# Electric, Magnetic, and Gravitational Fields

1. Dec 7, 2003

### Foon

I'm having trouble with the second part to this question and would really appreciate help, it comes with a diagram that I've tried my best to copy so I hope it will help you help..me ;

Thanks

Question:

A ping pong ball of mass 3.0 x 10^-4 kg hangs from a light thread 1.0m long, between two parallel plates 10.0cm apart. (figure)

http://foonyboi.250free.com/guh.jpg

When the potential difference across the plates is 420V, the ball comes to equillibrium 1.0cm to one side of its original position.

a) Calculate the electric field strength between the plates.

b) Calculate the tension in the thread.

Question b is what I'm having trouble with. I'm not sure where to start actually. I know that the force of gravity is acting on it and that Fg = Fe, which you can than equate qE = mg (the E being the electric field). I'm not sure if that applies or how it can help though.

Any help would be appreciated. Thanks again,

Foon

2. Dec 9, 2003

### Staff: Mentor

Nope. Consider all the forces on the ball. Remember: forces are vectors---direction counts! I see three forces on the ball: tension in the string, the electric field, and gravity. (What's the direction of each force?) Since the ball is in equilibrium, these must add up to zero.

Hint: treat the x & y components of the forces separately.