Physics 2 , two Styrofoam balls on insulated thread

In summary, the conversation discusses two small styrofoam balls with equal amounts of negative charge suspended on a vertical insulating thread. The force equilibrium between the balls and gravity is discussed, with the equation E = (kq1q2) / r^2 being used to calculate the number of electrons added to each ball. The conversation also addresses the incorrect use of units and the importance of considering the mass of the objects when calculating force.
  • #1
nathancurtis11
13
0

Homework Statement


Two small .1 gram stryofoam balls are strung like beads on a vertical insulating thread. The lower ball is glued to the thread but the upper ball is free to move. Imagine that both are given an equal amount of negative charge such that the upper ball is suspended about the lower with their centers 4.0 cm apart. (Assume that each ball repels the other as if it were a point charge located at its center.) Roughly how many electrons have been added to each ball? Express your result as a fraction of the total number of electrons initially inside each ball.

Homework Equations



E = (kq1q2) / r^2

One electron = 1.6 x 10^-19 Coulombs

k = 9 X 10^ 9 N m^2 / c^2

The Attempt at a Solution



Assumed the force was now close to zero if the top ball is now suspended in the air instead of being pushed upward. But then you just get both charges to be zero which would be incorrect. Not sure any other way to do this.
 
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  • #2
Don't forget gravity. The upper ball (that is free to move) is in a force equilibrium.
 
  • #3
So would force be -9.81 then?? so -9.81 = (kq1q2) / r ^ 2 ?
 
  • #4
A force is not a number, it has units.
What is the gravitational force on the styrofoam ball?
 
  • #5
-9.81 Newtons.
 
  • #6
so -9.81e-6 N c = ( 9e9 N m^2 / c ^2 (q^2) ) / ( .04m ^ 2)

solve for q from there?
 
  • #7
nathancurtis11 said:
-9.81 Newtons.
How did you get this value? It is wrong.

Another issue: If you set those forces equal, they should have the same sign.
 
  • #8
I thought since the ball was stationary then the force by the lower ball on the upper ball had to be exactly equal to the force by gravity, therefore would be the gravitational constant of 9.81
 
  • #9
The gravitational constant is something different.
Check the equation for the gravitational force. And please do not forget units. They are exactly the right tool to avoid the mistakes you make.

In particular, do you expect all objects on Earth to have the same force? An elephant as much as a mouse? Do you see the issue?
 

Related to Physics 2 , two Styrofoam balls on insulated thread

1. How do the Styrofoam balls repel or attract each other?

The Styrofoam balls repel or attract each other due to the principles of electrostatics. The balls have opposite charges and according to Coulomb's Law, opposite charges attract each other while like charges repel. This is due to the redistribution of electric charges on the surface of the Styrofoam balls when they come into contact with each other.

2. Why are the Styrofoam balls suspended on insulated thread?

The Styrofoam balls are suspended on insulated thread in order to prevent any interference from external electric fields. Insulators, such as the thread, do not allow electric charges to flow through them, thus keeping the Styrofoam balls isolated from any external electric fields that could affect their behavior.

3. What factors affect the distance between the Styrofoam balls?

The distance between the Styrofoam balls is affected by the amount of charge on each ball, the length of the insulated thread, and the dielectric constant of the surrounding medium. The force of attraction or repulsion between the balls decreases as the distance between them increases, so these factors can impact the distance between the balls.

4. How can the movement of the Styrofoam balls be explained by the principles of physics?

The movement of the Styrofoam balls can be explained by the principles of electrostatics and Coulomb's Law. The balls have opposite charges that create an electric field between them, causing them to attract or repel each other. This force of attraction or repulsion causes the balls to move and interact with each other.

5. What other materials can be used to demonstrate the same principles as the Styrofoam balls?

Other materials that can be used to demonstrate the same principles as the Styrofoam balls include other insulators, such as plastic or rubber, and objects with different charges, such as balloons or magnets. These materials can also exhibit the effects of electrostatics and Coulomb's Law, allowing for similar demonstrations of the principles of physics.

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