Question

Two particles with charges +2q and +q are separated by a distance r. The +2q particle has an electric field E at distance r and exerts a force F on the +q particle. What is the electric field of the +q particle at the same distance and what force does it exert on the +2q particle?

I think it should be E/2, F/2 but that's wrong. What should it be?

Answer 1

Newton's third Law !!!!
force of A on B = force of B on A

but anyway
distance R from 2Q
E = 2 k Q/R^2
F = E Q = 2 k Q^2/R^2

distance R from Q
E = k Q /R^2 half so agree
F = k 2Q Q /R^2 but the same force because half the field but twice the charge