Magentic Field Detector

Hi

I know it's possible to buy EMF field detectors to detect fields created by electric current, but is it possible to buy a detector to assess the strength of non-electrical magnetic fields. The closest I've been able to get with this is by monitoring the change of a digital compass when it comes into contact with a field. I can find no precision instrument to do this. Does anybody know if one is available?

Thanks

Regards,

Pan

Reply to
Pan
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Just wondering, how would you have a non-electrical magnetic field? Any magnetic field would have some electrical properties. Unless I am missing something

Reply to
SQLit

I'll second that. Even the magnetic field from e.g. a bar magnet comes from moving charge.

Pan, are you looking to detect only 'dc' components of magnetic fields, like from our planet, or a piece of loadstone sitting on a table? In fact, if you flesh it out a bit more what you're after, it may help.

j
Reply to
operator jay

Google 'Hall Effect' or 'magnetometer'.

Reply to
Paul Hovnanian P.E.

Here's a whole selection of DC Gaussmeters (no affiliation)

If you want accuracy, they get expensive.

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Beachcomber

Reply to
Beachcomber

I understand that the magnetic and electrical fields are exclusive and different. See Faraday's and Maxwell's laws.

A non-electrical magnetic field would be one associated with a material with a magnetic or paramagnetic property. An electrically induced magnetic field is one created by moving charges (current in a coil)

Reply to
hob

You understand incorrectly- however, at DC or low frequency AC, the electric and magnetic fields can be considered independently. I suggest that you look at Maxwell's equations again (Faraday is imbedded in these). As to non--electrical field - this has nothing to do with the magnetic or paramagneticl properties of the medium.

Reply to
Don Kelly

My understanding of electric and magnetic fields being exclusive and different phenomena is 100% correct.

First, the point of referencing Faraday's and Maxwell's laws relating magnetic field to electric fields was that within those Laws, magnetic fields and electric fields 1) exist exclusively as distinct physical phenomena, and 2) must exist distinctly, exclusively and different phenomena, in the equations for those equations even to exist mathematically, i.e., electric and magnetic fields exist separately, exclusive and different phenomena. One look at the equations, and you see two entities. That means there are two entities, not one of differing form.

To shut down any further discussion of them not being separate and independent fields, look at Gauss' and Coulomb's laws. Electric-field-only laws, and for Gauss, also a magnetic-field- only law. Separate and distinct, exclusive and different.

Yes, one phenomena may create the other - BUT ONLY if there is relative motion. And for one to create the other, that requires -and means - that they are exclusive and different fields, not one phenomena.

a)

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"Electric currents or moving charges create magnetic fields Magnetic fields exert forces on moving electric charges"

separate and distinct - one will create the other if MOVING - NOT MOVING - separate and distinct, exclusive and different.

b) Faraday's Law rests on their being exclusive and distinct fields (and note that there is no frequency limit in the equation.)

Faraday's law (M. Faraday) The line integral of the electric field around a closed curve is proportional to the instantaneous time rate of change of the magnetic flux through a surface bounded by that closed curve; in differential form, curl E = -dB/dt, As Faraday's Law states, there are independent and separate, exclusive and different fields -

Note the words "electric field" and "magnetic flux through a surface" (Phi =BA, i.e., magnetic flux is the product of the average field times the area the MAGNETIC FIELD penetrates)

The two exclusive and different fields must exist for the equation to be valid. And it is valid. c) Maxwell -

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same criterion for the equations to exist - the electric and magnetic fields must be exclusive and different, as indicated in the various equations

FWIW -

Note that what you posted contradicts itself

1)

you are speaking of the [magnetic] field. Your statement is not incorrect - however, your statement admits that the magnetic field exists exclusively and different from any electric field.

(That is because they ARE independent at all frequencies. There is no frequency component in Faraday's and Maxwell's Laws. You may be confusing inductance with magnetic field, or possibly the E, B and H vectors?)

---------- For the experimental proof usually used in basic physics lab:

Is there a magnetic field in a capacitor? A magnet does not move near a charged capacitor. A magnet moves near another magnet. The magnetic fields exist, and are not affected by the charge in the capacitor

Is there an electric field in a stationary magnet? An electric field does not change near a stationary magnet. And electric field changes near a charged ball. The electric fields exist, and are not affected by the magnet.

