Aptitude Test Problems in Physics Science for Everyone by S Krotov ( PDFDrive.com )

254 Aptitude Test Problems in Physics

3.12*. Since the sheet is metallic, the charges
must be redistributed over its surface so that the
field in the bulk of the sheet is zero. In the first
cpproximation, we can assume that this distri-
bution is uniform and has density —cr and cr over
the upper and the lower surface respectively of the
sheet. According to the superposition principle,
we obtain the condition for the absence of the field
in the bulk of the sheet:

q a

=-- 0.

4ne 012 eo

Let us now take into consideration the nonuni-

formity of the field produced by the point charge

since it is the single cause of the force F of inter-

action. The upper surface of the sheet must be attract-

ed with a force aSql(4ne 0 / 2 ), while the lower surface

must be repelled with a force crSql[4n8 0 (1-1-d) 2 1.

Consequently, the force of attraction of the

sheet to the charge is

F—

aSq^ q 2 Sd

41(e0/2 [

I

(1+0) 2 8:0 180 / 5 '

3.13. It can easily be seen that the circuit diagram

proposed in the problem is a "regular" tetrahedron

Fig. 209