Coulomb's Law lab , , it do tommorrow

1. Sep 23, 2009

Suy

Coulomb's Law lab , plz help, it do tommorrow

1. The problem statement, all variables and given/known data
Here is the picturehttp://img407.imageshack.us/img407/933/48460315.jpg [Broken]
0.2 1.4
0.4 1.4
0.8 1.35
1.0 1.35
1.2 1.3

This is the procedure:
Set up apparatus. Charge the sphere on the block by induction. Observe what happens to the suspended sphere as the charged sphere is brought close to it. Allow the suspended sphere to come into contact with the first.
Allow suspended sphere to return to rest position. Bring the furst sphere towards the suspended sphere again to make mesurements.
2. Relevant equations

Fe=mg*d/L
this is the equation that teacher gave us, but i don't understand this equation at all.

3. The attempt at a solution
I don't really understand the equation Fe=mg*d/L, why is it d/L(distance/length of string, right)
and why we don't use Fe=(kq1q2)/r2
Also, I need 3 source of error , not including human error,
but i have no clue what is it, it do tomorrow , if anyone can help me///

Last edited by a moderator: May 4, 2017
2. Sep 23, 2009

rl.bhat

Re: Coulomb's Law lab , plz help, it do tommorrow

Resultant of mg and Fe makes an angle θ with the vertical.
Draw the figure and find out what is d/L stands for and how it is related to Fe and mg.

3. Sep 23, 2009

Suy

Re: Coulomb's Law lab , plz help, it do tommorrow

I still don't get it.....

Attached Files:

• Untitled.jpg
File size:
2.2 KB
Views:
82
4. Sep 23, 2009

rl.bhat

Re: Coulomb's Law lab , plz help, it do tommorrow

If Fx and Fy are the components of a force F, and if θ is the angle between F and Fx, then tanθ = ?.

5. Sep 23, 2009

Suy

Re: Coulomb's Law lab , plz help, it do tommorrow

is it right?, but how do u find y

6. Sep 23, 2009

rl.bhat

Re: Coulomb's Law lab , plz help, it do tommorrow

I can't see the attachment.
If θ is very small, tanθ is nearly equal to θ, and d/L is θ.

7. Sep 23, 2009

Suy

Re: Coulomb's Law lab , plz help, it do tommorrow

http://img200.imageshack.us/img200/3631/34175284.jpg [Broken]
do you mean tanθ=(y/x)

Last edited by a moderator: May 4, 2017
8. Sep 23, 2009

rl.bhat

Re: Coulomb's Law lab , plz help, it do tommorrow

No. The resultant of Fe and mg will be along the string of length L. In this position string makes an angle θ with the vertical and the suspended sphere displaced a small distance d, then tanθ = d/L = Fe/mg.

9. Sep 23, 2009

Suy

Re: Coulomb's Law lab , plz help, it do tommorrow

Can u draw a picture for me, i am confused....
sorry, but i really don't get it..

10. Sep 23, 2009

rl.bhat

Re: Coulomb's Law lab , plz help, it do tommorrow

Sorry. I don,t know how to draw the picture. Anyway I will explain from your picture.
You have shown the separation of two positions of the suspended charged sphere as x or d.
In the equilibrium position of the sphere, F acts horizontally and mg acts vertically.
The relation between the length of the arc d, radius L and subtended angle θ is d = Lθ.

11. Sep 23, 2009

Suy

Re: Coulomb's Law lab , plz help, it do tommorrow

Fnially, thx a lot,
now i understand what you mean,
since tanθ=F/(mg) and tanθ=d/(L), so F/(mg)=d/L, if i solve for F, F=mg*(d/L)
but i don't understand why do we use Fe x-component
http://img200.imageshack.us/img200/3631/341752 [Broken]

Last edited by a moderator: May 4, 2017
12. Sep 23, 2009

rl.bhat

Re: Coulomb's Law lab , plz help, it do tommorrow

Where are we using Fe x-component?

13. Sep 23, 2009

Suy

Re: Coulomb's Law lab , plz help, it do tommorrow

http://img200.imageshack.us/img200/3631/34175284.jpg [Broken]
this picture, i guess i am wrong..

Last edited by a moderator: May 4, 2017
14. Sep 23, 2009

Suy

Re: Coulomb's Law lab , plz help, it do tommorrow

i have 2 more question, the teacher asked to graph force vs r and straighten the graph,
Do i use 1/r2 ?
and F=mg*(d/L),
g=9.81m/s, or -9.81m/s?

a point charge of -0.35 nC is fixed at the origin. where must and electron be placed in order for the electric force acting on it to be exactly opposite to its weight?
To balance the force, mg=k(q1q2)/r2
q1=-0.35 nC
but i am not sure how to find q2 and the mass

Last edited: Sep 23, 2009
15. Sep 23, 2009

rl.bhat

Re: Coulomb's Law lab , plz help, it do tommorrow

I don't understand what he means by straighten the graph.
g = 9.8 m/s^2

You have that the weight of the electron is equal to the force between q1 and q2.
Mass of the electron and its charge is given in any book.

16. Sep 23, 2009

Suy

Re: Coulomb's Law lab , plz help, it do tommorrow

I think he wanted us to make a straight line, not a curve
i remember the relation between F and r is
The gravitation attraction force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation distance.
is this same as electrostatics,because they have similar equation

17. Sep 24, 2009

MattH25

Re: Coulomb's Law lab , plz help, it do tommorrow

to straighten your curve you have to use the inverse of radius sqaured (1/r2)