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 Feb 27th 2018, 11:50 AM #1 Junior Member     Join Date: Nov 2017 Location: The Jurassic Forests Posts: 25 Electric Fields Can anyone explain how to find electric fields caused by multiple charges at a specified position? I keep getting wrong answers when I try to do those problems, if someone could just break down how to go about them it'd be a big help! Thanks!
 Feb 27th 2018, 01:56 PM #2 Senior Member   Join Date: Aug 2010 Posts: 357 The "electric field" is a vector field that, at each point, if multiplied by a charge, gives the force vector on that charge at that point. If you have q1 at point A, charge q2 at point B, charge q3 at point C, etc., then the electric field at point P is the sum of forces on a unit charge by charges q1, q2, and q3. Letting V1 be the vector from point P to A, The force on the unit charge at P due to charge q1 is q1V1/|V1|, where |V1| is the length of vector V1. Similarly if V2 is the vector from point P to B, the charge on the unit charge at P due to charge q2 is q2V2/|V2| and if V3 is the vector from point P to C, the charge on the unit charge at P due to charge q3 is q3V3/|V3|. The electric field due to all three charges, at point P, is the vector sum q1V1/|V1|+ q2V2/|V2|+ q3V3/|V3|. topsquark and TyrannosaurusWreck like this.
 Feb 27th 2018, 02:36 PM #3 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 993 Two notes to add the Halls excellent summary. The process is called superposition, which is a posh word to say that you can find the field due to each charge individually (or acting on their own) and add them up. But this must be vector addition since E (the electric field vector) is a vector. Hall mentioned three charges Q1, Q1 and Q3 but hidden is a fourth one the unit test charge. This is not considered to contribute to the sum. topsquark and TyrannosaurusWreck like this.
Feb 27th 2018, 04:21 PM   #4
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 Originally Posted by TyrannosaurusWreck Can anyone explain how to find electric fields caused by multiple charges at a specified position? I keep getting wrong answers when I try to do those problems, if someone could just break down how to go about them it'd be a big help! Thanks!
For just this purpose I created a website to explain things for people such as yourself. I recommend reading this page:

Laws of Electrodynamics

It has everything you might want to know. If not then let me know and when I'm able to I'll add it in.

 Feb 27th 2018, 05:31 PM #5 Junior Member     Join Date: Nov 2017 Location: The Jurassic Forests Posts: 25 Thank you all! Your explanations made a lot more sense than my textbooks, but for some reason I'm still not getting one problem... I'll post it tomorrow with my attempt at it.
Feb 27th 2018, 05:33 PM   #6
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 Originally Posted by Pmb For just this purpose I created a website to explain things for people such as yourself. I recommend reading this page: Laws of Electrodynamics It has everything you might want to know. If not then let me know and when I'm able to I'll add it in.
I couldn't find electric fields, what are they under on your site? Sorry!

Feb 27th 2018, 06:03 PM   #7
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 Originally Posted by TyrannosaurusWreck I couldn't find electric fields, what are they under on your site? Sorry!
When you have a problem like that use the web page search function.

But you can just scroll down to the section on Coulomb's law. Then see Eq. (7)

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