Is My Railgun Defying the Laws of Physics?

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Discussion Overview

The discussion centers around a homemade railgun that appears to be functioning contrary to expected physical principles, specifically regarding the direction of force as predicted by the right-hand rule. Participants explore the setup, mechanics, and potential errors in the construction or understanding of the device.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes their railgun setup and notes that it shoots in the "wrong" direction, suggesting it defies the laws of physics.
  • Another participant questions the claim of defying physics, asking for more details about the setup.
  • A humorous remark is made about the possibility of making sign errors in the setup, indicating that a detailed description is necessary for troubleshooting.
  • Concerns are raised about the low velocity of the ball, with suggestions that the setup may not be level, allowing gravity to influence the ball's movement.
  • A participant explains their understanding of the right-hand rule and how the current and magnetic field should theoretically produce a force away from the observer.
  • Questions arise about the current limiting factors from the car battery and the safety of the setup.
  • Another participant posits that a force acting away from the observer could still result in the ball rolling towards them under certain conditions.
  • Discussion includes considerations of the geometry of the setup and how the electromagnetic force might act relative to the ball's contact points with the rails.
  • A participant theorizes that the torque created by the force acting below the axis of rotation could explain the ball rolling towards the observer.
  • There is a suggestion that the majority of current may be conducting over the bottom surface of the ball, affecting the Lorentz force's direction.
  • One participant raises the possibility of the Faraday cage effect due to the use of a metal ball in the setup.

Areas of Agreement / Disagreement

Participants express varying interpretations of the railgun's behavior, with no consensus reached on the underlying reasons for the unexpected direction of the ball's movement. Multiple competing views and hypotheses are presented throughout the discussion.

Contextual Notes

Participants note the importance of the setup's geometry and the potential influence of the ball's contact points on the observed behavior, but these factors remain unresolved. The discussion also highlights the need for clarity regarding the current flow and its effects.

Cbixel
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I just made a rail gun using two ring stand poles, a steel ball, and a car battery. Maximum velocity is about 5 cm/sec.

What is strange is that it is shooting the wrong way. Defying the laws of physics and right hand rule. Any ideas ?
 
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How do we know that is "defying" the laws?
You don't bother to give any details.
 
Cbixel said:
What is strange is that it is shooting the wrong way. Defying the laws of physics and right hand rule. Any ideas ?

Usually this happens because you've made an odd number of sign errors. Professional scientists and engineers have gone through years of formal training to ensure that they know to always make an even number of sign errors. (Note that making zero errors is a special case of making an even number of errors, and is generally considered to represent an unrealistic and unnecessary ideal).

OK, kidding, aside... You'll have to tell us a lot more about your setup before we'll be able to tell you why your getting the result you're observing.
 
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5cm/sec ?
 
I would definitely like to see that setup. The only thing I can imagine is that you put the ring poles in parallel to create the rail. But what creates the force on the ball?

Also, yeah, 5cm/s? That's 0.18km/h. Are you sure your rail isn't just not completely level and the ball just rolls due to gravity?
 
So, its pretty level. The ball will not roll without a current applied. If I give it a push either direction, it rolls about the same distance before stopping, so its reasonably level.

So, the right hand rule says the force should be away from me. I am at the end of the two rails that is attached to the battery, so:
  • The current goes out the right rail (mag field up between rails).
  • The current comes back on the left rail. (mag field up between rails)
  • Current through steel ball is from right rail to left rail.
  • Right Hand Rule (Current from right to left through ball, plus mag field up = force away from me)
Switching the direction of current in the whole system should not have any effect, as both mag field and current through ball change directions, so force should still be away from me.

Perhaps I have created a left handed rail gun...

Anyway, if I put a small wire across the two rails, the wire moves away from me as would be expected. I am looking at torques on the the ball...
 
What is limiting the current from the car battery? You're not shorting it out, are you? That can be pretty dangerous...
 
I think that a force acting away from you can result in the ball rolling towards you.
 
The cause for that above behavior is very different though. If the spool didn't have that footstand to keep it a centimeter off the paper, the effect wouldn't happen.
 
  • #10
I am not saying that this is the case here. I have no idea of the geometry or about where the electromagnetic force acts, relative to the supporting surfaces.
Just a possibility.
I was thinking about a situation with the resultant of the electromagnetic force acting somewhere below the line of contact between the ball and the supporting edges. If there are any edges.
 
  • #11
I think I have it figured out. The force is, in fact, in the expected direction, or away from me. But, because the ball is between two rails, the axis of rotation is at the elevation of the points of contact between the ball and the rails. Thus, the force, away from me, is acting below the axis of rotation and creating a torque that spins the bottom of the ball away from me and the ball rolls toward me. This is different than the string on the spool demonstration, where the force is above the axis of rotation.

To test this theory out, we placed two tables 6 inches apart, and placed a bowling ball in the gap. A piece of tape attached onto the bottom of the ball and pulled toward us horizontally did, in fact, roll the ball away from us.
 
  • #12
Also, we think the majority of the current must be conducting over the bottom surface of the ball, and thus, this is where the Lorentz force is acting.

I am just glad I did not accidentally create the first "left handed railgun" ever.
 
  • #13
Mightn't you be dealing with the Faraday cage effect since you are using a metal ball?
 

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