The two fields exist independently, and are exclusive and different phenomena.

Reply to
hob

I understand that Einstein wrote a paper showing that what we consider to be magnetic phenomena actually arise from relativistic consideration of electric phenomena. All there are are electric fields. Magnetic fields are a handy modeling concept. How would you reconcile this with your 100% different hypothesis?

I understand that all magnetic fields are due to charge in motion. Ferromagnetic, paramagnetic, diamagnetic, and ferrite materials all have magnetic field resultant from moving charge, e.g. electron orbits, electron spin, nucleus spin. Can you provide an example of a magnetic field that arises from other than charge in motion?

j
Reply to
operator jay

My apologies if I didn't make what I wanted clear in my original post. I already have a meter here for detecting EMF fields given off by AC electric, but I had no way to pick up naturally occurring magnetic fields like those given off by standard magnets or audio speakers. So I was wondering if it was possible to buy a detector for picking up these fields as well.

It seems from what I've read here that this type of magnetic field is actually based on DC electrical properties and can be detected by a DC Gaussmeter. So I'll try and get hold of one of those :) Thanks to all of you for your help.

Regards,

Pan

Reply to
Pan

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These are hardly physics notes for physicists or electrical engineers. Have you got a real reference? One that does not have current inducing current (untrue) for example. As for the fields and equations mentioned here- static or quasistatic conditions hold. However, the electric and magnetic fields are actually coupled. Hence at higher frequencies one must treat the two as a pair, not mutually exclusive. As for frequency components- whenever there is a time variation, and a sinusoid is involved, then there is a frequency component. After all this is the basis of the use of frequency domain analysis for steady state sinusoids- it saves work.

Faraday's Law has a d(phi)/dt component - i.e. time varying. For a sinusoid of fixed amplitude, this term is directly proportional to frequency. That is: e=-d(N*phi)/dt for a flux phi =PHI*cos(wt) and fixed number of coil turns N becomes e= w*N*PHI*sin(wt) where PHI is the peak flux and w is the radian frequency =2*pi*f. Note the magnitude is proportional to w. Definitely a frequency component. Works in the curl E form as well. Faraday's law doesn't say that they are independent and separate. Look at it: curl E =-dB/dt implies that the E and B are coupled-not independent. Since the only measures of the electric and magnetic fields at a point in space are E and B, this does seem to indicate an interdependence in one direction, of the two fields (or regions of electric and magnetic influence). There is also a second term in curl B which depends on dE/dt -providing a reverse interdependence.

Please note that the general situation does include the time dependent terms. If you had read what I said, you would have realised that I indicated that for DC or low frequency (eg slow rate of change with time) the static equations are quite valid (theoretically but still an approximation at any frequency above 0) and the magnetic and electric fields can be considered as independent with what might be considered insignificant error- Hence Faraday and Ampere's Laws as well as standard circuit analysis are valid. Your "proofs" are based on the static conditions - which is where we agree. In that case the fields are independent. This is what you are trying to tell me but you are extending this to all frequencies. That is where I balk.

As for E and B, etc. To me the E vector and the B vectors are the measure of the electric field at a given point. This is all you can "measure" to show that there is a field. A field is simply a region of influence which is a pretty amorphous definition.

Sorry, I have long forgotten most of my EM theory- last used to pass a grad course and since then only used the static case as you have used it for more mundane power and machines work. --

Don Kelly @shawcross.ca remove the X to answer

Reply to
Don Kelly

I have not seen that paper - however, he had written several papers trying to marry the several forces into one unifying theory, and some papers were incorrect and discarded.

Your argument is circular - You could have just as well said that electric fields are a handy modeling concept, used to describe magnetic fields. Both magnetic and electical fields are artificial concepts use to describe observed phenomena. Think of force - force does not exist except as a concept used in mathematics to predict behavior. You cannot see it, hold it, or observe it directly.

Besides, electric and magnetic fields each have distinct different characteristics

How would you reconcile this with your 100%

Your logic is improper by implying a fact you stated is proven; thus your conclusion is in error - I do not accept one of your "facts", i.e., "Magnetic fields are a handy modeling concept. " Thus I do not accept your implied conclusion that my statement needs reconciling.

