This experiment posed two hypothesis:
1. The interaction between objects that have been rubbed is due to a property of matter that we will call charge. There are two types of electrical charge that we will call, for the sake of convenience, positive charge and negative charge.
2. Charge moves readily on certain materials, known as conductors, and not on others, known as insulators. In general, metals are good conductors, while glass, rubber, and plastic tend to be insulators.
To test hypothesis 1 we did an experiment using scotch tape
First we took two pieces of tape and attached them to the table. After pulling the two pieces off and putting them next to each other, with the non-sticky side of the tape facing each other, we noticed that the tape would repel from each other. Also, as the distance between the tape became smaller, the repulsion seemed to go up.
Then, we put two pieces of tape on the table and labeled them "B" for bottom and another two pieces, labeled "T" for top, over B. When we peeled B off of the table and T off of B we found that the two Bs and the two Ts were repelled by each other, but a T and a B were attracted to each other.
From these experiments we concluded that the hypothesis was true because for the Bs, as well as the Ts, the procedure to charge them was the same, and so they must have the same charge. However, the procedure to charge B and to charge T was different and so we got a different result when putting B and T close together. If there were more than two charges there would have been more than two results for the two procedures of charging B and T
Experiment 2: Charging Styrofoam Balls with Rods
This experiment was used to give us more experience studying the interactions between charged objects
First we charged the styrofoam balls by touching them to a plastic rod, that had been charged by rubbing it against fur. When we brought the plastic rod near the balls a second time the balls were repelled by the rod because both the rod and the styrofoam balls had the same charge.
Then we charged a glass "rod" with polyester, giving it a positive charge. When the glass rod was brought close to the styrofoam balls the balls were attracted to the glass because the balls were still negatively charged from the plastic rod.
To test hypothesis 2 we did an experiment using induction to charge objects
Experiment 3: Insulators, Conductors, and Induction
First, we charged a plastic rod with fur and brought it close to an uncharged styrofoam ball. The ball and the rod had no interaction. We then switched the styrofoam ball for another metal coated styrofoam ball, which was also uncharged; when the charged plastic rod was brought close to the uncharged metal coated ball the ball became attracted to the rod. We tried this same process with a metal sphere that was uncharged and had the same result.
These results lead us to the conclusion that the styrofoam is an "insulator" while the metal is a "conductor." The metal, having the ability to move or spread charge more easily, was able to be charged by induction from the rod. So the neutral charge in the metal, equal positive and equal negative, was able to move, or be polarized when the rod came near it. So the negative charge of the rod caused the positive charge in the metal to "move" to the side closest to the rod. This is why the metal objects were attracted to the rod even though they had not been charged by touching the rod to them. The styrofoam does not have this ability, and therefore that is why there was no reaction between the uncharged styrofoam and the charged rod.
I don't remember exactly when but we did define:
1. electric charge: the amount of matter that interacts electrically in an object
- an intrinsic property that defines how it interacts electrically
- Mason made the point that charge is to electricity and magnetism what mass is to forces and basically everything learned in 4A
2. coulomb: the basic SI unit for charge which is denoted by C.
Because of the humidity in the air we could not perform the experiment for Coulomb's Law, so instead we used a physics program to examine the effect of distance on the force between two point charges.
In the program we set up two point charges with the same charge and like signs, which would cause them to repel from each other. We then had the program give us a graph of the acceleration for each point charge because the force by, Newton's Second Law, is proportional to the acceleration times a proportionality constant, which is mass.
When then examined the graphs for different situations:
We noticed that when we doubled the magnitude of the charge on one particle the the force on both particles becomes double. We therefore concluded that the force is proportional to the product of the charges.
We also noticed that when we increased the distance between the two, the force became less and the graphs for the acceleration, and therefore the force, became 1/(x^2). From this we concluded that the force is proportional to 1/(distance^2).
Therefore, Coulomb's Law is:
F = k(q1q2)/(r^2)
Where k is the proportionality constant = 9.0E9 (N x m^2)/C^2
and r is the between the point charges
We then looked at Coulomb's Law as a vector quantity by examining the direction of the unit vector.
The unit vector is a vector that has a magnitude of one and gives the direction of the force. In the activity two point charges q1 and q2 are at a distance d from each other. We then examined how the direction of the unit vector changes, the direction of the force, changes when the signs of q1 and q2 are changed.
-x <------------q1----------------------q2---------------> +x
When q1 is positive and q2 is negative, when the charges are opposite:
the unit vector of q1, which denotes the direction of the F2 on 1 ,points in the -x direction, while the unit vector of q2, which denotes the direction of F1 on 2 points in the +x direction
F2 on 1 <----------q1 q2--------------> F1 on 2
When both charges are positive or both are negative, they have like charges:
the unit vector of q1 points in the +x direction, while the unit vector of q2 points in the -x direction
q1------------> F2 on 1 F1 on 2<--------------q2
BUT, forces can move along their line of action so we can redraw the last picture like this:
F1 on 2<------------q1 q2------------->F2 on 1
Therefore, no matter what the charge combination, like or opposite, the unit vectors will always point AWAY from two point charges.
We then tested our skills by calculating the force on two point charges by using trig and then by using unit vectors and discovered that unit vectors are easier.
bam <----there Joy are you happy?
P.S. Joy smells; Dante rocks
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