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Along the line joining two charges q1 and q2, the electric field (rightward taken positive) is plotted against the distance measured from q1. From the shape of the graph, which statement is correct?

1. q1 and q2 are both positive and q1 < q2
2. q1 and q2 are both positive and q1 > q2
3. q1 is positive and q2 is negative with q1 < q2
4. q1 and q2 are both negative and q1 < q2

Correct answer: q1 and q2 are both positive and q1 < q2

Solution

Because the field reverses sign (passes through zero) at a point between the two charges, both charges must be of the same sign; the graph shows them positive (field points away from each near them). The null point is closer to q1, which means q1 is the smaller charge, so q1 < q2.

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