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Asked in GSEB Board July 2017 · Potential of point charges
Given: charges Q and 9Q of the same sign, 4 m apart.
Idea: first find the null point, where the two fields are equal and opposite; then add the two potentials there as scalars.
For like charges the null point lies between them. Let it be r from Q: (kQ)/(r²)=(k 9Q)/((4-r)²).
Taking square roots: 4-r=3r, so r=1 m from Q and 3 m from 9Q.
Potential there: V=(kQ)/1+(k 9Q)/3=kQ+3kQ=4kQ.
So V=4kQ V — the field vanishes at that point, but the potential does not.
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