When the charges are located in some other medium rather than vaccum then the force between the charges will be :
————— 1.4
Where 
is the absolute electric permittivity of the medium.
The force between the charges held at same distance from each other in vaccum is :
————– 1.5
Dividing equation 1.5 by 1.4
—————- 1.6
Where 
is the relative electrical permittivity. It is also called as dielectric constant of the separation medium between charges, and is denoted by k.
As a result of eq 1.6 it is clear that the dielectric constant of the medium is the ratio of the absolute electrical permittivity of vaccum to the absolute electric permittivity of the medium.
In other words, dielectric constant is the ratio of the force of attraction or repulsion between the two similar point charges to the ratio of the force of attraction or repulsion between point charges separated by some distance.
The value of constant k depends on the type of medium that you are using. The values of k for different mediums is given below:
1. For Hydrogen k is 1.00026
2. For Glass k is from 3 to 4
3. For Vaccum k is 1.00000
4. For Mica k is from 5 to 6
5. For Water k is 81
Considering equation 1.6 :-
Using this equation in equation 1.4
So, it is clear that the charges separated using a particular distance, and water as a medium(k=81) is only 1/81 of the total thrust between them when they are present in air.
