As we know that potential energy done of particle a point is defined as the work done by external source to carry particle from infinity to that point. Now suppose the non-polar particle is in liquid. They acquire some potential energy. It means that I would have to do work to put the non-polar particle from infinity to that location where particle is. Why do I have to do work to put the particle. Which forces do I have to overcome to put the non-polar particle at that position?
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With a GIVEN electric field, the potential energy is "minus the work done by the field". In the example of only 2 charges in the Universe, with one of them fixed in space, the potential energy of the second one is the work done by the field created by the former.
Of course there are more examples, but the work depends on the field there is. You have to "overcome" that force: the electric force that is mean to be. To place the first charge in the universe is costless; placin a second one requires overcoming the force of the first one, and so on. That's why you want to calculate the fields.
For your example, you'd have to calculate the field from infinity to the liquid (physically, infinity can be some meters, for a point charge). The field in the liquid is what tells the work done. If the liquid is neutral and you have no field, there will not be work done, and so neither will be electostatic potential energy.
