# Static Electricity and Algebra Problems

1. Jan 30, 2012

### mattyc33

1. The problem statement, all variables and given/known data
I have a few problems that I found in my review package that I just couldn't figure out. I've been trying all day as I'm used to the normal F=kqq/r^2. These are higher levelled problems, but my prof says that they will be on the midterm which is coming up next week. If someone could help me that would be really great.
1) After rubbing a balloon against someone's hair, a balloon has accumulated a static electric charge. The air inside the balloon has the same density as the air outside of the balloon, and the balloon itself weighs 3.36mN . How many free electrons are there on the surface of the balloon if an electric field of magnitude 0.614x10^12N/C is required to keep the balloon floating at a constant height?

2) A particle with charge of magnitude 2.01x10^-10C is suspended motionless in the air by an electric field.If the electric field is 7.61x10^-19N/C downwards, what is the mass of the particle?

3) Point charges - q and - q are positioned as shown.
http://www.flickr.com/photos/74717767@N03/6772867909/
Find an algebraic expression for the electric field at point P, in terms of the given parameters.

4) Find an algebraic expression for the electric force due to the two points charges on the -axis on a point charge -9q at P , in terms of the given parameter.
http://www.flickr.com/photos/74717767@N03/6772903769/

2. Relevant equations

F=kq1q2/r^2
E=F/q

3. The attempt at a solution
1) I've converted megaNewtons to Newtons, but I just cannot find in my textbook how to calculate number of free electrons. I have the mass of an electron but I don't exactly know how to approach this type of question.

2) I see that I can find the number of Newtons in this problem. I do not have acceleration or any other variables, so I do not see how I can find mass.

3)I'm pretty sure that I have to algebraically make an expression that goes into the equation for electric field. I'm not given very much information so once again, I don't really know how to approach this one.

4) Same problem as the question above.

If anyone could help me I would really appreciate it. I'm not used to this level of electricity so if possible could you please explain. Thank you very much.

2. Jan 30, 2012

### tiny-tim

hi mattyc33!
why do you need their mass? you have the mass of the balloon

(and it's millinewtons, named after Newton's baby daughter, Millie Newton )
the acceleration is 0
you're given all the charges and distances …

what more do you need?

3. Jan 30, 2012

### mattyc33

Well thank you,

I just can't get the proper equations and stuff down. This is not homework, I am trying to do this for my own benefit and there is no solution page given by my prof.

I thank you very much for your answer but if anyone else could contribute and help me a bit more in-depth that would be awesome.

Thanks.