When two charges Q1 and Q2 are place near each other, a force of attraction or repulsion is produced between them as per Coulomb’s law. At the point occupied by charge Q1 we can imagine that the electrical state of space is caused by other charge Q2. This is due to electrical field caused by Q2.
“The property of an electric field that causes it to exert a force on a change introduced into it is called the electric field strength.”
This is given by the relation,
E=F/Q newtons/coulomb
Potential at any point in an electric field is defined as the work done in joules in moving a unit positive charge of one coulomb from the point at the infinity to that. The potential difference between two points in the field is the work done in joules in moving a positive charge of one coulomb from the point of lower potential to the point of higher potential. The potential
difference is independent of the routine taken by the positive charge too move to new position.
If the potential difference over a distance dl is taken by dV, and the field strength is E,
dV= -E*dl or E= -dV/dl volts/metere
This was that the field strength at any point is equal to the potential gradient at that point. The negative sign indicates that the work done is the positive when the charge is moved from lower to higher potential.