge of my ball and magnet (what's
the
> correct unit for charge?), and I measure r in meters, then F will come out
> in Netwons? While we're on the subject, what would be a suitable range of
> magnitude for q and Q? (The ball is 80g in mass.)
>

btw, I wouldn't mix magnets and electrostatic charges. The math can get very
complex.
Stick to two charges balls and it's not that hard.

Gail
--
#macro G(H,S)disc{0z.4pigment{onion color_map{[0rgb<sin(H/pi)cos(S/pi)*(H<6)
cos(S/pi)*(H>6)>*18][.4rgb 0]}}translate<H-5S-3,9>}#end G(3,5)G(2,5.5)G(1,5)
G(.6,4)G(.5,3)G(.6,2)G(1,1)G(2,.5)G(3,.7)G(3.2,1.6)G(3.1,2.5)G(2.2,2.5)G(9,5
)G(8,5.5)G(7,5)G(7,4)G(7.7,3.3)G(8.3,2.7)G(9,2)G(9,1)G(8,.5)G(7,1)///GS

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> Coulombs (C)

OK, so tell me people - *is* there a "u" in that or not? ;-) Some seem to
think there is, others not...

> >While we're on the subject, what would be a suitable range of
> > magnitude for q and Q? (The ball is 80g in mass.)
>
> Quite small. The electrostatic forces are very strong

So I recall... "A person jumps off the top of a building. It takes him 30
seconds to accelerate down to the bottom under gravity, but only a fraction
of a second for electrostatic forces to bring his bode to a half again.
[Presumably rearranging it beyond recognition in the process!]"

> eg. If you give one ball a charge of 5*10^-5 C and the other a charge
> or -5*10^-5C and put them a metre apart the force between them is 22.5 N
> That's equivalent to the gravitational force exerted by the earth on an
> object of mass 2.2kg.

Ah... yes, *charge balls*... I had _better_ remember to give them OPPOSITE
charges... presumably they'll repell instead of attract otherwise? (Hmm...
that might actually be useful later on...)

> If you charge a plastic rod by rubbing it with fur you can typically get a
> charge
> of 10^-9 C

So I take it a 1C is a fairly large charge then? (I remember hearing that 1
Farrid is larger than any capacitor ever built - "built" being the word!)

> Gail

Thankyou very much!

So, in summary, I have three "magnets" (at least, fixed points which I want
to "attract" a moving particle). Right... so I need r in meters, q and Q
with opposite sign and at around about 10^-7 C or so, and the formula will
give me an answer in Newtons which should be halfway sane (assuming I make
sure that r stays away from zero!) Right, will try...

Thanks again for all the people who bothered to help a hapless half-brain!
Andrew.

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> btw, I wouldn't mix magnets and electrostatic charges. The math can get
very
> complex.

Mmm... complex is bad... (Unless it involved the square root of -1 ;-)

> Stick to two charges balls and it's not that hard.

Sounds good to me...

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Andrew Coppin wrote:

> So I take it a 1C is a fairly large charge then? (I remember hearing that 1
> Farrid is larger than any capacitor ever built - "built" being the word!)

http://www.partsexpress.com/pe/showdetl.cfm?&User_ID=8556534&St=3685&St2=71679322&St3=65133449&DS_ID=3&Product_ID=118270&DID=7

-- 
Ken Tyler

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