Two sirens at a distance apart are soundings with the same frequency, when a person standing between them runs towards one siren then he hears beats, Why?

A and B are two point charges kept at a certain distasnce apart. If the charge on A is reduced to one-quarter, that on B doubled and at the same time distance between them reduce to one-half of its previous value, how does the force between them change?

Two identical metallic spheres, having unequal opposite charges are placed at a distance of 0.50 m apart in air. After bringing them in ir contact with each other, they are placed at the same distance apart. Now, the force of repulsion between them is 0.108 N. Calculate the final charge on each of them.

Write 'Yes' for correct statements and 'No' for incorrect statements. The Ampere is that current which when flowing in two infinite parallel wires one meter apart produces a force between them of 2 x 10-7 N/m.