Question

For the figures below, calculate the potentials VA, VB and hence the potential difference VAB

Answer 1

potential due to a charge q at a distance of d is given by

k*q/d

where k=9*10^9

a) potential at A=9*10^9*10^(-6)/(10)=900 volts

potential at B=9*10^9*10^(-6)/(30)=300 volts

hence Vab=Va-Vb=900-300=600 volts

b)

potential at A=[9*10^9*10^(-6)/(10)] + [9*10^9*10^(-6)/(30)]

the first term is due to the charge at 10 m from A and second term is due to charge at 20+10=30 m from A

Va=900+300=1200 volts

potential at B=[9*10^9*10^(-6)/(10)] + [9*10^9*10^(-6)/(30)]

the first term is due to the charge at 10 m from B and second term is due to charge at 20+10=30 m from B

Vb=900+300=1200 volts

hence Vab=1200-1200=0 volts