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I Determining what side of a magnet is north and south

  1. Apr 18, 2016 #1
    I have an issue in determining what side of a magnet is north and south. The only data I have is that garnered from a lab on induction, and I have taken note of the all the polarities of the circuits.

    The magnet was not labeled, but we noted that one end was "flat" and the other was "jagged", and labeled it accordingly.

    One part of the lab dropped the magnet through a counterclockwise coil. We hooked the coil up to a voltage sensor, setting the left side as positive and the right side as negative. We dropped the magnet through the top, and received a voltage-time graph.

    When the flat end was dropped into the coil from the top, the voltage-time graph peaks positive, then peaks negative.
    When the jagged end was dropped into the coil from the top, the voltage-time graph peaks negative, then peaks positive.

    The question I need answered is, from this data:
    Which end is north and which is south?

    I hesitatingly determined that the flat end was south, but I would like some confirmation on this, as I don't fully understand this.

    I have attached this picture if any part of my inquiry is unclear.

  2. jcsd
  3. Apr 18, 2016 #2
    In our school days a bar magnet would be given to determine which end has north pole and which end is south?
    we would hang the magnet by a thread tied in the middle and leave it to rotate in the earth's magnetic field, after a few minutes it will stay in a north-south position; and the end which was pointing north(geographical) will be marked north and the other end South pole.
    Just to confirm it we will use a small compass neddle and normally it will be staying in north soth postion-
    then we will bring one side of the 'magnet' whose pole is to be determined to the say north end -if there is attraction- then it must be south and if repulsion it must be north pole.
    repulsion being the surest test of polarity.
    in your diagram for the readings you can only say that the poles/ends are opposite polarity and it does not help to confirm the pole-type.
  4. Apr 19, 2016 #3


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    It can take a long time for a large lump of magnetised metal to swing round and 'point' NS. Using a cheap and cheerful plotting or hiking compass will give you the location of the poles on the object in no time at all. Other methods that have been described above are also long winded in comparison - although interesting, of course.
  5. Apr 19, 2016 #4
    Couldn't knowledge of how magnetic fields point at north and south poles not be able to be used to determine polarity? Considering magnetic fields move outwards from the north pole and inwards towards the south pole, couldn't one figure out the magnetic flux, and thus the emf, and thus the current? With the knowledge of which way the current moves (from the emf graph), couldn't we figure out which pole it is?

    I do not have access to this magnet anymore, so I'm trying to figure the poles out from the info I have.
  6. Apr 19, 2016 #5


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    You may, but you need to make sure you know the polarity of your ammeter as well, i.e. you need to know if the ammeter records a "positive" current, which way is the direction of the induced current in a coil. This will then tell you the direction of the induced magnetic field, and via application of Lenz's Law, you can deduce the direction of the original magnetic field.

  7. Apr 19, 2016 #6
    An ammeter was not used, rather we used a digital voltage sensor. I have included the polarities of the voltmeter, and how they were connected to the coil, thus a current going from + to - on the voltmeter would be recorded as a positive peak, and a current going from - to + would be recorded as a negative peak?

    I understand that the principles can be used to figure it out, but I'm not quite sure on how to actually apply and figure it out correctly.
  8. Apr 19, 2016 #7
    You need to figure out the direction of the current in the coil based on the readings of your voltmeter.
    When you see the coil from the top, is the current going clockwise or counter-clockwise.
    You know the current conventionally goes from the positive end to the negative one.
    So follow the positive end and see how the current circle through your coil.
  9. Apr 19, 2016 #8


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    You may need to accept the fact that you may never know the answer to this particular problem because you may not have remembered / recorded everything in exact enough detail. But not to worry. Once you have read all you need to, you will be well placed to know next time, about the Physics and a bit more about doing experiments. Both can be tricky, first time through. :smile:
  10. Apr 19, 2016 #9
    When the flat end is dropped, the voltage peaks positive, then negative. Therefore, for the first peak, that means current is moving from + to -, in a counterclockwise fashion. That means, due to the right-hand rule, the magnetic field from the wire is pointing upwards (towards the sky) on the inside of the coil. Thus, the magnetic field from the magnet must be opposite to that created by the wire, and magnetic field of the magnet should be pointing downwards (towards the ground). The north side of a magnet creates field outward, and thus it would be the one to create a field downwards. Thus, the flat end of the magnet is the north pole.

    Is this logic and application of principles correct?
  11. Apr 19, 2016 #10
    It looks OK. If you know that the top of the coil becomes N pole when the magnet approaches from the top, you know that the approaching coil is N as well (so they repel).
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