Force on a Point Charge in an Electric Field

[wheybags]

Homework Statement

A negative point charge of 10 ^(−6 )C is situated in air at the origin of a
rectangular coordinate system. A second negative point charge of 10^(-4) C is
situated on the positive x-axis at a distance of 50 cm from the origin. What is
the force on the second charge?

Homework Equations

Coulomb's law?

The Attempt at a Solution

I don't know ;_;

Coulombs law gives the force between two points, buy I need the force on one of the points. My notes say :

If multiple charges exist, the fore
on anyone charge equals the vector sum of the
forces exerted on it by the other charges.

But I have no idea what that means. Help please?

 

[hx35]

F = (k_eq₁q₂)/r²What your notes say is basically all forces of other charges will affect that one charge.

 

[wheybags]

Could you explain how you got that please?

 

[berkeman]

Homework Statement

A negative point charge of 10 ^(−6 )C is situated in air at the origin of a
rectangular coordinate system. A second negative point charge of 10^(-4) C is
situated on the positive x-axis at a distance of 50 cm from the origin. What is
the force on the second charge?

Homework Equations

Coulomb's law?

The Attempt at a Solution

I don't know ;_;

Coulombs law gives the force between two points, buy I need the force on one of the points. My notes say :

But I have no idea what that means. Help please?

hx35 is just reminding you to use Coulomb's law. The two charges are the two charges that you are given in your problem. The force is symmetric between the two charges (the force is equal and opposite on the two charges).

 

[wheybags]

hx35 is just reminding you to use Coulomb's law. The two charges are the two charges that you are given in your problem. The force is symmetric between the two charges (the force is equal and opposite on the two charges).

I don't really get what you mean.

 

[berkeman]

I don't really get what you mean.

Why not?

What is k? What is the value of q1? What is the value of q2? What is the value of r? So what is the force?

 

[wheybags]

Why not?

What is k? What is the value of q1? What is the value of q2? What is the value of r? So what is the force?

I still don't know how to find the force on only one point.

 

[berkeman]

I still don't know how to find the force on only one point.

You are finding the force on both points when you use that equation. The force experienced by each point.

Alternately, you can find the electric field generated by the other point, and use your own charge value as the "test charge" in that electric field at that distance away from the other charge to find the force on you, F=qE. It's the same thing.

 

[wheybags]

You are finding the force on both points when you use that equation.

What? The question asks me to find the force on the second charge.

 

[berkeman]

What? The question asks me to find the force on the second charge.

The force on the 2nd charge is the same (but opposite direction) as the 1st charge. There are only 2 charges in this problem, right?

If you have multiple charges in a question, you have to add them up as a vector sum of forces at your test charge location.

Please read the wikipedia article -- hopefully it will help to clear up some of the confusion:

http://en.wikipedia.org/wiki/Coulomb's_law

This force (F) acting simultaneously on point charges (q1) and (q2), is given by

(emphasis is mine)