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Four point charges of equal strength but two different polarities are arranged on the corners of a square as shown. The strength of each charge is measured in nC and can be varied using the slider. You can mouse-down to read the magnitude of the field at any point in the animation (position is given in meters and electric field strength is given in newtons/coulomb). Restart.

Find a formula for the electric field midway between charge 2 and 3 (indicated by the small black dot) as a function of the strength of the charges. Include a vector diagram that also shows the direction of the resultant field.

Problem authored by Chuck Bennett and Wolfgang Christian.