I have a question which asks how a compass needle would align when placed between 2 current carrying solenoids. Textbook Question

The answer is apparently:

The north pole of the compass would point up the page. The compass would align with the magnetic field (which points upwards), produced by the two solenoids whose inner ends both act as south poles by the right-hand grip rule.

This is conventional current by the way. Even with this explanation it still doesn’t make sense to me. For the left solenoid, the magnetic field lines would point to the left. For the right solenoid, it would point to the right.

The understand that the inner sides of the solenoids are the south poles. What I don’t understand is how the magnetic field is pointing upwards? What field is even pointing upwards?

My guess would be that if the South Pole is above the point ‘z’, then there is somehow a North Pole below the point then, this means that the arrow of the field points upwards and hence the north end of the needle points upwards?

I seriously don’t know.


1 Answer 1


The magnetic field is usually represented by field lines, which are loops going through the solenoid. Magnetic field lines do not cross (or a compass would have to point in 2 directions at once). The field lines come out of the outer ends of both solenoids and loop around (above or below) to come back in past point Z (for those on the lower half of the page) and into the inner ends of the solenoids.

The field lines are directed up the page at Z, so that is the direction a compass at Z would point.

  • $\begingroup$ So the loops of each go all around and at Z they start going upwards. Thanks makes sense now. $\endgroup$ May 15, 2020 at 3:46
  • $\begingroup$ @Peter won't the magnetic field produced by the wire considered? like it should also have an effect na? $\endgroup$ Jan 15 at 12:42
  • $\begingroup$ @DarshitSharma, note that the current in both those wires is up the page, so their magnetic fields at Z (directions given by the right hand grip rule) would cancel. $\endgroup$
    – Peter
    Jan 15 at 13:10
  • $\begingroup$ @Peter Yes so no magnetic field which means that needle can point in any direction, right? $\endgroup$ Jan 15 at 13:20
  • $\begingroup$ @DarshitSharma, not any direction. The direction described in the answer. Another consideration is that there will be many turns in the solenoid so its field will be much stronger that the field from 2 wires at points reasonably close to the solenoids., and this would apply even if the geometry of the wires was changed. $\endgroup$
    – Peter
    Jan 16 at 1:45

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