A charge Q = - 822 nC is uniformly distributed on a ring of 1.9 m radius A charg

Question

A charge Q = - 822 nC is uniformly distributed on a ring of 1.9 m radius A charge q=+537 nC is placed at the center of the ring. Points A and B are located on the axis of the ring, as shown. In Figure, the work in eV (as a whole number) done by an external force that transports an electron from B to A is: In the figure Q =4.7 nC and all other quantities are exact. What is the F Times 10^5 where F is the force on the charge Q? Express the answer with two decimal places. Point charges, Q_1=+40 nC and Q_2=-70 nC, are placed as shown in Figure. An external force transports an electron from point A to point B. What is the work in eV (as a whole number) done by the external force?

Explanation / Answer

1) Work done a to b is WQab = kqQ[(1/ra)-(1/rb)] = (9*10^9*1.6*10^-19*822*10^-9)[1/3 -1/4] =0.986*10^-16 J

Work done a to b is Wqab = kqQ[(1/ra)-(1/rb)] = -(9*10^9*1.6*10^-19*537*10^-9)[1/1.8 -1/3.2] =-1.88*10^-16 J

Work done by an external force= Wnet =WQab + Wqab = -0.894*10^-16 J = 558.75 eV

3) work done = change in potential

potential at point A = kQ1/0.6 - kQ2

=600 - 630 = - 30V

potential at point B = kQ1 - kQ2/0.6

=60-1050 =-990V

VB - VA = -960V

so work done = 960 eV

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