Two ferromagnetic balls, rolling down a ruler

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Discussion Overview

The discussion revolves around the behavior of two ferromagnetic balls rolling down a slightly inclined ruler towards a strong ceramic magnet. Participants explore the forces at play, particularly focusing on induced magnetic fields, polarization, and the interactions between the balls as they approach the magnet.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether induced magnetic fields are polarizing the balls in a way that affects their motion towards the magnet.
  • Another participant suggests that if induced EMF were the only factor, the balls would not stop completely, as gravity would continue to act on them.
  • There is a proposal that the induced magnetic field could be strong enough to magnetize the balls, creating permanent dipoles that affect their interaction.
  • A participant describes how, as the balls roll closer to the magnet, they become polarized and attract each other, potentially forming a single object that cannot roll effectively.
  • One participant introduces a related scenario involving a magnetic bowl, discussing how eddy currents can create forces that affect the motion of a ferromagnetic ball, although the relevance to the original scenario is questioned.
  • Another participant agrees with a previous explanation and elaborates on the interaction between the balls, suggesting that induced currents will lead to opposite magnetic fields that attract the balls to each other.
  • There is a challenge regarding the nature of eddy currents, questioning whether they provide a restoring force or merely a damping force, particularly when the ball is stationary.
  • A participant introduces the idea that if the balls were superconductive, the eddy currents could persist indefinitely, potentially altering the dynamics of the situation.
  • A question is raised about the assumption of friction being sufficient to prevent the balls from sliding without rolling.

Areas of Agreement / Disagreement

Participants express various hypotheses and challenges regarding the forces acting on the balls, with no clear consensus reached on the mechanisms involved or the implications of eddy currents.

Contextual Notes

The discussion contains assumptions about the behavior of the balls, the nature of induced magnetic fields, and the effects of friction, which remain unresolved and may influence the interpretations presented.

Saketh
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Here's the situation:

We have a ruler, inclined slightly with a strong ceramic magnet at the end. Now, we put two identical ferromagnetic (steel, for example) balls on the top of the ramp, touching each other. We let the balls roll down the ramp, and, perhaps against intuition, the balls stop before touching the magnet.

I'm trying to figure out why this is, but I am not sure what's causing the forces in the end. Are the induced magnetic fields polarizing the balls in a certain way? I have a grasp of what's going on, but the conceptual details are escaping me.

Thanks in advance.
 
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If it is induced EMF, the ball wouldn't stop completely, since if it does stop completely, there will no longer be any EMF, and gravity will pull the ball down.So, it shouldn't be just induced EMF... other than that, I'm clueless as well.
 
Last edited:
I thought so as well, but then I thought that maybe the induced magnetic field is sufficiently strong to magnetize the balls, which would create permanent dipoles.

I'm still not sure, though.
 
I am assuming that the balls are rolling down the ruler at the same time and are in a single file line.

In the above situtation what happens is as the balls roll down the slightly inclined ramp the approach the magnet. The closer the balls get to the magnet the stronger the polarization of the steel balls. As the balls polarize they then attract one another. Once the two balls are attracted strongly enough they balls essentially become one object looking very much like a peanut, which as you might guess does not roll down hill end over end.
 
I'm not sure if it's relevant to here but if you have a round and uniform magnetic bowl then if you drop a ferromagnetic ball into it the ball will hover in the air. the reason for this is that the magnetic field of the bowl create eddy currents on the surface of the ball which in turn create magnetic fields which push the ball out of the bowl.
 
I believe quinn have provided a good explanation, if the interpretation of the set-up is correct.
daniel_i_l said:
I'm not sure if it's relevant to here but if you have a round and uniform magnetic bowl then if you drop a ferromagnetic ball into it the ball will hover in the air. the reason for this is that the magnetic field of the bowl create eddy currents on the surface of the ball which in turn create magnetic fields which push the ball out of the bowl.
How do Eddy currents provide anything more than a damping force? You are claiming they provide a restoring force. You will then need to address the point raised by tim above: when the ball is hovering above the cup (i.e., stationary), there should be no induced eddy current, and hence no upward force on the ball.
 
quinn said:
I am assuming that the balls are rolling down the ruler at the same time and are in a single file line.

In the above situtation what happens is as the balls roll down the slightly inclined ramp the approach the magnet. The closer the balls get to the magnet the stronger the polarization of the steel balls. As the balls polarize they then attract one another. Once the two balls are attracted strongly enough they balls essentially become one object looking very much like a peanut, which as you might guess does not roll down hill end over end.
Your assumptions of the system are correct.

Here's what I thought -- is this correct?

Induced currents in the balls will polarize one of the balls. This induced magnetic field in the one ball will induce a current and an opposite magnetic field in the other ball. The balls will magnetize with opposite dipoles -- thus they will attract each other.
 
Gokul43201 said:
How do Eddy currents provide anything more than a damping force? You are claiming they provide a restoring force. You will then need to address the point raised by tim above: when the ball is hovering above the cup (i.e., stationary), there should be no induced eddy current, and hence no upward force on the ball.
Your're right - but if the balls are superconductive then the eddy currents caused initially will continue for ever.

quinn: are you assuming that there's enough friction so that the balls can't slide without rolling?
 

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