# SAT Subject Physics Practice Question 240: Answer and Explanation

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**Question: 240**

**11.**

An electric dipole, a pair of equal but opposite charges. Two isolated point charges are fixed in the positions shown on the *y* axis; the positive charge is located at the point (0, *b*) and the negative charge is located at the point (0, –*b*).

Which graph best illustrates the electric potential along the *x* axis, from *x* = –*a* to *x* = *a*?

A.

B.

C.

D.

E.

**Correct Answer:** A

**Explanation:**

Every point on the *x* axis is equidistant from the source charges. For any given point, *P*, on the *x* axis, let *R* denote its distance from the +*Q* charge and from the –*Q* charge. Then the potential at *P* is *k*(+*Q*)/*R* + *k*(−*Q*)/*R* = 0. Therefore, the potential is zero everywhere along the *x*-axis, so graph A is the answer.