Strength

– Strength defines the ‘power’ of the force field.
– The greater the Strength the bigger the effect on particles towards or away from the Force Field.

– In this example the little square was set as particle emitter and the central circle (the ‘F’) as Force field (Shape: Point).

Positive strength: all newtonian particles are blown away from the Force point.
Negative strength: all newtonian particles are attracted to the Force point.

– In the next example note how newtonian particles are blown away from a positive (+0.5) value of Strength while are attracted by a negative one (-0.5).
– When particles reach the (negative) Force point, they cross it for inertia. Some particles are bigger and coloured so you can follow better their movement:

Blender Force Field Strength