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Homework Help: Strenght of magnetic fields on magnet

  1. Nov 22, 2006 #1
    In lab, we used a bar magnet and a device that can measure magnetic fields. We put the measuring device near the north part of the pole and it measured 9T. We did the same to the south, but it gave us 75T. I was expecting it to give somewhere around -9T, is that what is suppose to happen? We repeated the process, but it still gave us the same values.

    Can someone help me understand what happened? Thx
  2. jcsd
  3. Nov 22, 2006 #2


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    Staff: Mentor

    It should be the same at the two surfaces, but near the surface, the field is so high that something else may be going on. What is the specified range of measurement for your measuring device? How is it constructed? Did you make measurements at, say, 1cm intervals for the 10cm area above each pole? How do those measurements compare? How big is the magnet and its poles physically?
  4. Nov 22, 2006 #3
    Er i dont know much about it. The shape was kinda like a gun where at the tip, it can measure magnetic fields in two ways. It can be by axial and i forgot what the other one was called. When we measured it it was about 4 cm away from the magnet. We did the same distance for both poles but we got some wierd value.
  5. Nov 23, 2006 #4


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    Homework Helper

    It probably was a Hall probe.


    There are usually markings on the probe that enables one to hold it such that the magnetic field is perpendicular to the conducting plate. Some probes have two perpendicular plates. One is held perpendicular to the direction of the bar magnet. This will then measure the axial component of the magnetic field - in the direction of the axis of the bar. The other plate is perpendicular to the first and it measures the the transverse component of the magnetic field. To get the strenght of the field one therefore need to combine these two components.

    If you did not observe the orientation of the probe, that is you rotated it about its own axis as you measured the two different poles, the two readings would be different at the two poles.
    Last edited by a moderator: Apr 22, 2017
  6. Nov 24, 2006 #5


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    Homework Helper

    Point your index finger away from you. Hold the palm of your hand up against it. The finger represents one magnetic field line coming from one of the poles of the bar magnet. The palm of your hand is the conducting plate of the hall probe. The magnetic field line goes throught the plate and loops around and entering the magnet on the other side. If you loop your finger like that you will see that it points in the same direction again, meaning that if you held the hall probe up against the other pole the magnetic field will cross it in the same direction. This will then induce an electric potential in the same direction over the plate in both cases. That is why the sign of the measurement do not change at the two poles.
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