# What's wrong with this diagram of magnetic field lines?

1. Jan 7, 2014

### magnetics

I was looking through a book on magnetism and it had the attached diagrams of field lines and stated that the classical diagram of field lines exiting the N pole and entering the S pole is incorrect. They claim their diagram is the correct representation of the magnetic field lines.

Their rationale is that if you take a long bar magnet, there is a strong magnetic attraction at the ends but very little in the middle.

But isn't this observation due to field gradients? That is, the field lines curve at the ends of a bar magnet and the field is strongly inhomogeneous, while at the middle of the bar magnet the field lines are almost parallel and the field is almost homogeneous.

Thank you.

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2. Jan 7, 2014

### sophiecentaur

The book is wrong. Read three others and take a majority decision!

3. Jan 7, 2014

### magnetics

Well thanks for that sophiecentaur.
I know the book is wrong, what I was hoping for is a clear explanation as to why it's wrong :)

4. Jan 7, 2014

### ZapperZ

Staff Emeritus
What am I missing here?

I looked at the attachment, and it had the phrase "Mistaken Concepts of Magnetism" in the title of the figure. If you took this from the book, what is wrong here? The picture IS showing the mistaken concepts of the magnetic field of the earth and a bar magnet. So the book is correct that these are mistakes!

Zz.

5. Jan 7, 2014

### sophiecentaur

It's wrong because the fields are not like that. It's a bit of a waste of time trying to find what's wrong about something unless you are trying to put someone right. Neither of us can tell the author how he's wrong so just ignore it. Presumably you can get a 'clear explanation' of the right description (any decent text book). Maxwell's Equations do not describe it like that - see what they, correctly, predict. (This link makes a start at it). If, like me, you would rather not go in that deep, just take it as correct.
The flaw is in how he's drawn the lines between the two intermediate poles. Why don't the lines go from the inner N to the inner S? If they do what the diagram says at that point, why don't they do it along the whole length of the magnet - wherever you choose to introduce a gap?

6. Jan 7, 2014

### ZapperZ

Staff Emeritus
But the author IS trying to show that the fields are not like that! Isn't that what "Mistaken Concepts of Magnetism" accompanying the figure would imply?

Zz.

7. Jan 7, 2014

### sophiecentaur

You are absolutely right. But diagrams like that in books should always have a bold cross over them to avoid anyone taking them seriously. Let's face it, he may just as well have drawn a picture with pink elephants pulling at the lines with their trunks.

8. Jan 7, 2014

### ZapperZ

Staff Emeritus
But that's missing the point here, isn't it? We're talking about whether the book is correct or not. The issue of whether it is clear or not is a different matter. And obviously, to me, it was VERY clear since I saw the title of the figure right off the bat.

It makes this whole thread rather moot, because there's nothing wrong with this book. The magnetic field lines ARE wrong, and the author of the book clearly stated that these are wrong as well. In other words, the book hasn't stated something wrong.

What is wrong is the starting premise of this thread.

Zz.

9. Jan 7, 2014

### magnetics

Sorry, maybe I should have totally ridiculed the authors from the start so as to not give the wrong impression, but I try to always see things from the other person's perspective.

On the previous page of the book they show the Earth's and a bar magnet's field lines in the classical way which they then describe as incorrect - this is what they say is mistaken. The picture I attached is the authors idea of a correction and I explained their rationale as described in the book.

Admittedly the book was first published in 1974, but where did they go wrong?

10. Jan 7, 2014

### Crazymechanic

what do yu mean they show the classical way and then go on to say that's incorrect , are you saying that the authors first make a picture of how things should be and say it's wrong and then show how things shouldn't be and call that also wrong , then what is right ?

I think it would be great if you could show the previous page you were talking about , there have been many physics books over the years .

Why would the authors show a picture of field lines they think should be correct and then go on to put a title that goes like " Mistaken concepts" ? Sounds very strange.

11. Jan 7, 2014

### Drakkith

Staff Emeritus
Where did they go wrong? At the very beginning. You can easily see the field lines of a bar magnet by using iron filings.

12. Jan 7, 2014

### magnetics

Another question could be, why is there very little magnetic attraction in the middle of a bar magnet? This is what I was trying to draw out.

13. Jan 7, 2014

### ZapperZ

Staff Emeritus
Sorry, but you're making very little sense here.

After read what you've written, I'm leaning towards giving the benefit of a doubt in the book's favor.

Zz.

14. Jan 7, 2014

### Crazymechanic

Maybe just write down here the title of the book and the authors and publishing year.And I'm sure Zapper or others will tell either the book is wrong or your take on the book is wrong.

the weakest part of the field so to say is in the middle because fields of same strength but opposite poles comes together the same distance and cancel out , break the magnet in half and you get two new magnets each with a N and a S.and now the middle point is again between those two.

15. Jan 7, 2014

### magnetics

Here it is in black and white, see attached.
Yes, they go on and describe what subscribers to the Physics Forums would call the impossible and the implausible!

#### Attached Files:

• ###### DavisNS22.jpg
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16. Jan 7, 2014

### Drakkith

Staff Emeritus
What book is this?

17. Jan 7, 2014

### Crazymechanic

ok give us the name of the book and the authors, so you say that the first picture you attached in the Original Post is the one that the authors say is the right one?

18. Jan 7, 2014

### ZapperZ

Staff Emeritus
They simply can't get any more obvious than highlighting it in the TITLE of the chapter!

I still do not see what is wrong with this book. What you had described and shown so far seemed to reveal more of your misunderstanding of the authors were trying to do and show.

Zz.

19. Jan 7, 2014

### sophiecentaur

I think the clue is in the typeface used. It says 'pulp'.

20. Jan 7, 2014

### magnetics

It's called "Magnetism and its effects on the living system". By Davis & Rawls. First published in 1974 by Acres USA.

It's not a text book on physics and I think the disbelief that someone could have even published such nonsense has hijacked the original premise of my question.