# Homework Help: Sphere Charge Hanging on Thread (Electric Field Question)

1. Dec 19, 2011

### HarleyM

1. The problem statement, all variables and given/known data
Charge 1 (0.15 kg) in the following diagram is hanging from a light insulating thread, as shown in diagram (red is what I have added myself)
Charge 2 is brought close and pulls charge 1 , deflecting the string away from the vertical axis by 40 degrees. The charges have the same magnitude, charge 1 is negative.

A) Find the tension in the thread
B) Find the charge on 2.

2. Relevant equations
Fg=mg
Fe=Kq2/r2

3. The attempt at a solution

Vertical

Fnet=0
0=FtCos40 + (-mg)
Ft=mg/cos40
Ft=1.92

Horizontal

Fnet=0
0=Fe+(-Ftsin40)
0=kq2/r2+(-1.92sin40)
q=√(1.92sin40)(0.42)/(9x109)
q=+/- 4.7x10-6C

How does this look?

2. Dec 20, 2011

### Simon Bridge

Is there anything about that which concerns you?
How could you work out for yourself if you got the right answer?

FWIW: without crunching the numbers, your reasoning looks sound.
Comment the actual paper you hand in.

3. Dec 21, 2011

### ehild

the answer q=+/- 4.7x10-6 C is not correct. Read the problem again. It says
Do the charges attract or repel? What is charge 2 then?

ehild

4. Dec 21, 2011

### HarleyM

Well I wrote +/- because it was a square root, the charge is actually positive I know because it attracts charge 1

Thank you for the input however I feel more confident with my physics knowledge everyday !

5. Dec 21, 2011

### ehild

Then it was correct but it was not the solution yet.

It was a nice solution. I especially liked the picture. You can be confident in your knowledge!

ehild

6. Dec 21, 2011

### Simon Bridge

It is common for the math to give a choice of solutions - you pick the one that makes sense in terms of the physics.