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Hi, I' did this problem but for some reason I'm not getting the right answer. If you could check to see where I made my mistake I'd really appreciate it. Thanks!

The problem:

A poitn charge (m=1 g) at the end of an insulating string of length 55 cm is observed to be in equilibrium in a uniform horizontal electric field of 12000 N/C where the pendulum's position is as shown in the figure, with the charge 12 cm above the lowest (vertical ) position. If the field points to the right, determine the magnitude and sign of the point charge.

My Answer:

I know the charge is postive because it is repelled by the electric force.

Because the system is in equilibirum, I know that

Ftx = Fe (force of the elctric field)

Fty = mg

Fty = mg

= (.001 kg) (9.8)

=.0098

To find Ftx we need the angle.

theta = invscos (43/53)

theta = 35.77

Fty = Ft cos (35.77)

.0098 = Ft cos (35.77)

Ft = 0.012078

Ftx = 0.012078 sin 35.77

Ftx = 0.00706

Fe = 0.00706

Fe = qE

0.00706 = q * 12000

q = 5.88 * 10 ^-7

Correct Answer is : 7.8 * 10 ^-7

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# Homework Help: Point Charge Check?

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