## Saturday, July 16, 2016

### Unit 1 Work Product (part 2) - 2016 E&M Modeling Instruction Workshop

To recap, go back and read the last post!

After the discovery we began some of our deployments. The idea of conductors vs. Insulators came up. Especially on these problems:

After this we came back to the pith ball and did an experiment. We hung the pith ball from 2 strings and wanted to see if there was a relationship between the distance two equally charged objects were brought together and the force between them. We charged the pith on a string and another pith on a stick with a fun fly stick. We then touched them together to be sure the charges on them were equal. We then slowly brought the stick pith toward the hanging pith. Since we knew the mass we could draw a force diagram to help us calculate the force of repulsion between the piths. See below.
The video of us taking data can be seen here. We used video and freeze frame to take our measurements. When we processed our data we found that the force seemed to increase at a square of the amount the distance decreased.
We then discussed the experiment. Laura asked if only 1 electron transferred. Probably not. Probably millions. Which makes talking about charge difficult. It's like asking someone how far their Grandma lives away from them in inches! So scientists made up a unit for charge called the Coulomb. A coulomb is the charge of 6.25x10^18 electrons. Which means the charge of one proton or electron is 1.6x10^-19 C.

After that we went back to the lab and came up with a new equation:
Since the charge signs don't really match our idea of the signs we use for forces we decided to make it an absolute value. We also weren't sure if the charges should be multiplied or added at first. I looked at it like this:

Since each charge acts on each other charge it is a multiplication problem. (Top drawing has 4 charges on left and 2 charges on right. Each charge on the left feels a force from each charge on the right and vice versa. A weird thing though. In the bottom drawing where 6 charges are separated equally, 3 and 3, There is a repulsion of 9 arrows, where in the top drawing there is only 8 arrows. This is very interesting to me and I will need to think on it more. This has to be why Don had us touch the two pith balls together in the lab before taking measurements.)

Lastly our formula needs a proportionality constant because most equations need one. It is called k for obvious reasons (/sarcasm) and it's units are weird and make very little sense as an idea unlike many of the constants we have encountered thus far in physics.

The whole equation in all is called Coulomb's Law. It looks very similar to the Universal law of Gravitation. Wonder if that means something?