Magnetic field - General current in a cube

AI Thread Summary
The discussion focuses on determining the magnetic field in a cube with current entering and exiting at opposite corners. A specific case was solved using symmetry, yielding a magnetic field of zero, but participants express uncertainty about generalizing this result. Key factors include the path of the current and its density, which remain unspecified, complicating the analysis. The consensus suggests that the magnetic field's behavior at the center may depend on the entry and exit points, as well as which edges are active. Considering the edges as resistances could provide insights into current distribution and help tackle the general case.
pieuler
Messages
3
Reaction score
2
Homework Statement
Given a cube carrying current, such that the current enters and leaves the cube at any two arbitrary points, prove that the magnetic field at the centre of the cube will always be 0.
Relevant Equations
Biot Savart Law
Ampere Circuit law
I could solve a similar (rather, a specific case of the above) where the current entered through a
corner and left from the corner opposite to it along the body diagonal of the cube. For this specific case, I was able to easily exploit symmetry to deduce the answer (0). However, I cannot think of any suitable technique (eg. symmetry) for the general version.

PS. I have basic knowledge of calculus and vectors, but not so much of 'vector calculus'. Any intuitive guidance/hints/solutions are appreciated.
 
Last edited:
Physics news on Phys.org
I think we need to know more information about the given current density. Saying that it just enters at one point and exits at another point is not enough. We need to know what is the in between path (is it a straight line? it is some sort of helix? or what else, they can be infinite ways for the inbetween path) and how the current density varies spatially in this path.

If you have some image or figure you can post here please do it, many times one picture worth thousands of words.

I don't think we can prove that the magnetic field at the center will always be zero, regardless of the details of the current density.
 
Current is constant and uniform. No specific details about current/current density were given so I believe it's supposed to be constant and uniform(like no variations whatsoever). Current flows along the edges only.

IMG_2890.jpg

Basically, any two completely random points on the edges of the cube. The setup is like a cubical cage made of wires. So that tells you that the current flows only along the edges.
 
I still don't see how it would be possible to prove that the magnetic field would be zero at the center, my opinion is that it depends on the entry point, on the exit point and which edges (or parts of edges) are "active" that is they carry the current.
 
Delta2 said:
I still don't see how it would be possible to prove that the magnetic field would be zero at the center, my opinion is that it depends on the entry point, on the exit point and which edges (or parts of edges) are "active" that is they carry the current.
How about we consider the edges to be resistances (equal of course) and then think about the current distribution? That is one way of doing the specific case I mentioned, so it might help in the general case too.
 
Thread 'Variable mass system : water sprayed into a moving container'
Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
Thread 'Minimum mass of a block'
Here we know that if block B is going to move up or just be at the verge of moving up ##Mg \sin \theta ## will act downwards and maximum static friction will act downwards ## \mu Mg \cos \theta ## Now what im confused by is how will we know " how quickly" block B reaches its maximum static friction value without any numbers, the suggested solution says that when block A is at its maximum extension, then block B will start to move up but with a certain set of values couldn't block A reach...
Back
Top