- Electricity and Magnetism 75
ing each other along the equator if one
hemisphere is uniformly charged with a
surface density al and the other with a sur-
face density a 2.
Fig. 763.7. The minimum strength of a uniform
electric field which can tear a conducting
uncharged thin-walled sphere into two parts
is known to be E 0.
Determine the minimum electric field
strength E 1 required to tear the sphere of
twice as large radius if the thickness of its
walls is the same as in the former case.
3.8. Three small identical neutral metal
balls are at the vertices of an equilateral
triangle. The balls are in turn connected
to an isolated large conducting sphere whosecentre is on the perpendicular erected
from the plane of the triangle and passing
through its centre. As a result, the first and
second balls have acquired charges q 1 and
q 2 respectively.
Determine the charge q 3 of the third ball.
3.9. A metal sphere having a radius r 1
charged to a potential cp, is enveloped by a
thin-walled conducting spherical shell of
radius r 2 (Fig. 77).