I know that if any insulator or dielectric is placed between the electric charges then coulomb force decreases by factor known as dielectric constant.So, what will the effect when any conductor is placed?
[Physics] What will be the effect on Coulomb force if any conductor is placed between the electric charges
conductorscoulombs-lawelectrostatics
Related Solutions
The Coulomb force in a medium with relative dielectric constant $\epsilon_r$ is given by your first equation. Only from this follows the electric field strength of a spherical symmetric free charge $Q$ in the dielectric with $$E=\frac{Q}{4\pi\epsilon_0\epsilon_r r^2} \tag{1}$$ which, with the electric displacement $D=\epsilon_r \epsilon_0 E$, results in the correct Gauss Law $$ \int_{sphere} \epsilon_r \epsilon_0 E da=Q \tag{2}$$ This is equivalent to the differential form of Gauss's Law, the Maxwell equation in a dielectric $$ div (\epsilon_r \epsilon_0 \vec E)=\rho$$ where $\rho$ is the free charge density.
Note added after a comment by Zhouran He: In Coulomb's Law for the electric force $F$ exerted by a free charge $q_1$ on a second (test) charge $q_2$ in a dielectric with relative permittivity $\epsilon_r$, only the charge $q_1$ as the source of the force field can be considered to be reduced by the polarization charges of the dielectric to the $q_1/\epsilon_r$ so that the vacuum Coulomb law can be used with this net charge. Even though the charge $q_2$ is also surrounded by polarization charges, the force $F$ exerted by the net charge $q_1/\epsilon_r$ works on the free charge $q_2$. One can alternatively consider $q_2/\epsilon_r$ to be the net charge exerting the force $F$ on the free (test) charge $q_1$. The free charge $q_2$ sees a net charge $q_1/\epsilon_r$ exerting a force $F$ on it according to Coulombs vacuum law. The polarization charges induced by itself around it don't exert a force on itself. The same reasoning applies with interchanged roles of the charges. Thus the second form of Coulombs Law for a dielectric is correct.
I wouldn't say that the force law has changed. In every medium, the force as a function of electric field is the same:
$$\vec{F}=q\vec{E}$$
The difference is that, in a dielectric medium, the electric field looks different than in a vacuum. This is because the dipoles in the medium are influenced by the electric field, and orient themselves in the configuration of minimum energy (namely, so that their positive end is pointing in the opposite direction of the electric field). The dipoles themselves generate an electric field, and in this configuration, their contribution partially cancels the external electric field. So the net effect of a dielectric medium is that electric fields are smaller within it than in a vacuum.
Best Answer
When a conductor is placed there would kind of same phenomenon. The electrons will develop on the side of positive plate. But here unlike dielectric there will be complete accumulation of charges on the surface. Hence there will be complete cancellation of Electric field inside the conductor. Hence the force will decrease more.