Question about electrostatics

[StephanieZ]

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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!

 

[IAmTheCheese]

Ok, I am going to take a wild guess and say that the electric force vector between R and S was, in fact, the direction of the electric field. This is an important concept, because IIRC the electric field actually shows the direction a POSITIVE charge would travel. Since electrons are negatively charged, they are going to travel OPPOSITE the direction of the electric field.

It is also important to understand that the electric field is not a force. Actually, E = F/q, or Electric field = Force / Charge.

For more information, please visit my good friend Wikipedia:

http://en.wikipedia.org/wiki/Electric_field

 

[Trisphorin]

The question is bit unclear. But i think the answer is correct because the vector you are describing has to be electric field line. if i remember correctly, electric field line of positive source charge goes away from the source charge and electric field line of negative source charge goes toward the source charge. The fact that the vector goes from S to R indicates that S must be positive and R must be negative source charge because the line is going away from S towards R. Thus if a electron was placed in the middle, it would be attracted by S and repelled by R and therefore move toward S.

Hope this helps!

 

[StephanieZ]

Thanks for the replies. Now that I think about it, the vector has to be indicating electric field lines, but it is unclear because the question doesn't say anything about a field and the vector itself has the letter F over it and a question prior to that one (but using hte same picture) talked about forces. Maybe the F was never supposed to be force? Maybe it was supposed to F for field? That makes sense, actually, but my brain totally interpreted it as force. Damn you MCAT, why do you have to be so sneaky??

 

[Rabolisk]

This isn't an MCAT trying to be sneaky. It's just bad job by Kaplan.

 

[Trisphorin]

^^

 

[Bayonetwork]

Definitely a poorly written question but if you think about it the only forces in physics that would act on an electron would be an electric or magnetic force. Their is no mention of an emf source so that leaves you with an electric force assumed to be given by an electric field (spheres). Given the answer choices this makes the most sense and you know the rest.

Definitely weird that the force vector had an F over it. Electric fields are usually designated E and magnetic fields B.