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 Posted: Mon, 27th Sep 2010 21:56 Post subject: Physics question |
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2 point charges:
q1=-1 * 10 ^-6 C
q2=-4 * 10 ^-6 C
We are searching for the point P, where field intensity E = 0 N/C, that should be between those two charges (they are 30cm apart from each other)
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ixigia
[Moderator] Consigliere
Posts: 65115
Location: Italy
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| Posted: Mon, 27th Sep 2010 22:12 Post subject: |
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Three years ago I could have resolved it quite easily, now...not so much 
 
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| Posted: Mon, 27th Sep 2010 22:59 Post subject: |
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Don't be shy!
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| Posted: Tue, 28th Sep 2010 09:01 Post subject: |
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I'm not sure what the field intensity is?(N/c= Newton/coulomb?) But why should it be zero anywhere? To get metastable states you need atleast 3 charges I think.
Edit: Of cause You only need 2 charges for setting the field equal to zero, but they should still have opposite sign.
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| Posted: Tue, 28th Sep 2010 09:47 Post subject: |
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No i think hes set that question up right, with the negative charges otherwise i think there would be an issue with field intensity being zero? Its been a loooooooooooooooooong whiiiiiilllllleeeeeee. If it were opposite fields though you can be sure the outcome is not a +
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| Posted: Tue, 28th Sep 2010 10:11 Post subject: |
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Coulomb's law:
+ force equation for charges:
Since the principle of superposition is valid, you can find the total electric field and set it to 0.
I did it like this:
And I got curious result...
Try searching the physics forum...
Also try this: http://demonstrations.wolfram.com/LinesOfForceForTwoPointCharges/
Cool stuff for Mathematica if you have one...
EDIT:
Hmmm... I may have missed sth... but that's the way it should be...
"Quantum mechanics is actually, contrary to it's reputation, unbeliveably simple, once you take the physics out."
Scott Aaronson | chiv wrote: | | thats true you know. newton didnt discover gravity. the apple told him about it, and then he killed it. the core was never found. |
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Rofl_Mao
Posts: 3187
Location: Nederland
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| Posted: Tue, 28th Sep 2010 10:54 Post subject: |
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| ixigia wrote: | Three years ago I could have resolved it quite easily, now...not so much
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9 years for me 
Question looks pretty straightforward... unless there's a snake underneath the grass. As they say in holland.
| Lopin18 wrote: | I think you played too much Fallout 3, Pedo Perk acquired.  |
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| Posted: Tue, 28th Sep 2010 10:56 Post subject: |
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So it's quite possible that it is not on the line? Because that was the 2nd question... the first question that was given:
q1 = -1 *10^-6 C
q2 = 2 * 10^-6 C
and the point P is right in the middle of them (30cm from q1 to q2),
what is E at position of P
I'm not sure if we calculated that right because I used this formula:
E=(k*Q)/r²
where k = 7.97*10^9
Q=q1 (later q2)
r=0.15
After I got E for Q1-P and E for Q2-P I just added the results as E are just vectors, right?
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| Posted: Tue, 28th Sep 2010 12:56 Post subject: |
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OMFG YOU ARE SCIENTISTS
MY RESPECTS
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| Posted: Tue, 28th Sep 2010 13:01 Post subject: |
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Here E is a scalar. You can add as you like because the theory electricity is linear.
If you're looking for the intensity then it is proportional to |E|^2. It is the intensity you measure in an experiment.
Your formular is correct aslong as you remember that r is the spacing between the points. Often r is taken to be the length from some arbitrary zero point. Perhaps it would be better to make the substitution r-> r - r', where r is the vector from zero to the point you're measuring at and r' is the vector from zero to your charge particle.
edit: I haven't checked the numerical constant . I would guess it needs some units.
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| Posted: Tue, 28th Sep 2010 13:47 Post subject: |
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| psychokillergr wrote: | OMFG YOU ARE SCIENTISTS
MY RESPECTS |
Euhm... no  Still thanks, actually it's for a friends physics test and I just wanted to help him by posting here as I know that there are some physicists here like dingo who'll find this stuff rather amusing than hard
@atropa: thx! |
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| Posted: Tue, 28th Sep 2010 17:28 Post subject: |
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Pumpy you got k wrong, it's not 7,97*10^9 look at my scanned paper in the upper right corner...
"Quantum mechanics is actually, contrary to it's reputation, unbeliveably simple, once you take the physics out."
Scott Aaronson | chiv wrote: | | thats true you know. newton didnt discover gravity. the apple told him about it, and then he killed it. the core was never found. |
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TSR69
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Posts: 14962
Location: Republic of the Seven United Provinces
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| Posted: Tue, 28th Sep 2010 20:15 Post subject: |
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| iconized wrote: | Hmm 19 years ago for me when I did electrical engineering 
Looking at the problem I think we should try to solve it in 2D, so Q1 is at 0 on the x axis and Q2 is at 0,3 on the x axis.
Using Coulomb's law it get this:
E = (kQ1/(r^2)) + (kQ2/((r+0,3)^2)) = 0 |
What I did, but I didn't try with same numbers... tho the scaling should be right... Hmmm let me try that one again...
EDIT: No... I got the same result : x=-0.06+-0.12i :\
"Quantum mechanics is actually, contrary to it's reputation, unbeliveably simple, once you take the physics out."
Scott Aaronson | chiv wrote: | | thats true you know. newton didnt discover gravity. the apple told him about it, and then he killed it. the core was never found. |
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TSR69
Banned
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Location: Republic of the Seven United Provinces
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| Posted: Tue, 28th Sep 2010 20:40 Post subject: |
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Hmm that horrible formula of mine gives complex solutions.
@dingo_d: Seems you're right.
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| Posted: Wed, 29th Sep 2010 09:52 Post subject: |
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Well r=+-inf is a solution. Of cause 2 negative E fields will never equal 2 zero, unless they are both zero at some point. For point charges this is infinity!
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TSR69
Banned
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Location: Republic of the Seven United Provinces
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| Posted: Wed, 29th Sep 2010 14:41 Post subject: |
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If r -> ± ∞ then E -> 0 but 2 negative (or positive) point charges will never neutralise each other.
Formerly known as iconized
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