# "Strength" of magnets: is there really such a thing?

1. Sep 14, 2014

### JeffEvarts

Ever get a simple question that leads you down a rabbit hole?

A friend of mine asked me "how much stronger are rare earth magnets than regular iron magnets", and I was given pause for thought, since I realized that I didn't really know much about how the relative "strength" of magnets are measured. I can now assure you that googling for "magnet" or "rare earth magnet" is the path to madness and pain. Wikipedia's content is useful, but reading the entry on Oersted requires more physics than I remember.

The above search for data did seem to indicate that different materials do make stronger magnets. I was not suprised.

But various comparatives of magnetic strength have apparently unrelated data: A "small" magnet, a "refrigerator" magnet, etc.

I came away wondering "what DO we mean when we talk about how strong a magnet is", and I came up with two organic ways of thinking about it:

• Suppose you had two cubes that were magnetic, and they were joined at their face. The amount of force required to separate them (probably divided by the area they were joined at) would indicate how strong the magnets were
• Suppose I had a long cylindrical bar magnet. I take a loop of wire, move it 1 meter along the magnet, and measure how many coulombs of current I drove through the circuit, modified by the resistance of the wire.

Convinced now that there is such a thing as "how strong is a magnet", that materials can make a specific difference, and that this is probably not related to size or configuration, I proceed with my question:

Presuming maximum "magnetization" of the material and identical physical configuration, what is the relative magnetic strength of:
1. A lodestone (Fe3O4)
2. An iron magnet (Fe)
3. A Nd2Fe14B magnet

Anyone have real (or computable) data?

-Jeff

P.S.
This seems like kind of a noob question, but some cursory searches didn't give me any clear answers.

2. Sep 14, 2014

### enorbet

IIRC Oersted is specific to motors and generators being defined as a dyne per unit pole. AFAIK, raw magnetic strength is measured in Gauss, one Maxwell/cm^2. Gauss takes into consideration area and therefore physical shape and therefore any focusing because of it, as opposed to dissipation.

Even if I am off a bit you may find this helpful
http://en.wikipedia.org/wiki/Orders_of_magnitude_(magnetic_field)

3. Sep 14, 2014

### elegysix

A few days ago I was looking at some magnet retailer's website. Apparently they quantify 'strength' by pull force. And they defined pull force as the force required to separate the magnet from a flat sheet of steel (or something similar). They have different types on there, and may be able to give some insight. Check out their specs page, which shows different grades and strengths, etc.

Last edited: Sep 14, 2014
4. Sep 14, 2014

### JeffEvarts

Apparently enorbet and elegysix conidered my post tl;dr. Just in case other did too, let me post JUST the question I asked:

What is the relative magnetic strength of:

A lodestone (Fe3O4)
An iron magnet (Fe)
A Nd2Fe14B magnet

Anyone have real (or computable) data?

5. Sep 14, 2014

### Vanadium 50

Staff Emeritus
The problem is that you usually need two numbers to specify the strength - the field strength and the distance at which it is measured. The shape of the magnet is critical.

6. Sep 14, 2014

### elegysix

IMO, because there are varying 'grades' of magnets of each material, each with different strengths, the question has no clear answer. Maybe you are interested in the magnetic saturation point, or maximum magnetic flux density for each?

Last edited: Sep 15, 2014
7. Sep 14, 2014

### enorbet

Perhaps OP considered it too long to read the linked article and especially it's table. While it doesn't mention lodestone it does give relative strengths for Iron and Neodymium-Iron-Boron.

8. Sep 15, 2014

### JeffEvarts

The K&J magnetics page contains only Nd. The Wikipedia page mentions Nd and "refrigerator magnets", which I assume are ceramic ferrite.

Anyone actually know? What is the relative magnetic strength of:

A lodestone (Fe3O4)
An iron magnet (Fe)
A Nd2Fe14B magnet

-Jeff

Thanks to those that have replied! I appreciate I'm not asking a simple question.

9. Sep 15, 2014

### davenn

relative to what ??

did you not read Vanadium50's response ?
you haven't specified the factors that determine the field strength

and I will add size/ mass of any of those magnetic materials
without the appropriate info you question is unanswerable

Dave

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