Prove that the magnetic field at the centre of the sphere is zero when____

  • Thread starter lindloda
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  • #1
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A metallic sphere of radius R and specific resistance ρ is taken and two wires( negligible resistance) connected to it at random positions (say at an angle θ apart from each other.both the wires are perpendicular to the surface of the metallic sphere. Prove that the Magnetic field B at the centre O is zero when a current I is turned on in the circuit.


This situation is kind of like -
imagine a hollow pendulum . now its connected to one thread(suppose its a wire.) ...we connect another wire to this pendulum at a random position. and using a cell create a potential difference between the two ends of the wires such that a current flows through the wires, into the sphere ...all around the sphere and back to the other wire

What I did is assume to the sphere to be made of two different resistors connected in parallel. one piece subtends a solid angle dΩ at the centre and hence the other piece will subtend an angle 4∏- dΩ.
After that ...I have no clue.

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The Attempt at a Solution

 
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  • #2
rude man
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Hints for you:

1. supposing the connections were made at 180 deg apart, like north & south poles, what would be the mag field at the center of the sphere?

2. Now assume the connections are made randomly. What can you say about the current density (amp/m2) in any direction but passing thru the center, compared to the current density of 1 above?
 
  • #3
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I know its really easy for north and south pole like situation, but
This is really different...and i'm not sure I follow your hint...
 
  • #4
rude man
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Well, what do you conclude is the B field at the center of the sphere when the connections are made to 180 deg. opposite poles, and what is your rationale?
 
  • #5
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Are you serious ?


I know it's OBVIOUSLY ZERO for any position.


Read the title pls.
And then as a mental exercise I invite you to think what does "Prove the mgn. field is zero" means ..

[Hint:
It means the magnetic field IS ZERO !]

I know it is zero.
I just need the solution .
I cant find it anywhere.

I hate to be rude but
I dont need cross questioning ..
All I want is the solution.
 
  • #6
rude man
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Are you serious ?


I know it's OBVIOUSLY ZERO for any position.

If it's OBVIOUSLY zero for any position then you must know WHY it's obvious! Saying I know it's zero because my prof says so is not proving it.

I suggest showing mathematically that the B field is zero along the axis connecting contact points 180 deg. apart, then follow up with an argument why it should apply to any two connections.
 
  • #7
Vanadium 50
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All I want is the solution.

Too bad. That's not what PF is about.
 

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