Student 1: \"The net electric force on the +q charge is now threetimes as large

Question

Student 1: "The net electric force on the +q charge is now threetimes as large as before, since there are now three positivecharges exerting forces on it." Student 2: "I don't think so. The force from the +Q charge onthe left will cancel the force from the +Q charge on the right. Thenet electric force will be the same as in part A." 1) Do you agree with either student? Explain. 2) Indicate the direction of the net electric force on the +qchrage. Explain. 3) What, if anything, can be said about how the magnitude ofthe net electric force on the +q charge changes when the two +Qcharges are added? Explain. I would appreciate it if you draw out the vectors too.Thanks.

Explanation / Answer

1. No. The two added charges are not pulling in the samedirection as the original, so their forces will not add directly.Student 1 is wrong. But the forces from two added charges are notdirectly opposed to each other, so they cannot cancel each other.Student 2 is also wrong. . 2. The direction of the net electric force is down. The middlecharge pulls straight down. The other two charges pull down and tothe left, or down and to the right. .         The left and rightparts of those forces will cancel each other... but all the "down"parts of the forces combine. . 3. The magnitude of the net electric force is greater than themagnitude of the original force (with one charge) but less thanthree times the magnitude of the original force. That is all thatcan be said; you can't be any more specific than this.

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