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March 24th, 2011, 07:51 PM   #1
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Negative/Positive Electric Fields

Hi, I was wondering if someone could help me get over some confusion regarding electric fields.
I was doing a question involving the formula F=qE
The electric field needs to be found
F= 6.69x10^-16 N
Q= -1.6x10^-19 C (electron)

In calculating this, I end up with the electric field -3559 N/C. This is negative because the charge used is negative. However, the answer is just 3559 N/C. I looked things up online for this question and people simply put a positive value for the charge even though it's an electron.

I can't really find any information on why this is and would appreciate any input. Thanks!

EDIT: In thinking about this, it's probably in the question's wording, as it is asking for the strength, which probably implies magnitude. But I'm still confused as to whether you can really have negative or positive fields? The only site I found commenting on it said there's really no sense in saying it is positive or negative...
On the other hand, it is a vector...

Very confused about whether fields can be negative..
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March 24th, 2011, 08:13 PM   #2
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Re: Negative/Positive Electric Fields

From those numbers, I'm getting an electric field of 4181 N/C. Perhaps you meant for the force to be 5.69 N?

Regardless, the reason the positive value is reported is that the force and electric field are vector quantities in three-dimentional space. There is in general no single preferred direction, and a negative vector pointing in one direction is precisely equivalent to a positive vector in the opposite direction. So when talking about vector quantities, it is customary to report only the magnitude (always a positive quantity), and possibly the direction.

The exception to this is when a preferred basis has been defined, in which case a vector quantity might be referenced by its components in each of the three basis elements. In this case, some or all of these components might be negative.

Does this answer your question?
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March 24th, 2011, 08:19 PM   #3
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Re: Negative/Positive Electric Fields

Yes, sorry, that was a typo! I meant 5.69x10^-16N

That answers my question, thanks a lot for your help!
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