enter image description here When switch is closed the ring will jump upward, but why?

I know that when the switch is closed a current is established in the solenoid. The current flow is from + to - so that the direction of the magnetic field in the solenoid is downward (according to right hand rule). The magnetic flux increases according to Lenz's law. So, the ring should produce a magnetic field in the opposite direction. In my book it is stated that the ring will feel a force upward. I know that the induced current which is in a magnetic field will experience a force but I don't know how to identify the direction of this force. How can I know that it's upward; which rule should I use?


closed as off-topic by John Rennie, sammy gerbil, Jon Custer, Chris, M. Enns Feb 25 '18 at 2:35

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – John Rennie, Jon Custer, Chris, M. Enns
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 1
    $\begingroup$ Hi Elize and welcome to the Physics SE! Please note that we don't answer homework or worked example type questions. Please see this Meta post on asking homework/exercise questions and this Meta post for "check my work" problems. $\endgroup$ – John Rennie Feb 22 '18 at 9:20
  • 1
    $\begingroup$ I am not asking for homework check my question is how can I find the direction of a force which a ring would feel when it is located in a changing magnetic flux just I need the rule $\endgroup$ – Elize Feb 22 '18 at 9:25
  • $\begingroup$ See hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html $\endgroup$ – sammy gerbil Feb 22 '18 at 11:33
  • $\begingroup$ Following @sammygerbil link (Lorentz Force), I think you also need to assume that the magnetic field is not pointing directly downwards near the ring. Rather, the field is downwards and radial (inward). The radial component is the one that will push the ring upwards. Possibly, the graded top construction of the iron core is meant to emphasize this field orientation. $\endgroup$ – npojo Feb 22 '18 at 18:07
  • $\begingroup$ What is radial component ?I haven't seen such a thing as radial component $\endgroup$ – Elize Feb 23 '18 at 7:16

You should consider a circular current carrying loop as a bar magnet. The coils of the solenoid can be simplified as a bar magnet with north pointing down (As magnetic field is down). And the ring as a bar magnet with north pointing up(As the induced current has a magnetic field up). They clearly repel.


Not the answer you're looking for? Browse other questions tagged or ask your own question.