What is the condition required to choose potential at infinity to be zero?

What is exarch condition?

What is the condition of resonance?

Two charges 5 xx 10^-8 C and -3 xx 10^-8 C are located 16 cm apart. At what point(s) on the line joining the two charges is the electric potential zero? Take the potential at infinity to be zero.

Two charges 6xx10^-6C and -4xx10^-6C are located 10 cm apart. At what points on the line joining the two charges is the electric potential zero? Take the potential at infinity to be zero.

Two charges 3xx10^-8C and -2xx10^-8C are located 15 cm apart. At what point on the line joining the two charges is the electrical potential zero? Take the potential at infinity to be zero.

What was the condition on the earth then?