## shivam_bhalla 3 years ago A magnet of pole strength m and length l is broken into two pieces. The pole strength of each piece is _________

1. shivam_bhalla

@experimentX

2. shivam_bhalla

Options a) m b)m/2 c) 2m d)m/4

3. experimentX

I don't know the answer to be exact. let's try to think it in terms of current.|dw:1336268902939:dw|

4. shivam_bhalla

LOL. I have the answer with me. It is m/2 . But I don't know why. Is there any formula for magnetic pole strength. Because I don't seem to find one. Moreover I have only 20 minutes to leave for my exam centre and this is one of the sample questions

5. shivam_bhalla

Any logic you would apply. I found only One formula which is m = pl m-->magnetic moment p-->pole strength l--->separation of charges. According to this I should get 2m but don't know why answer is given as m/2

6. experimentX

I was guessing the same. If we remove the two loops above ... the field would reduce by 2

7. shivam_bhalla

How about the above formula Because m = NIA A--->area of cross-section I--->current Can you clarify how N would be affected on cutting the loop into 2

8. experimentX

seriously i dont know the formula ... to be honest i don't know anything about magnetism. My prediction is solely on addition of vector fields. Perhaps N = N/2 .... since current in the loop will not change and A is also same.

9. shivam_bhalla

Ok. I found it. http://www.whatitequals.com/content/magnetic-moment-solenoid-magnetic-moment-loop N = Number of turns It would become N/2. Now applying this in the first formula I would get m/2 = p (l/2) So p_1 = p_2 ?? p-->pole strength Am I going wrong anywhere??

10. experimentX

new m = N/2 IA = old m/2

11. shivam_bhalla

@experimentX , Can you make it little more clear what you are trying to convey ?

12. experimentX

since the no of turns are halved, the magnetic moment will also be halvened. and we have pl = m => which makes pole strength half it's original value.

13. shivam_bhalla

@experimentX , I found this too http://www.pmtprep.com/posts/list/magnetism-a-magnet-of-pole-strength-m-and-magnetic-moment-m-1007572.htm;jsessionid=8B8406110F8322A5B9694123C241B5AE.node1#1234418 which says magnetic pole strength remains constant irrespective of separation between the poles

14. shivam_bhalla

@experimentX L also becomes l/2

15. Romero

then the strength is half.

16. experimentX

http://en.wikipedia.org/wiki/Magnetic_moment#Magnetic_moment_of_a_solenoid consider solenoid instead of magnet.

17. shivam_bhalla

@experimentX , I considered the same bro

18. Romero

permanent magnet right?

19. shivam_bhalla

yes

20. Romero

maybe your formula is wrong. Check your book. But I remember that the strength was relative to the length.

21. Romero

And you can physically show that. When you break a big magnet the total B has to add up the same for both magnets.

22. Romero

So if it was cut in half then both have an equal B that add up to the B of the original magnet

23. shivam_bhalla

Which one @Romero . my source for m =NIA p=mL is wikipedia http://en.wikipedia.org/wiki/Magnetic_moment#Magnetic_moment_of_a_solenoid http://en.wikipedia.org/wiki/Magnetic_moment#Magnetic_pole_definition Ok. Thanks @experimentX and @Romero . The answer is http://www.pmtprep.com/posts/list/magnetism-a-magnet-of-pole-strength-m-and-magnetic-moment-m-1007572.htm which says magnetic pole strength doesnot change with change in length. It is only dependent on the area of the crossection of magnet. :) Looks like the sample paper answer key is wrong. Thanks for taking your time

24. Romero

But when you cut them in half you actually have more surface than before cutting it.

25. shivam_bhalla

@Romero ,In the formula it is area of crossection. See the diagram |dw:1336270957783:dw|

26. experimentX

yea ... it seems plausible. LOL, there was L (length) ... i was thinking it current. lol

27. Romero

But the answer given in the website refers to just surface area in general. You're right just pointing out that you take note on how you write things and word it correctly.

28. shivam_bhalla

@Romero , yes they mentioned "surface area of poles" which should imply the "crossectional area" and their answer seems to match with that implication. Anyways. Thanks for all your help @Romero . I got to leave for my exam now :D

29. experimentX

best of luck!!!

30. shivam_bhalla

Thanks @experimentX