# Homework Help: Help computing unknown charge

1. Jan 28, 2012

### jesse james

Physics Help Computing Force

1. The problem statement, all variables and given/known data

Two insulated balls with a mass of 0.8g each are suspended on 25 cm threads from the same point on the ceiling. Each ball has a equal positive charge. The angle formed between the two threads is 36 degrees. What is the charge on each ball?

2. Relevant equations
unknown

3. The attempt at a solution
I am unable to even begin to start on this problem any help would be much appreciated even if just telling what equations i need to be working with I assume I need to be something witht he equation of gravity and force

2. Jan 28, 2012

### engphy2

Re: Physics Help Computing Force

Based on reading your problem i think is it about computing the coulomb force. Am i right?

3. Jan 29, 2012

### jesse james

1. The problem statement, all variables and given/known data
2 equally charged spheres both weighing 0.8g are hanging from 25cm of thread.... As a result of the equal positive charge placed on each ball they spread apart forming an angle of 36 degrees between the threads. What is the charge on each ball?

**almost identical to the question posted and solved here**

2. Relevant equations
F=k*q1*q2/r^2

3. The attempt at a solution
this is how I worked through this using the problem posted above as a guide but I am getting the wrong answer

sin 54° = Ty/T
T = (m*g)/sin 54°
T=0.0008*9.8/sin 54°
T = 0.0097

Tx = T*cos 54°
Tx = (0.0097)*cos 54°
Tx = 0.0057 N

**so that math computed "Tx" and looking from the other problem that looks to be "F" in the equation listed for finding the charges I need**

**now shows equation to get the distance**

1/2*r = sin 36°
r = 1.18

**im pretty sure that is wrong but not sure how exactly but that number is way too big if my string is only 25cm**

ok so now I believe thats all the info i need to plug into the equation.

Tx = k*q2/r2
q = (Tx*r2/k)1/2

i put it into excell first figuring the Tx*r^2/k ....then getting the square root of that number..end up with

q=9.4*10^-7
which i believe is something like 94 uC

not sure on the conversion or really any of it as the listed answer is 83 nC..pretty sure i am messing up with my radius and maybe other places aswell ...anyhelp would be greatly appreciated :)

4. Jan 29, 2012

### JHamm

Re: Physics Help Computing Force

What forces are acting on each ball? Remember for equilibrium the sum of the forces is zero.

5. Jan 29, 2012

### ehild

Hi jesse james,

where that 54° angle comes from? The balls "spread apart forming an angle of 36 degrees between the threads."

ehild

6. Jan 29, 2012

### jesse james

i tried with 36 and was still coming up with wrong asnswer anychance someone could work out the equations so I could understand i bit better

7. Jan 29, 2012

### ehild

36 °is the angle between the threads. So what is the angle one thread encloses with the vertical?

ehild

8. Jan 29, 2012

### jesse james

i really dont know how to figure out the angles...i seen from the sample problem that he used 70° in his first equations and didnt know how he got it

9. Jan 29, 2012

### ehild

I guess you study Maths and Physics. What about trying to solve the problem by yourself?
Those charged balls are in equilibrium. What does it mean on the forces exerted to one of the balls?

ehild