Why is this the case..
I would have thought it would have followed the E field?
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An electric field, or rather, electric field lines radiate from positive charges and flow into negative charges. We all know that opposite charges attract, thus, for a fixed positive charge to attract a negative charge, the negative charge must be compelled to move in the opposite direction of the electric field lines. This is the convention we use. Based on that, it is not hard to see that the electron would accelerate leftward - against the direction of the electric field.