Once again, you have used the same faulty logic. No, all magnetic fields are NOT due to charge in motion. Your second statement is correct - the magnetic fields of most naturally occuring materials are derived from moving electrical charges.

Can you provide an example of a moving electical field that does not create a magnetic field? Coherent spin manipulation in strained semiconductors?

All magnetic fields listed in the electromagnetic equations do not require an electric field. They have to be separate and distinct entities to exist.

1) You can check most basic physics texts to verify the difference between the two kinds of fields: The difference between a magnetic field and an electrical field is that the magnetic field is a dipole field (That means that every magnet must have two poles.) and the electrical field is a monopole field (an electrical field of either a positive (+) or negative (-) charge can stand alone. 2) And they are defined as distinct in the fundamental laws of electromagnetics. That alone is proof enough that they are distinct.
Reply to
hob

oh, forgot something

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Reply to
hob

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Current does not induce current? Charge creates electrical field, charge moves, by definition creating current. Field associated with the charge moves. Moving field cuts nearby wire. Charge induced in wire by moving electrical (not magnetic in this example) field. Varing strength of field as r^2 and/or geometry creates charge flow in nearby wire. Basic experiment at the University for second semester physics students.

This assumption is incorrect, thus the conclusions based on it are incorrect. They MAY be coupled. One is a monopole, and the other is dipole. Either may stand alone without the other (moreso electrical, the other especially in quantum physics)

But that is a special case of sinusoid, which is the special case of time variant, which is the special case of magnetic and elctric fields interacting. It is not the general condition.

yes, in the sense of time as a linear element of measure and specific infinitesimal time location. The differential dphi/dt means: phi measured exactly at any infinitesimally small unit of time - rather than measured at the average delta phi/delta t The use of the dt does not mean varying any more than distance D over time T means varying speed.

E.g. if you are driving along at 60 mph without your velocity varying, dx/dt =60mph. if you are steadily accelerating from 0 to 60 mph in two minutes, at 1 minute dx/dt = 30 mph, while up til then, delta x/delta t =15 mph.

For a sinusoid

There is no such thing as a sinusoid of fixed amplitude - the amplitude of a sinusoidal wave varies sinusoidally with linear time.

In your example, the magnitude at any "t" is proportional is to "Sin wt", not "w". Your "w" describes the length of period of the sinusoid, not the magnitude.. i.e., for the eqaution

E= d*N*dphi/dt.

The equation for "phi in t" which is differentitated with respect to t can be: a constant such as 1 (giving zero Electrical field), t (giving a steady field), t^2 (giving a field rising with time t), R cos (wt) -(giving a field that varies sinusoidally from R to -R of the period wt.)

(BTW , isn't the minus actually Lenz's law rather than Farday's, which has no minus?)

That equation defintitely and unequivocably says they are independent. (That is not the same as saying they may interact or often interact. They usually do interact. But they are independent.) If they were "coupled", which I take you to mean one is created by and depends on the other, it must read

curl E = -d F(B,E)/dt rather than curl E = -d B/dt

(I think the field vectors were E and H - B is magnetic flux density )

I did see where you said that the general laws of electromagentism are approximations at lower frequencies, and their error increases as frequency increases. Which is wrong. That frequency realtionship is a phenomena associated with inductance.

Those laws are valid everywhere in the known universe - at any frequency and at any magnitude.

I still feel you are confusing inductance with magnetic fields. A magnetic field's existance is not dependent on frequency. Inductance is. All that you have posted regarding frequency and magnetic fields looks true for inductance, but it is patently incorrect for magnetic fields.

I would hold that the definition of a field is quite specific, even though general in nature.

And remember that electrical fields are monopole, and magnetic fields are dipole

Actually, I had to do some teaching in the more esoteric aspects of fields in the past year. If only I had the command of the subject back when I took fields that I had last semester. :-)

Is it that we get smarter, or that we just accept such things easier?

Reply to
hob

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Moving charge- i.e current- produces a magnetic field which if varying with time, produces an electric (potential) field as per Faraday. If the varying magnetic field cuts a wire, it doesn't cause a charge "flow" or current in the wire, but merely a non-zero electric field in the wire- and this doesn't necessitate a current or charge flow. To be pedantic, a voltage is induced- not a current - and this voltage will exist whether or not a current (or even the wire) exists. It is common but incorrect usage to say that a current is induced. Your second semester physics experiment detected the induced voltage by measuring a current produced by the voltage. Fair enough.

