Calculating Electric Force with Coulomb's Law: Balloon and Denim Example

[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 separated 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?

 

[NYROCKFAN]

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

 

[Doc Al]

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

 

[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

 

[Doc Al]

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?

 

[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.

 

[NYROCKFAN]

There are no initial charges given.

 

[Doc Al]

That's the thing... I have no IDEA what the charge is on the denim or the balloon.

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.

 

[NYROCKFAN]

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

 

[Doc Al]

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

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

 

[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 separated 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).

 

[Doc Al]

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.)

 

[NYROCKFAN]

Ok... thank you!