Q: If we model the electron as a uniform sphere of radius r, spinning uniformly about an axis passing through its centre with angular momentum L= h/(4*pi), and demand that the velocity of rotation at the equator cannot exceed the velocity c of light in vacuum, then the minimum value of r is--
(a) 19.2 fm (b) 0.192 fm (c) 4.8 fm (d) 1960 fm (e) 480 fm

Question

Q: If we model the electron as a uniform sphere of radius r, spinning uniformly about an axis passing through its centre with angular momentum L= h/(4*pi), and demand that the velocity of rotation at the equator cannot exceed the velocity c of light in vacuum, then the minimum value of r is--
(a) 19.2 fm     (b) 0.192 fm    (c) 4.8 fm                    (d) 1960 fm   (e) 480 fm

anonymous · Answer

Where are u stuck at?

anonymous · Answer

used the following relation : $$I \omega = h \div 4 pi$$
where $$I=2mr ^{2}\div5$$
 $$v=\omega r$$
and v= c (spped of light )
put the values of h, m, and c.
got minimum radius = $$6.24\times 10^{20} m$$..........but it doesnt match any of the options and the correct answer is supposed to be $$480 fm$$

anonymous · Answer

why would that be the min. radius.. that would be the max radius right?

anonymous · Answer

besides you did some calculation mistake.. i got the exact answer 480fm :P check calculation again... how in the world would you get a 10 raised to 20?? :O :O :O :O :O :O.. what a big electron is that :O :O :O :O :O :O :O

anonymous · Answer

rishavharsh:

Maybe you could just post, what you _actually_ typed in to get the minimal radius? Maybe you made an arithmetic mistake or you got a physical constant wrong or something like that...

anonymous · Answer

@Mashy you were right, i made a calculation mistake. i got the answer using the same procedure. thank you