# Help with Coulomb's Law

1. Nov 29, 2004

### NYROCKFAN

This is the question:

A balloon rubbed against denim gains a charge of -8 x 10^ -6 C. What is the electric force between the balllon and denim when they are seperated by 0.05 m?

That's all I know... I need to find the force between the two, all I know is the formula (Coulomb's Law), the distance, the constant (8.99 x 10^9), and that the balloon gains a negative charge. How do I figure it out? Is there a formula to calculate what the initial charges for the balloon and denim are?

2. Nov 29, 2004

### NYROCKFAN

Help... this question is on a graded packet due tomorrow... any ideas?

3. Nov 29, 2004

### Staff: Mentor

Assume that before all the rubbing, the charge on balloon and denim was zero. Hint: Does the net charge change?

4. Nov 29, 2004

### NYROCKFAN

The net doesn't change...

So do I plug in zero for q1 & q2 in this equation?

Fe = (8.99 x 10^9)(q1)(q2)
___________________
0.05^2

5. Nov 29, 2004

### Staff: Mentor

q1 and q2 are the charges on the two objects. You know the charge on the balloon, but you need to figure out the charge on the denim:

If the net charge is zero, what must be the charge on the denim?

Last edited: Nov 29, 2004
6. Nov 29, 2004

### NYROCKFAN

That's the thing... I have no IDEA what the charge is on the denim or the balloon. I just know that the balloon gets a specific charge added to the initial charge when the two objects meet.

7. Nov 29, 2004

### NYROCKFAN

There are no initial charges given.

8. Nov 29, 2004

### Staff: Mentor

Sure you do. It tells you right here:
A balloon rubbed against denim gains a charge of -8 x 10^ -6 C.​
That's the charge on the balloon.

9. Nov 29, 2004

### NYROCKFAN

In the book it says that charge was added to it's initial charge (we isn't known).

10. Nov 29, 2004

### Staff: Mentor

So... your statement of the problem in your first post is incomplete? What's the exact statement of the problem?

11. Nov 29, 2004

### NYROCKFAN

A balloon, when rubbed against a piece of denim, gains a charge of -8 x 10^ -6 C (therefore,the final charge is -8 x 10^ -6 C more than the initial charge). What is the electric force between the balloon and denim if they are seperated by 0.05 m?

That is the exact wording,,, and the book gives no mention of what the final or initial charges for the two objects were (and I've read the entire unit on electric force trying to see if it was mentioned somewhere).

12. Nov 29, 2004

### Staff: Mentor

How strange. Obviously you cannot calculate the force without knowing the charges involved. (As opposed to handing in a blank worksheet, I would calculate the force assuming that the initial charges were zero and add a note explaining the need for that assumption.)

13. Nov 29, 2004

### NYROCKFAN

Ok... thank you!