# Finding charge - problem

1. Mar 24, 2015

### MiniOreo1998

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. Charge 2 is brought close and pulls charge 1, deflecting the string away from the vertical axis. The charges have the same magnitude, but charge 1 is negative.

A) Find the tension in the thread

B) Find the charge on charge 2. Is it positive or negative? Explain your reasoning.

I'm satisfied with my answers, however, I'm almost certain I haven't "listed" everything correctly. (example, putting FT in the wrong place where a different notation should be listed...)

I'm taking notes from the tips that I've been given in previous threads (just want to make it clear that I'm actually storing this information in order to avoid having to scramble every time a new problem arises).

2. Relevant equations

Fg = m g
Fe= K q2 / r2

3. The attempt at a solution

A)

Ft = m g
Ft = (0.15) (9.8) / cos 40
Ft = 1.9

The tension in the thread is 1.9 N

B)

Ft = k q2 / r2
Fe = (9.0 x 109) q2 / 0.42
q = √ (0.402) (1.9 sin40) / 9.0 x 109
q = 4.7 x 10-6 C

Charge 1 (given as negative) attracts Charge 2, thus charge 2 is positive.

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2. Mar 24, 2015

### BvU

Ft = m g ​
on one line, and then
Ft = (0.15) (9.8) / cos 40​
on the next makes my skin crawl. The 1/cos(40) seems to come out of the blue sky. Something like
Fty = m g
| Ft | = (0.15) (9.8) / cos 40​

is more understandable. You can then continue with
Ftx = | Ft | sin(40) = k q2 / r2 etc​