The Experimental and Historical Foundations of Electricity - Unicamp

7.10 Can Two Bodies Electrified with Charges
of the Same Sign Attract One Another?
Experiment 7.23
WerepeatExperiment6.5(Figure6.7). Butnowthenegativestrawismoved
even closer to the strip of the negatively charged electroscope. We observe that
for distances smaller or equal to a certain value, of the order of 2 to 4 cm, the
strip is no longer repelled by the straw, being attracted by it. The strip touches
the straw and remains attached to it.
Experiment 7.24
We repeat Experiments 4.7 and 5.23 (Figures 4.18 and 5.27). But now
the negative straw is moved even closer to the negative disk of the pendulum.
We observe that the inclination of the pendulum from the vertical increases
when the distance between the straw and the pendulum goes from 15 to 5 cm,
approximately. This shows that the intensity of the repulsive force increases
when the distance between them decreases in this range.
However, for distances less than or equal to certain value, of the order of 5
cm, there is no longer a repulsion between the negative straw and the negative
disk. At these small distances they attract one another. The disk touches the
negative straw a second time and is again repelled by it.
After 2 or 3 of these contacts between the negative straw and the disk of the
pendulum, the same phenomena are once more observed, but over somewhat
different distances. When the disk has a greater degree of electrification, the
repulsion between the disk and the negative straw can be observed at a distance
larger than before, of the order of some 20 cm. The intensity of the repulsive
force increases when this distance is decreased between 20 cm and a lower limit
of 2 or 3 cm. When the distance between the straw and the disk has a value
less than or equal to this lower limit, there is again an attraction between them,
and the ACR mechanism comes into play.
These experiments can be understood on the basis of the principles discovered
so far.
Let us suppose that a body �, an insulator, has been charged negatively by
friction. It is close to a neutral body ��, a conductor, which has no net charge.
There will be an attractive force between them. This attractive force is due to
the polarization of body �� in the presence of body �, as in Figure 7.29. We
represent their attractive force by �� > 0 (Figure 7.32 (a)). We now electrify
body �� with a charge of the same sign as the charge of the attracting body
�. This can be done, for instance, by the ACR mechanism. This will generate
a new force between them. It is repulsive and it will be represented here by
�� < 0. In Figure 7.32 (b) we present this new force of repulsion, disregarding
the previous attractive force due to the polarization of the conductor. The new
negative charge on the conductor is represented in the middle of body �� only
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