Sphere Charge Hanging on Thread (Electric Field Question)

[HarleyM]

Homework Statement

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.

Homework Equations

Fg=mg
Fe=Kq²/r²

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?

 

[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.

 

[ehild]

the answer q=+/- 4.7x10-6 C is not correct. Read the problem again. It says

The charges have the same magnitude, charge 1 is negative.

Do the charges attract or repel? What is charge 2 then?

ehild

 

[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 !

 

[ehild]

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

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

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

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

ehild

 

[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.