# Some problems in electic field

1. Oct 9, 2011

### wowowo2006

hey i got some problems in doing this question
http://i4.photobucket.com/albums/y117/tkm20022001/AL%20PHY/phy03mc_32_33.jpg" [Broken]
the official ans is A
i dunno why the 2nd statement is wrong. i think it's right as it;s at equilibrium state if not the charge will accelerate due to e field presents and may affect the charge distribution which is not at a equilibrium
moreover i dunno why the 1st statement is true, the - charges should be denser at the inner surface near the + charge, why there is still evenly distrubuted + charges on outer surface denser even when the - charges are denser at some place.
i am just studying some basic physics
and my mates explain this to me with shielding effect and gauss surface. i don't know about this . Could someone help me solve this question and give me some simple explanation about shielding effect and gauss surface and how they can help me get the answer.

Last edited by a moderator: May 5, 2017
2. Oct 9, 2011

### Staff: Mentor

Think of the charge within the cavity as being fixed in place by other forces. When they speak of electrostatic equilibrium, they mean all the charges within and on the conducting shell.
The charges on the inner surface arrange themselves to cancel any field from the charge within the cavity--the field within the conducing shell must be zero. That effectively shields the outer surface from any effect of the charge within. Since it's a spherical surface, the induced charge on the outer surface distributes itself uniformly.

3. Oct 9, 2011

### wowowo2006

sorry i don't know what does 'The charges on the inner surface arrange themselves to cancel any field from the charge within the cavity' mean. does it mean that the e field within the cavity and the metal shell , both e field is zero or only with the shell. and why does the 'self arrangement of + charges' is uniform, is it just because the ball is a perfect sphere?. i don't know exactly why the unevenly distributed - charges can in a equilibrium state when the + charges besides them is evenly distributed. Could you give me more explanations plz

4. Oct 9, 2011

### Staff: Mentor

Sorry about that. I meant that the charges on the inner surface of the shell arrange themselves to ensure that the field is zero throughout the material of the shell (and all points beyond), not within the cavity itself.
The combined field of the charge within the cavity and the charge on the inner surface of the shell is zero for all points beyond the inner surface. Thus as far as the outer surface goes, the charges on the inside exert no effect.