# Bar magnet.

1. Jun 28, 2013

### BIT1749

Bar magnet.........

why are the magnetic poles of a magnet are located somewhere within the magnet,not at the ends?

Last edited by a moderator: Jun 28, 2013
2. Jun 28, 2013

### Staff: Mentor

I'm sorry, but that doesn't make much sense: the entire magnet is a magnet.

3. Jun 28, 2013

### SteamKing

Staff Emeritus
The poles of all my magnets are at the ends. Why are your magnets different?

4. Jun 28, 2013

### Staff: Mentor

Might be a definition issue.

5. Jul 20, 2013

### BIT1749

at the ends but a little inside....why the ratio of magnetic length to geometrical length of a bar magnet is always near 0.85?????

6. Jul 20, 2013

### BIT1749

i asked about poles which are present only at two points,just inside the ends.....not all over the magnet......

7. Jul 20, 2013

### mrspeedybob

You could make the poles wherever you want. Putting them at the ends is just what manufacturers usually do. If a particular application required them to be at the corners or perhaps some arrangement of multiple poles then that's how they would be made. Take sheet magnets for example, they are usually made with alternating stripes of NSNSNS for the width of the sheet. I'm guessing they do this to create a field that is relatively strong at the surface but diminishes very quickly with distance.

8. Jul 24, 2013

### BIT1749

A BAR MAGNET has only two poles,north & south,just inside the ends .The ratio of magnetic length to geometrical length is always 0.85...why is not different for different BAR MAGNETS?

9. Jul 24, 2013

### CWatters

If you look at the field lines for a bar magnet you can see they start to spread out before leaving the end of the magnet rather than at the very end. It's just an "end effect". If the ends were a different shape it probably wouldn't be 0.85.

If the air was replaced by more iron that would also effect the equivalent length.

10. Jul 24, 2013

### zoobyshoe

I think what you're talking about is the fact the field lines begin to fan out and curl back in space before the end of the physical magnet, like this:

http://www.bbc.co.uk/bitesize/ks3/s...y_forces/magnets_electric_effects/revision/3/

I question whether it's possible to determine an objective "magnetic length" for any permanent magnet, and I suspect whoever came up with that ratio had some specific purpose in mind for which they could stipulate criteria. Regardless, with any magnet, the full intensity does not reach all the way to the ends: some of the lines obviously start to curl back before the ends.