# How to image the magnetic field

1. Oct 6, 2014

### jollage

Hi all,

I'm dealing with a homework connected to the magnetic field. But I can't image how these things are generated.

I can image that for the electric field, one can generate it by just applying a voltage different across two plates and you will get an electric field pointing from one plate to another. For the magnetic field, I guess it's basically the same thing, but I can't image how it's generated. Let's say, I have two plates, between which certain plasma or fluid fill, in this case, how can I fix the strength of the magnetic field at one plate to be B and the other to be 0, at the same time, the direction of magnetic fields are actually parallel to the plates? Sorry since I know this question maybe sound very strange....

Thanks.

2. Oct 6, 2014

### Staff: Mentor

You don't generate a magnetic field with plates. You generate it with current-carrying wires. One of the most efficient ways is with a current passing through a wire coil or solenoid.

3. Oct 6, 2014

### Staff: Mentor

No worries. The magnetic field is a little harder to visualize than the electric field since it doesn't originate from + charges and end at - charges like the electric field does. Unfortunately, I don't know of any way to visualize a magnetic field that helps you understand how it is generated. For example, current running through a straight wire will generate a magnetic field that runs in circular loops around the wire. Since the field lines are circles, they never enter or leave the wire, making it difficult to understand how they are generated.

The only thing I can suggest is to memorize the right hand rule so that you'll know the orientation of the field lines.

It may help to understand that the electric and magnetic lines you see in images don't really exist. They are only there to help you visualize how a charged particle will react when placed in an electric or magnetic field. The lines are like contour lines on a map or latitude and longitude lines on a globe. You don't see these lines when you go outside, as they only exist to help you understand how to get around.

4. Oct 7, 2014

### jollage

Thank you very much berkeman and Drakkith

I just want to make sure another thing regarding how to fix the strength of the magnetic field on the boundary of plate. The plasma or fluid and the plate are made of obviously two different materials, so I guess the strengths of the magnetic fields generated (by the current) within these two materials are also different, right? Now in the problem, the plasma is in motion, resulting in a magnetic field also changing with respect to the space. My question is that is the magnetic field on the contact of the plasma and the plate constant? Since the plate isn't motion, I guess the magnetic field there should be not changing, right?

Thanks

5. Oct 7, 2014

### Staff: Mentor

Can you say more about what you are trying to do? The interaction between a magnetic field and a plasma can be quite complex. Do you have any diagrams or pictures that would help us to understand what you are working toward?

6. Oct 7, 2014

### Staff: Mentor

What current? You've said nothing about any sort of current in your setup, just something about 2 plates with a plasma or fluid between them. I'm in agreement with Berkeman. It would greatly help if you could tell us or show us the exact setup of the problem.

7. Oct 12, 2014

### jollage

Hi Drakkith and berkeman, sorry for the late reply and the unclear sentences.

The current I'm talking about is following your thread (you mentioned how to generate the magnetic field by the current). Actually, all these questions are coming from my mind, just slightly related to the homework. What I eventually want to know is, except the complex physics of the plasma, that

If the magnetic field is perturbed, let's say the magnitude of this perturbation is b, then what's the boundary condition of this perturbed magnetic field on the interface between the fluids (or plasma) and the plates?

Note that b<<B, where B is the unperturbed magnetic field, and B is steady in time.