Question

In electric field how we define high potential and low potential and why the electrons move from the region of lower to higher potential whereas positive charge moves from higher to lower potential

Answer 1

So a high potential is a high voltage; low potential is low voltage. A high potential can injure or kill you, or could cause great damage to property; or it could provide the energy to do a great deal of work, like turning a large motor. A low potential would have little effect on you, and perhaps you could not even feel any current it would produce when you touch it. Low potentials ordinarily do not provide energy for much work, but there are exceptions such as a car battery, which can produce several hp. for a very short time.

Now about the second part of question ,This is entirely a matter of convention. It is the fundamental concept we follows.And its explanation is beyond your syllabi which involves maxwell's equation and all.