The electric field near the surface of Earth points downward and has a magnitude
ID: 1999778 • Letter: T
Question
The electric field near the surface of Earth points downward and has a magnitude of 140 N/C. What is the ratio of the magnitude of the upward electric force on an electron to the magnitude of gravitational force on the electron?
What charge should be placed on a plastic ball of mass 4.44 g so that the electric force balances the weight of the ball near Earth's surface?
- I tried to solve the second question by setting the upward force = downward force. so the question I used was mg=qE, then solved for q.
q= mg/E = 9.8m/s^2*.00444kg/ 140 N/C = 3.108e-4 C <----- this answer was incorrect.
Explanation / Answer
second question )
net force on ball is zero
mg + qE = 0
q = -mg/E
q = -3.108e-4 C ( you are missing negative sign )
for force to be in upward direction charge should be negative