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Simply so, are electric fields always positive?

Electric Field can never be negative . As electric field is Force experienced by charge divided by magnitude of charge . Hence Electric Field also becomes positive . So , electric field is always positive .

Additionally, at what point is the electric field zero? There is no zero-field point for a pair of equal-magnitude-but-opposite-sign charges. Electric field is zero in that because the sum of electric field vectors have same intensity and direction, but are opposite. That is halfway between two like charges.

Correspondingly, what are electric fields caused by?

Electric fields are caused by electric charges, described by Gauss's law, or varying magnetic fields, described by Faraday's law of induction. Together, these laws are enough to define the behavior of the electric field as a function of charge repartition and magnetic field.

Can the electric field be negative?

Electric field is not negative. It is a vector and thus has negative and positive directions. An electron being negatively charged experiences a force against the direction of the field. For a positive charge, the force is along the field.