Understanding Electric Field Direction Around Point Charges

[JooJupe]

I don't know, when given a point charge (sign is indicated = positive or negative), which direction the field lines go



Although I NOW know that if the point charge is positive, field lines are outwards, and if negative then they are inwards...HOWEVER, I have come across an example or two where the point charge is surrounded by an electric field with the same sign (due to another source) - and I would much rather learn how to understand the consequential direction of the field lines, than just learn the diagrams


Thank you :D

 

[zhermes]

Hi JooJupe, welcome to PF. I'm a little confused on your question; are you asking how to find the resulting field lines from adding each-field from two separate charges?

 

[JooJupe]

Hi JooJupe, welcome to PF. I'm a little confused on your question; are you asking how to find the resulting field lines from adding each-field from two separate charges?

Hi, sorry for the poor initial explanation - here it is in a clearer version;

I have an exam, and one of the common questions is a picture of a point charge - the question asks 'Given that the point charge is Positive (and they also ask negative), draw equipotential lines and electric field lines.' Now, by looking over model-solutions I have concluded that if the point charge is positive, then the lines are directed away from the point charge - and if the charge is negative, then the lines point towards the centre of the charge. However, I have also come across a question that has a 2nd part to it;

1. Given that there is a point charge in space, draw electric field lines and equipotential lines;

a) given that it is positive

b) Given that it is positive, AND the charge is inside a positively charged space.

I just don't get the second part - if the charge is positive and the surroundings are positive, then the radial lines point outwards...from what i gather from model solutions - however, what if the charge is negative AND the surroundings are negative?? thanks :)

 

[Rayquesto]

I notice that sometimes these vector directions are confusing and you get stuck, but I've always stuck to a visual rule that I'll never forget, because it always works, except for this really complicated problem on a test I took (or maybe the reason why I got my vectors wrong was because I was tired). But anyway, imagine the point charge as the only object being effected by motion. Then imagine holding the point charge as though it were a positron and letting go. if the charge that's relative to the point charge positron is an electron, the point charge will go in the direction of that electron, thus a vector is drawn in that direction. If the charge relative to the point charge positron is a positive proton, then the point charge when let go is going to be going in the direction opposite of the proton. These directions represent whether the point charge is being affected in the negative or positive direction.

 

[zhermes]

1. Given that there is a point charge in space, draw electric field lines and equipotential lines;
...
b) Given that it is positive, AND the charge is inside a positively charged space.
...
if the charge is positive and the surroundings are positive, then the radial lines point outwards...from what i gather from model solutions - however, what if the charge is negative AND the surroundings are negative?? thanks :)

That's a really strange question. 'Surroundings are positive' as in there is a uniform charge density throughout space in that region? If so, then that doesn't really change anything---because of gauss' law, a perfectly uniform charge background wouldn't contribute any electric field.

And by symmetry, replacing all positives with all negatives will be exactly the same, except with the direction of lines reversed.

 

[ashishsinghal]

Show the diagram, then we could help.