Question

when is an electric field said to be 'uniform'?

Answer 1

if the electric current do not fluctuate it is called uniform current

Answer 2

If a positive charge is placed at a point halfway between the plates then it will move toward the negative electrode (electric attraction) along the electric field lines. The field lines are parallel to each other and at right angles to the plates. They go from the positive plates and move toward the negative plates.
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