## Abhisar one year ago Force between 2 identical short bar magnets whose centres are r metre apart is 4.8 N when their axes are in the same line. if the separation is increased to 2r metre then the force b/w them is?

1. Abhisar

@MrNood

2. Abhisar

@irishboy123 @Rushwr

3. MrNood

I am not entirely sure on this but I THINK htat the magnetic attraction force follow the 'inverse square law' (similar to gravity) so the force is proportional to 1/s^2 where s is the distance if oyu double the distance therefore you 1/4 the force

4. Abhisar

Ye, that's what I thought but it's not the correct answer. Then I thought since magnets does not occur as monopole the variation must be 1/r^3 but the correct answer is 0.3 which means the variation is 1/r^4. i am not able to understand how?

5. MrNood

OK - soz - not able to help further

6. Abhisar

Thanks doe c:

7. Elsa213
8. Abhisar

Ye, i saw that. They have solved it wrong...

9. Elsa213

v.v lemme look again :3

10. imqwerty

but that looks alright

11. Elsa213
12. Abhisar

Haha, nuuu it is wrong doe. The answer should be 0.3 N btw.

13. Abhisar

See, that pdf file also says that it should be 1/r^4 i.e. o.3 N

14. Elsa213

So your making the question already knowing the answer? o.e

15. imqwerty

:) so why do we it inverse square law

16. Abhisar

Irishboy !!!!!!!!!!!!!!!!

17. Abhisar

Save me !! c:

18. IrishBoy123

@Abhisar fraid i am clueless on how this becomes $$x^{-4}$$. i would bet on inverse square especially as these are pole to pole......

19. Abhisar

I think it has to do with the fact that unlike electric poles there are no magnetic monopoles.

20. MrNood

This is from Wikipedia - the formula doesn't reproduce here - but you can see it is 1/r^2 If both poles are small enough to be represented as single points then they can be considered to be point magnetic charges. Classically, the force between two magnetic poles is given by:[2] F={{\mu q_{m1} q_{m2}}\over{4\pi r^2}} where F is force (SI unit: newton)qm1 and qm2 are the magnitudes of magnetic poles (SI unit: ampere-meter)μ is the permeability of the intervening medium (SI unit: tesla meter per ampere, henry per meter or newton per ampere squared)r is the separation (SI unit: meter). The pole description is useful to practicing magneticians who design real-world magnets, but real magnets have a pole distribution more complex than a single north and south. Therefore, implementation of the pole idea is not simple. In some cases, one of the more complex formulas given below will be more useful.

21. MrNood

this link implies that empirically the result is fopund to be 1/r^4, but does not offer any theory... http://www.exo.net/~pauld/activities/magnetism/forcebetweenmagnets.html