Railgun: Significant magnetic field where projectile is?

[greypilgrim]

Hi.
Very simplified schematics of railguns all look like this:

I have trouble properly understanding this. So apparently there is still a significant magnetic field where the projectile is, even though that's where the current stops flowing through the rails? Of course the magnetic field doesn't just stop there, there's a fringe field, but that has to be a lot weaker than the magnetic field between two wires far from the ends, doesn't it?

Would it make sense to build the projectile like this to move the crossing wire further inside the magnetic field:

Of course, the parts of the projectile (anti-)parallel to the rails would create an opposing magnetic field. But since this field is created by the projectile itself, it shouldn't be taken into account for the Lorentz force acting on the projectile. Or should it?

 

[renormalize]

Can you clarify the second configuration? Are the orange conductors that parallel the black rails insulated from those rails, or in electrical contact with them? If they're in contact, isn't this the same as the first configuration, just with slightly thicker rails ahead of the projectile?

 

[greypilgrim]

They are part of the projectile and insulated from the rails, apart from the very front where there's contact (at the places where the projectile in the initial configuration was).

EDIT: I uploaded a clarified version.

 

[DaveE]

But the orange current flowing anti-parallel to the black current will cancel, so no net increase in the field past the projectile.

 

[greypilgrim]

That's what I was wondering, does the field created by the anti-parallel parts count when calculating the Lorentz force onto the projectile? Wouldn't that mean that the orange part would create a net force on itself, contradicting 3rd Newton?

Imagine we only have the isolated orange part at rest, but with a huge inital charge separation that leads to a current. That shouldn't start moving, should it?

 

[DaveE]

does the field created by the anti-parallel parts count when calculating the Lorentz force onto the projectile?

It cancels the force from the black wire next to it. You can say that they each create an opposite force, if you like. I prefer to say that there is no net current in that area so no field is created, but it is ultimately the same thing.

 

[renormalize]

That's what I was wondering, does the field created by the anti-parallel parts count when calculating the Lorentz force onto the projectile? Wouldn't that mean that the orange part would create a net force on itself, contradicting 3rd Newton?

Yes, the "anti-parallel parts count". On your diagram, I've marked non-existent magnetic field loops with red Xs:

The field in that region is zero because the net current through those loops vanishes: the black and orange currents are antiparallel and exactly balance to zero. So the only region of magnetic field propelling the projectile is behind it. Your "alternative" is really just the standard railgun.

 

[greypilgrim]

Okay, thanks. I think my question comes down to the thought experiment in #5. I refined it a bit and formulated it more clearly in this other thread.

 

[berkeman]

Okay, thanks. I think my question comes down to the thought experiment in #5. I refined it a bit and formulated it more clearly in this other thread.

This thread here is now closed. Please continue the discussion in that other thread. Thanks.