magnetic flux and Hall Probes

[ans]

I'm planning an investigation on how magnetic flux density midway between opposite poles of 2 bar magnets varies with separation of the bar magnets (using a Hall Probe). [simply as shown in attachment]

 

To do this, I'll need to calibrate a hall probe (which I think is done by working out the p.d. (Hall pd) at a known magnetic field strength).

 

My initial thoughts were that I could use a long straight wire of known current, and use [math] B \ = \ \frac{\mu_0 I }{2 \pi \ r} [/math]however, this would not work would it? Due to non-uniform magnetic field.

 

Is the best approach to use a "Helmholtz coil" to get a uniform field- and use the formula given here: http://en.wikipedia.org/wiki/Helmholtz_coils ?

 

I've never come across this before, so any help / hints would be appreciated. Thanks.

 

PS - I apologise in advance if this thread is better suited elsewhere.

[ans]

attachment

[swansont]

There's actually two things you need to worry about - the relative accuracy and the bias.

 

Either the long wire or the Helmholtz coils will work. The coils give the advantage of a pretty uniform field over some volume, so knowing exactly where the measurement point is in the probe isn't as critical, but you have to make them. (Here's a hint: the + and - leads will contribute field, too, but if you run them next to each other, the field they add should cancel out and not do anything to spoil your calibration)

 

When your probe is sitting in the open, it will probably not read zero. Is this because of some offset in the equipment and/or is it due to a background field. You can check this with a "differential" measurement: try it with your calibration field in both directions and with different orientations of the probe.

 

If the probe reads +a and -a when you reverse the probe oreientation, you know you're reading a component of a background field (e.g. the earth, but there could be other magnetic items around as well). If it reads +a and +a, you have a bias in the equipment, and you can find the strength of this with a test field in both directions to get to what zero is. You can also add an external field until you get zero on the probe. If you read +a but -b, then you have both a background and a bias.

[Guest needy nerd]

i've been reading about other people who are doing the A2 OCR planning exercise and i've noticed your using helmholtz coils to calibrate the probe, where are you getting the info about this from? my teachers useless and has given my class no help or guideance as to where to begin. if you have any websites or books that you would recommend please let me know. i dont really know where to begin.

 

any help would be much apreciated.

[Johnny5]

I'm planning an investigation on how magnetic flux density midway between opposite poles of 2 bar magnets varies with separation of the bar magnets (using a Hall Probe). [simply as shown in attachment]

 

To do this' date=' I'll need to calibrate a hall probe (which I think is done by working out the p.d. (Hall pd) at a known magnetic field strength).

 

My initial thoughts were that I could use a long straight wire of known current, and use [math'] B \ = \ \frac{\mu_0 I }{2 \pi \ r} [/math]however, this would not work would it? Due to non-uniform magnetic field.

 

Is the best approach to use a "Helmholtz coil" to get a uniform field- and use the formula given here: http://en.wikipedia.org/wiki/Helmholtz_coils ?

 

I've never come across this before, so any help / hints would be appreciated. Thanks.

 

PS - I apologise in advance if this thread is better suited elsewhere.

 

 

I don't recall seeing that formula at wikipedia before. If I were you, I would attempt to derive it.

 

Why are you trying to compute the hall potential difference? Exactly what is the purpose?

 

At any rate, here is the definition of Magnetic flux density:

 

[math] \Phi_m = \oint \mathbf{B} \bullet d\vec a [/math]

 

The direction of the differential area vector with magnitude da, is normal to the surface you are integrating over.

[Guest mayor17]

Is there a formula relating current to force or magnetic flux density? if ther is any, the planning exercise is a piece of cake then.

[Johnny5]

Is there a formula relating current to force or magnetic flux density? if ther is any, the planning exercise is a piece of cake then.

 

I can make one for you. What is the question though? You are not the original guy are you?

 

Give me the "setup," then I can use electrodynamics to work out the B field formula.

 

If the set up is complicated, it will take me awhile to get the right answer for you.

 

But i know how.

 

Regards

[Guest imran2571]

i'm doing the a2 ocr planning as well. ive heard that u can use a helmholtz coil or a standard solenoid (or toroidal). ive found ways to connect the apparatus and i think i have a rough idea of the situation. however, i doubt that any of our college labs have a helmholtz coil. therefore, to my mind, i reckon we need to use the solenoid instead.

[Guest srawl]

Most of the plan is easy just the calibration bit need help on that plz

[Guest bennyginge]

Help! i am really confused, i don't know what a hall probe is let alone how to calibrate one. somebody needs to give me a hand. please!!!

[Johnny5]

Help! i am really confused, i don't know what a hall probe is let alone how to calibrate one. somebody needs to give me a hand. please!!!

 

A hall probe measures magnetic field strength.

[Guest bennyginge]

A hall probe measures magnetic field strength.

ok, so how do i calibrate it using helmholtz coils? anyone fancy emailing me a finished copy?

lol - only kidding but help would be nice