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---------- The relationships in Maxwell's equations are not dependent on dipoles or monopoles. curl E =-di(B)/di(t) (di to represent partial derivative) curl B= mu*J = permeability{conductivity*E +permittivity* di(E)/di(t)}=mu*(Jc +Jd) These imply coupling.

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With the exception of not using partial derivatives, we have been using dx/dt or whatever, correctly. ? Faraday's law is applicable at an instant. What is your point here?

Reply to
Don Kelly

look around for a Magnetometer... i believe the navy uses them to locate submarines and mines.

i have a small one that indicates the magnetic charge of a tape head

Reply to
TimPerry

I found something as I finished cross-checking defunct terms in my already completed closing post (some info other than the subject of this day's post) so I top posted the following FYI -

"Units of the electric field = [Newtons / Coulomb]"

"Units of the magnetic field = [Tesla] = [Newton second / Coulomb / meter] "

Different units.

I also noted that units of magnetic field _strength_ are amperes/meter.

(BTW - I love it how SI creates a strength unit without a unit of force, and using their chosen base unit of amperes, gives us the coulomb as ampere-seconds. Only in SI do we have strength without any unit of force and charge measured in seconds of charge flow. Worse, we told them it would happen.)

snipping, because the thread was interesting and it points out how we often see things differently.. best to end it amicably.... one last "non-field" observation below

I just noticed the word "magnetic" was misspelled in the header - rustier, indeed!

If you also had your "schoolin" in that era, you would

1) be amazed at how simple some of those old proofs have now become - a lot of the old proofs now use vector proofs from new theorems/new approaches in mathematics, and have abandoned the old long-and-tedious non-vector-mathematics proofs. 2) note that many of the magnetic units you had to memorize have been dropped in the SI consolidations of the nineties -

I think oerstads, maxwells, and gauss are no more in the world of commerce.

However, I still separate SI-used-for-commerce from metric-real-units-for-scientists-and-engineers - and thermodynamics is still easier to do in my head using British rather than SI/metric units.

You, also. Merry Christmas. Good talking with you.

Reply to
hob

---------- Yes- the two fields are, as you say, different. 'Tis obvious in the static case. I was not really contesting this. What I was trying to say is that there are some linkages- i.e. an electric field which produces a current which produces a flux producing an electric field and everything in quadrature.

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---------------- I had problems with the why of the change from coulombs to amperes as the base unit. Possibly it is that the Ampere seems to be a more "tractable?" term. As for the strength in ampere (turns)/meter - I used that for years. Could the problem be in both terminology and in the concept- after all, no one ever measures H directly and its use could be avoided but it is convenient to seperate the strength from the material - particularly when working with devices which have non-linear magnetic properties. A B-H curve is a lot easier to work with than any use of permeability in such cases.

------- That is true but, as I didn't need to use full blown EM theory in dealing with power systems (even lightning surges) or machines, I simply didn't keep up with this. As to proofs- I actually do not recall any non-vector proofs or usage and I have long lost my old copy of Jackson. Ardley uses vector forms as well as matrix expressions and tensors in some cases.

I dropped oerstads, abamps, etc, very shortly after being introduced to them. Now when I see "Gauss" I have to mentally convert it to SI. It was either the late 50's or early 60s that I was converted to the metric/SI system. However, part of the trend in SI was/is to honour the dead by renaming units -ie. Tesla instead of Weber/sq.meter or mho to Siemen when the original units were actually more descriptive.

I find that the metric/SI units are easier than the English units in dealing with mechanics or electromechanics.

I apologise for one statement that I made which implied exactly the opposite of what I intended. Certainly I accept Maxwell's equations at all frequencies. It is circuit theory which is an approximation to full blown EM theory. This is good but has limitations and is what has been called a "quasi-static approximation".

Reply to
Don Kelly

Thanks for the suggestion. I'll look out for one of those too :) Unfortunately, from what I can tell, both items do seem pretty expensive :)

Regards,

Pan

Reply to
Pan

also look for something called pipe locaters. the gas company used them to locate pipes

Reply to
TimPerry

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