Magnetic train/battery moving inside coil

[fog37]

Hello,

I have seen several videos online where a small AA battery with neodymium magnets attached to its poles manages to move inside a coil of copper. I think I understand how the battery moves. would a large coil be better than a small coil to make the battery move faster since the area of the coil increases the magnetic moment of the coil?

What if instead of a copper coil we had a tube of copper? I don't think the battery would move inside the hollow tube the same it moves inside the coil. Why?

 

[davenn]

I don't think the battery would move inside the hollow tube the same it moves inside the coil. Why?

think about what the coil is achieving ... then see if you can answer your Q

 

[fog37]

The battery and the local section of the coil form a closed circuit. Current runs through the coil and a magnetic field is generated inside the coil which generates forces on the magnets attached to the battery and propel it.

In the case of a hollow copper tube instead of a coil, current should flow on one of the walls of the hollow tube. The current on the tube should generate a magnetic field which would exist only outside of the tube. Is that correct?

both in the case of the coil and the tube there are eddy currents that cause retarding forces.

 

[davenn]

The battery and the local section of the coil form a closed circuit. Current runs through the coil and a magnetic field is generated inside the coil which generates forces on the magnets attached to the battery and propel it.

yes lots of turns and inductance

In the case of a hollow copper tube instead of a coil, current should flow on one of the walls of the hollow tube. The current on the tube should generate a magnetic field which would exist only outside of the tube. Is that correct?

but in the case of the tube, there is a short circuit between the battery ends
imagine it as a 1/2 or 1 turn coil with very little inductance and therefore very little magnetic fieldDave