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## Homework Statement

A sphere with radius 2.0 mm carries a 1.0 μC charge. What is the potential difference, VB - VA, between point B 4.0 m from the center of the sphere and point A 9.0 m from the center of the sphere? (The value of k is 9.0 × 10^9 N∙m2/C2.)

## Homework Equations

The formula for electrical potential difference: V = (ke x Q)/r

## The Attempt at a Solution

My professor said the answer is 1300V, but I'm having a hard time arriving at that answer. My understanding is that I should use the formula V = (ke x Q)/r first for point B and then for point A, and then I should subtract the answer I got for point A from the answer I got for point B. However, I'm not sure what to plug in for the "r" value of the formula. After converting from mm to m, the radius of the sphere is .002m. If point B is 4m from the center of the sphere and point A is 9m from the center of the sphere, do I subtract .002 from each of those numbers? Unless I have just incorrectly converted units, I am not getting an answer anywhere near 1300V. Is there some other formula I need to use?