The railgun revisited

[broli]

For some reason the railgun has always been a source of many debates and doubts even though it's probably one of the oldest and most simple demonstration of electromagnetism around.

If I've learned anything from all the years in this field is to question EVERYTHING, take nothing for granted and prove things for yourself. Ironically the Rail Gun was one of those things that were so simple and obvious as to not question it as so many have come and gone to explain it from the lorentz force side to the classical ampere's law side.

This thread will hopefully document a series of experiments I shall be performing on Rail Guns and perhaps show things that can be classified as "strange". So let me kick off with a question;

Consider a simple railgun with a long magnet underneath it. My question is, as you widen the the rails what happens to the force on the crossing rod? Does the force get weaker as you widen the rails, remain the same or get stronger?

[broli]

The level of engagement has really dwindled these past years on this forum. I'm all for open sourcing ideas and work in order to spark discussions and new ideas but this lack of it sort of defeats the point when all you do is give. The whole point of this community is to feel as if there is a "community" and to step out of that lone wolf status that many feel they are in when doing things in this field yet if anything I feel more alone in this field than anything else.

[broli]

Here is the first experiment on the rail gun with different rail width:

https://youtu.be/TAHW9AyThsE

I used a 1m long array of magnets in order to eliminate edge effects, the magnets are stuck to a bar of steel which is underneath them as you can see from the photo.

My lab supply can only handle 5A, being ferrite magnets the force is not too big but it's certainly there. But still In conclusion I could not spot a difference in force.

Does this not bring up some questions? How can the rod be arbitrary long and still have a force acting upon it, the magnetic field of the ferrite magnets should have long wrapped back around producing a Lorentz force in the opposite direction, yet we see movement.

[Low-Q]

Hi there! Long time no write :-)
A railgun moves the projectile because the magnetic field along the rail and through the projectile has the same magnetic orientation that will force the projectile to move away from its own magnetic field.
The magnetbar you have there really does not much, but apparently it does.... The field that goes perpendicular to the rods magnetic field (When in the very middle of the magnetbar) does not do anything, but the magnetic field around the rod also interfer with the magnetic field that is not in center of the magnet bar. These magnetic "lines" that is not at the magnetbars center do suddenly not cross the fields around the rod perpendiculary, but from a given angle. That magnetic field will make the rod rolling.
It works just like a speaker driver with its coil inside a magnetic field. The longer the coil is, the less the force you get out of it becaus the magnetic field aroud the coil is straighter through the magnets field, and therfor less efficient. That means if the coil was infinite long, but still had the same electric current flowing through the coil, it would not make any force at all.


I bet if you remove the magnets, the rod are more willing to roll away from the powersupply - given that the rails are perfectly level.


To answer the wide versus narrow rails, I do not have a good answer, so it will only be a wild guess:
When the rails are close to each other, I think that the magnetic field around the rod is not as perpendicular to the bar so the force contributed by the rod is slightly less, or it is so focused in a smaller area it doesn't reach out as far towards the ends of the magnetbar, as when the rails are further apart. In addition, the magnetbar is now working with a magnetic field that is denser, and therfor it compensate for the inefficient magnetic field of the rod. Just a wild guess.


On the other hand, this experiment, even though it appears to have little loss in the system, have too great loss versus power, it might be hard to see the difference in force. So the only explanation, or correct result would be using math.


If you use a couple of 1,5V C-size batteries in parallell, you can easily get 30-40 amps throug the rail without hazardious results to the batteries - except they will get emty very soon. A direct short circuit of a single 1.5V AA cell do easily exceed 20 amps with a short thick copperwire.


Vidar

[broli]

Hey Vidar,

Thanks for pitching in. I'm well aware of how the railgun concept functions but there are things that are seemingly counter intuitive. To answer a few things, when the magnets are removed the rod does not have enough force to push even if I give it a gentle start, the magnets are definitely a big factor. The fact it moves in the opposite direction when polarity is reversed also proves this.

If I understand you correctly you also are implying the rod is moving due to the "edge effect" of the magnets. Well I can tell you the reason why I used a 1m long array was to eliminate this discussion. The FEMM simulations also show a near constant field, until you get a few cm's close to the edge then the field increases.

As I said to me the surprise was the fact the wide rails still produced a force since as illustrated the field lines should have long curled back and reversed the force.

Thanks for the battery idea but from here on I'm actually going to use coils.

There are many more interesting experiments coming.