Negative/positive magnetic field?

[2h2o]

Homework Statement

"In the data, some of the values are positive and some are negative. What does that mean for the polarity?"

The experiment was to measure the magnetic field, varying angular position, around a standard bar magnet. My field meter did not indicate signed values--that is, no negative values were output. The range of values simply ranged from 4.19T to 0.91T.

Homework Equations

The Attempt at a Solution

I'm having trouble understanding the meaning of a "negative" magnetic field. I didn't think there really was such a thing--just that magnetic fields are directional. That is, the field lines point from the N pole to the S pole. So by this reasoning, I wasn't surprised to not have any negative B values; until this analysis question.

What I'm thinking is that, if a sign indicator did exist in the data, it would indicate the orientation of the field. But then, what sign indicates which direction?

I'm just having trouble with what the concept of a "negative magnetic field" is. Any insights would be helpful. Thanks!

 

[Delphi51]

Everything you say is correct!
What kind of a "magnetic field meter" did you have? I have never seen one. Did it actually measure the field strength and direction? I'm thinking the question might have been made up before such a wonderful meter was invented, and was intended for use with a meter measuring a component of B, perhaps the vertical component, and you were supposed to say positive meant upward and negative downward.

I'm retired now. I remember in the 1970's reading about the discovery of the quantum magnetometer that measured quanta of magnetic flux passing through a superconducting quantum interference loop. Even that device only measured one component of the field, though you could use several of them to get the whole thing. The company I worked for was doing magnetic surveys of potential oil fields on the west coast of Canada.

 

[2h2o]

It is a "Magnetic Field Sensor." (link below) I found a product page for it. It uses the Hall effect to measure the radial and/or axial component(s) of B.

http://www.pasco.com/prodCatalog/CI/CI-6520_magnetic-field-sensor/

I still don't understand why I didn't obtain any signed values, but thanks for the insight as to direction. I think we were measuring the radial component; but I'm not certain of it. Assuming that we were, I still don't understand what direction corresponds to + or -.

For example, the bar magnet lies flat on a table; the N pole is pointed upwards as the observer looks at from directly above. Magnetic fields point from N-->S, so would that mean that + values are closer to the N pole than the S (and the inverse, - values closer to S)?

It seems somewhat arbitrary to me; like there should be a sign convention here that physicists have agreed on; but I'm not aware of.

 

[Delphi51]

One is sensitive to axial
field lines parallel to the length of the probe and the
other radial field lines perpendicular to the probe.

The axial mode certainly detects one component of the field. Radial isn't very clear; maybe 2D, maybe not. The manual suggests you should get negative and positive readings, but doesn't define what they mean. In the axial mode, if you got a posative reading, then reversed the orientation of the device, you should get a negative reading. It is possible you always held it the positive way.

The field from a bar magnet is a 3D pattern, spreading out from the N pole in every direction, then going around and back in the S side.

 

[2h2o]

Ok, thanks for your help.

Sounds like the problem was in the method of data acquisition. Unfortunately the instructions that I had available were not very clear about the technique of how to use the device.

"The field from a bar magnet is a 3D pattern, spreading out from the N pole in every direction, then going around and back in the S side."

It's easy to lose sight of that amongst all of the 2D field diagrams in every textbook... :-)

Cheers!