A positive point charge is surrounded by an equipotential surface \(A\), which
has a radius of \(r_{A}\). A positive test charge moves from surface \(A\) to
another equipotential surface \(B\), which has a radius of \(r_{B}\). In the
process, the electric
force does negative work.
(a) Does the electric force acting on the test charge have the same or
opposite direction as the displacement of the test charge? (b) Is \(r_{B}\)
greater than
or less than \(r_{A}\) ? Explain your answers.
Problem The positive point charge is \(q=+7.2 \times 10^{-8} \mathrm{C}\), and
the test charge is \(q_{0}=+4.5 \times 10^{-11} \mathrm{C}\). The work done as
the test charge moves from surface \(A\) to surface \(B\) is \(W_{A B}=-8.1 \times
10^{-9} \mathrm{~J}\). The radius of surface \(A\) is \(r_{A}=1.8 \mathrm{~m}\).
Find \(r_{B}\). Check to see that your answer is consistent with your answers to
the Concept Questions.