I\'m going through my kaplan MCAT physics book and one of the questions goes lik

Question

I'm going through my kaplan MCAT physics book and one of the questions goes like this. They show a picture of two circles, one labeled S and one labeled R, with a force vector extending from S to R. The question asks: 

"If an electron were placed midway between R and S the resultant electric force on the electron would be:"
A) towards R
B) towards S
C) up
D) down

Due to the fact that the force vector was pointing towards R, I reasoned that the two circles were unlike charges since they were attracting each other, but it didn't seem to indicate whether R or S was positive or negative. 

The answer at the back of the book says the electron will move towards S because unlike charges attract and repelled by R because like charges repel. How did they deduce that S was positive and R was negative? Does it have to do with the fact that the force vector is pointing away from S and towards R? I didn't think it mattered which direction the force vector was pointing. 

Please forgive me if this is obvious, but physics is by far my worst subject for MCAT. I would appreciate your help!

Explanation / Answer

Here you did not posted the diagram and vased on your explanation of the diagram, the vector given in the diagram should be a field vector but not the force vector. Because, if it will be a force vector, then the magnitudes of the chatges on either side should be different. Or, if the magivitudes of the charges are sam, than both will have same polarity. As it was given in the answer that the charges are hacing different polarity, based on the magnitude of the charge, the particle will move (as both ate at same distance). If the vector is a field vector, then as it is pointed towards R, R should be negative and S should be positive and consecutively electron move towards S (positive polarity)  

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