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Why does Electric Field Strength [Vector] point in the direction of a positive charge

mmmn

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Reply 1
Avatar for okey
okey
15
um opposites attrakt init
Original post by Mihael_Keehl
mmmn


I guess that would be the case if it was the positive charge was creating the electric field. :s-smilie:
Original post by Mehrdad jafari
I guess that would be the case if it was the positive charge was creating the electric field. :s-smilie:


EFS = Force experienced per unit positive charge so I suppose it is using a positive test charge to analyse what direction the force is travelling in?

Maybe I overcomplicated it..
Original post by Mihael_Keehl
EFS = Force experienced per unit positive charge so I suppose it is using a positive test charge to analyse what direction the force is travelling in?

Maybe I overcomplicated it..


If the negative charge is causing the field, the direction of the force will be towards the negative charge as the positive test charge will be attracted towards the positive charge.
Original post by Mehrdad jafari
If the negative charge is causing the field, the direction of the force will be towards the negative charge as the positive test charge will be attracted towards the positive charge.



I understand the latter part: but with EFS isn't it always the positive charge that is causing the field?
The electric field points away from positive charge.
Original post by Mihael_Keehl
I understand the latter part: but with EFS isn't it always the positive charge that is causing the field?


Yes, it could be, but the direction of the field lines will be away from the positive charge causing the field and towards the positive test charge, i.e the force will be towards the positive test charge (and not towards the positive charge causing the field). Fundamentally, the direction of the field lines has been defined to be always directed towards the negative charge, because Coulomb's equation F=Qq4πϵ0r2F=\dfrac{Qq}{4\pi\epsilon_0 r^2} , where one of the charges is negative, mathematically gives the negative sign for the force direction, even if the negative charge is the test charge.
(edited 8 years ago)
Original post by Mehrdad jafari
Yes, it could be, but the direction of the field lines will be away from the positive charge causing the field and towards the positive test charge, i.e the force will be towards the positive test charge (and not towards the positive charge causing the field). Fundamentally, the direction of the field lines has been defined to be always directed towards the negative charge, because Coulomb's equation F=Qq4πϵ0r2F=\dfrac{Qq}{4\pi\epsilon_0 r^2} , where one of the charges is negative, mathematically gives the negative sign for the force direction, even if the negative charge is the test charge.


Perfectly explained, excatly what i Was looking for.

Thank You.
Reply 9
Avatar for alow
alow
19
Convention.
Original post by Mihael_Keehl
Perfectly explained, excatly what i Was looking for.

Thank You.


No worries! Glad you found it satisfying.
Original post by Mehrdad jafari
Yes, it could be, but the direction of the field lines will be away from the positive charge causing the field and towards the positive test charge, i.e the force will be towards the positive test charge (and not towards the positive charge causing the field). Fundamentally, the direction of the field lines has been defined to be always directed towards the negative charge, because Coulomb's equation F=Qq4πϵ0r2F=\dfrac{Qq}{4\pi\epsilon_0 r^2} , where one of the charges is negative, mathematically gives the negative sign for the force direction, even if the negative charge is the test charge.


I must finally ask, from my reading I have assumed that the test charge taken to help us determine the direction of the field lines is positive, so is this assumption wrong since your last statement say we can take the test charge to be negative?
Original post by Mihael_Keehl
I must finally ask, from my reading I have assumed that the test charge taken to help us determine the direction of the field lines is positive, so is this assumption wrong since your last statement say we can take the test charge to be negative?


Do you man the test charge in the equation I wrote? The direction of the field lines is independent of whether the test charge is positive or negative, but not on the force acting on the test charge in the field.
Original post by Mehrdad jafari
Do you man the test charge in the equation I wrote? The direction of the field lines is independent of whether the test charge is positive or negative, but not on the force acting on the test charge in the field.


Sorry if i made it unclear lolzz, in your last line you said "even if the negative charge is the test charge."

what I meant to ask is that "Do we not always take the test charge to be positive?"
Original post by Mihael_Keehl
Sorry if i made it unclear lolzz, in your last line you said "even if the negative charge is the test charge."

what I meant to ask is that "Do we not always take the test charge to be positive?"


Well, me might take the positive charge to be the test charge in our examples, but what but I was referring to was when the negative charge was the test charge, in which case the negative test charge will move in the opposite direction to the field lines. I honestly don't see the reason as to why the electric field strength is defined with with respect to a positive test charge in the field that's not necessarily the case.

Edit: @Mihael_Keehl: I think the reason that the electric field strength is defined with respect to a positive test charge in the field is that regardless of whether the point charge creating the field, in both case the positive test charge will accelerate along the field lines.
(edited 8 years ago)
Original post by Mehrdad jafari
Well, me might take the positive charge to be the test charge in our examples, but what but I was referring to was when the negative charge was the test charge, in which case the negative test charge will move in the opposite direction to the field lines. I honestly don't see the reason as to why the electric field strength is defined with with respect to a positive test charge in the field that's not necessarily the case.

Edit: @Mihael_Keehl: I think the reason that the electric field strength is defined with respect to a positive test charge in the field is that regardless of whether the point charge creating the field, in both case the positive test charge will accelerate along the field lines.


Original post by Mihael_Keehl
Sorry if i made it unclear lolzz, in your last line you said "even if the negative charge is the test charge."

what I meant to ask is that "Do we not always take the test charge to be positive?"



Why are people talking about test charges? It's much simpler than that. It's convention that the electric field points away from positive charge and towards a negative charge. It's like saying why does the magnetic field point from north to south, it's convention (although you can't have one pole on it's own of course)
(edited 8 years ago)
Convention.
Original post by langlitz
Why are people talking about test charges? It's much simpler than that. It's convention that the electric field points away from negative charge and towards a positive charge. It's like saying why does the magnetic field point from north to south, it's convention (although you can't have one pole on it's own of course)


Yeah, I guess it was unnecessary talking about test charges but I was referring to when the force on a charge particle is in the opposite direction to the field lines, as in the case of a negative charge in the electric field.

Although I also believe it's much simpler, I don't think the convention itself was ruled out from nothing. I have not studied Maxwell's equations but I was reading that this was somehow related to the mathematics describing it in terms that field lines point away from the positive charge towards the negative charge, for which they have non-zero divergence at those points.
Original post by Mehrdad jafari
Yeah, I guess it was unnecessary talking about test charges but I was referring to when the force on a charge particle is in the opposite direction to the field lines, as in the case of a negative charge in the electric field.

Although I also believe it's much simpler, I don't think the convention itself was ruled out from nothing. I have not studied Maxwell's equations but I was reading that this was somehow related to the mathematics describing it in terms that field lines point away from the positive charge towards the negative charge, for which they have non-zero divergence at those points.


No I don't think so. The assignment of positive and negative is random in itself. They could equally have opposite names or be called cat and dog.
Original post by Mihael_Keehl
I understand the latter part: but with EFS isn't it always the positive charge that is causing the field?


Hmm no both positive and negative charges create an electric field

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