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11 March 2010

Mail Time: B field of a straight wire, and waves before magnetism?

It was good to hear from Fed Duay, a two-year veteran of my AP Summer Institute at Manhattan College*

* Which, for the uninitiated, is in the Bronx.  No, I don't get it, either.

Fed said:


I have two questions. We are doing the "B field of a straight wire lab", where we can use a compass aligned to the earth's magnetic field and trigonometry to find B's value; then we graph "B vs. 1/r" and use the slope to find the "vacuum permeability" value [or should we find the current as compare to the ammeter reading?]. However I have come across two values for the earth's field: 2x10^-5 T and 5x10^-5 T (or 20 microT and 50 microT respectively). What do you use for the earth's field value?

I actually do the experiment the other way -- I use the ammeter reading and mu naught to find the magnetic field.  I like either of the two ways you suggested.  That's one of the beautiful aspects of the graphical approach to laboratory... Depending on what you measure or what you look up, a single experiment can be done in a wide variety of ways.

As for the value of Bearth:  You're only finding the HORIZONTAL COMPONENT of the earth's magnetic field.  Along the east coast, the magnetic field points more down than north, at a "dip angle" that can be close to 70 degrees off of horiontal. 

The site will tell you the local magnetic field, including all components. I find at Woodberry Forest the northward magnetic field component is 2.0 x 10^-5 T.

(Fed continues with his second question:)

Also, I see that last year you covered part of waves before finishing magnetism; I am guessing that you needed to do the "standing wave lab on a string" before finishing EM. Is this the reason or is there something else I should be aware of?

Nothing other than personal preference is in play here.  The intricacies of electromagnetic waves aren't included on the AP physics B exam.  Certainly students are expected to know the EM spectrum, the visible wavelengths, which colors of light have higher frequencies, and so on; but the fact that electric and magnetic fields oscillate in accordance with Maxwell's equations is irrelevant at the physics B level.  Standing waves are in no way a prerequisite to magnetism.

I tried sticking in the wave section before magnetism because that breaks up the toughest parts of the AP course. Electricity and magnetism kick my students' butts, especially coming in the dead of winter when they're busy and in bad moods, anyway. I put waves in between, because waves have some cool demonstrations, are easily vizualizable, and are (comparatively) easy .

(You got a question you want answered in Mail Time?  Either post a comment, or email me at  Those who include an astute and witty criticism of the Cincinnati Bengals or Reds impending disastrous seasons are most likely to see their questions answered.)


  1. Why are magnetic field lines coming out of a magnetic south pole?

  2. You're right that magnetic field lines come out of a north pole and into a south pole. Thing is, earth's "North" pole is actually a magnetic south pole, and vice versa.

    Check out your compass. It "points" north... but if you put the compass in between a horseshoe magnet's poles, it will point to the south pole, as you'd expect.


  3. So geographic North is actually magnetic South. The picture is in error