# Homework Help: Ball suspended in a horizontal electric field

1. Oct 11, 2011

### cdriscoll

A table tennis ball of mass 10g carries a charge of 0.04 micro Coulombs. It is suspended at the end of a rine threa 25cm long and placed in a horiztonal electric field. The effect of the field is to move the ball's centre 1.5cm to on side of a vertical line through the point of suspension.

What is the force on the tennis ball?

What is the magnitutde of the electric field?

I'm not even sure how to start this one so any help would be really appreciated!

Last edited: Oct 11, 2011
2. Oct 12, 2011

### grzz

What is the formula for the electric force on a charge q due to an electric field E?

3. Oct 12, 2011

### cdriscoll

F= k q(1) q(2) / d^2 ?

4. Oct 12, 2011

### SammyS

Staff Emeritus
That's the magnitude of the force one charge exerts on another.

The question asked by grzz was

"What is the formula for the electric force on a charge q due to an electric field E? "

5. Oct 16, 2011

### cdriscoll

E=F/q ?

6. Oct 16, 2011

### SammyS

Staff Emeritus
Good enough.

How much is the force of gravity, which is downward. ?

7. Oct 16, 2011

### cdriscoll

9.8m/s/s....?

I'm really sorry I've been taking ages to reply I am so busy, but my assignment is due tomorrow so I'm screwed anyways.... thanks for trying sorry I am so bad at physics :( if you still want to try and explain though that would be good i just might be slow replying, school starts tomorrow

8. Oct 16, 2011

### grzz

Now check whether the gravitational force is negligible compared with the electric force.

9. Oct 16, 2011

### SammyS

Staff Emeritus
That's the acceleration due to gravity, g. The force due to gravity is the weight = mg .

The force due to gravity (the weight) is downward. You are given the mass, so you can calculate this.

The electric field is horizontal, so is the electric force.

The tension in the fine thread must cancel both of these. The tension (also a force) is along the same direction as the thread.

10. Oct 17, 2011

### grzz

I am revising my one suggestion.

The weight CANNOT be negligible because the weight must balance teh vertical component of the tension.

11. Oct 17, 2011