I recently came across a question regarding a current carrying wire between magnadur magnets.enter image description here The thing is that we were supposed to find the magnetic field direction. As the girl increased the current the reading on the balance increased so I assumed that the direction of force is into the page. However the marking scheme says that as this force is on the magnet, the force on the wire according to N3 is out of the page. I don't get that, like we never consider N3 in other scenarios, why here?


I assume that this is a plan view of the arrangement looking down?

With no current passing thorough the wire and the magnadur magnet arrangement sitting on top of the balance the balance reading is equal to the weight of the magnadur magnet arrangement.

If you used your finger to push down on the magnet the balance reading would increase.

You have been taught that when a current carrying conductor finds itself in a magnetic field a force acts on the conductor.

Have you every though about where that force comes from ie the force on the current carrying conductor is due to ????????? and then used Newton's third law to state that the force on ????????? is due to the current carrying conductor.

In this case the magndur magnets via the magnetic field they produce apply a force on the current carrying conductor and in turn the current carrying conductor exerts a force on the magnadur magnets via the magnetic field they produce.

It is often the case that the force on the magnets is not noticed due the arrangement ie they are heavy and on a bench with relatively large frictional forces acting.

Here is your sort of current balance in action.

  • $\begingroup$ yes but how are we supposed to know about the directions of the forces they both exert on each other? Why cant the wire apply a downward force on the magnet? $\endgroup$ – Megan mars Dec 17 '18 at 11:52
  • $\begingroup$ You have to make the assumption that you are looking down on the apparatus. If you know the current direction (left to right) and the direction of the force on the conductor (out of the page) you can they find the direction of the magnetic field. $\endgroup$ – Farcher Dec 17 '18 at 12:01
  • $\begingroup$ no but how do we know that its out of the page, that's what im confused about. The reading on the balance is increasing so why not into the page $\endgroup$ – Megan mars Dec 17 '18 at 13:06
  • $\begingroup$ The force on the magnet (reading on balance) is into the page so the force on the conductor is out of the page (Newton's third law). $\endgroup$ – Farcher Dec 17 '18 at 16:16

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