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Uniform Electric Field Upward Electric Force Equals Weight. HELP

by m_physics
Tags: w=mg electric field
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Jan23-09, 12:58 AM
P: 7
1. An object with a charge of -3.6uC and a mass of .012kg experiences an upward electric force, due to a uniform electric field, equal in magnitude to its weight. (a) Find the direction and magnitude of the electric field. (b) If the electric charge on the object is doubled while its mass remains the same, find the direction and magnitude of its acceleration.

2. E=kQ/r(sqrd)
k=8.99 x 10^9 N*m^2/C^2
magnitude of electron's charge (e): 1.60 x 10 ^ -19 C

3. Attempt
W= (.012kg)(10)

Upward force equal to magnitude in weight so...
W= W=kQ/r(sqrd)
.12N + (8.99 x 10^9) [(-3.6uC)(1.6 x 10^-19])/(r^2)
Solve for r

Plug r into E equation to find electric charge.
E=(8.99 x 10^9) [(-3.6uC)(1.6 x 10^-9)]/(-431.52^2)
E= -51.7824/-431.52^2
E= -2.78 x 10 ^ -4

I have no idea about part b. Is it always equal to gravity? Is that how they found the answer.

Correct answer from book:
a) (-3.3 x 10^4 N/C) y direction
b) (9.81 m/s^2) y direction
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Jan23-09, 01:08 AM
HW Helper
P: 4,435
In the electric field force on a charge is equal to E*q
Hence mg = E*q. Now solve for E.

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