This will give you the Form representation of the entity:
Entity["PhysicalConstant", "ElectricConstant"]
This will give you the Symbol associated with it:
Entity["PhysicalConstant", "ElectricConstant"]["Symbol"]
You can do this:
1/(4 Pi Sqrt[x^2 + y^2 + z^2] Entity["PhysicalConstant", "ElectricConstant"]["Symbol"])
or, you can just use the symbols as input--check out "Basic Math Assistant" under the Palettes menu.
