DAY 1 -
|Totally makes sense now.|
We started by reading a very convoluted article that treated complicated topics in E&M as if they were really quite elementary and simple. As a class we pretty much disagreed with that point! Particular treatment was given to the use of analogies in physics in particular the analogy of electricity as water flowing in a hose.
We also had a lengthy discussion on the word "model". Model has a multitude of definitions that get bandied about in science education all the time. We talked for a very long time without getting to any real conclusion. I like the idea that a model is just something that exists in the mind of the student that helps them make sense of the world. That is model with a lowercase 'm'. The models that we build in modeling instruction are Models with an uppercase 'M'. That way we can distinguish if we are talking about a simple model or a complex one with many representations that give us a full picture of a particular phenomenon.
After that we actually started with learning about charges and entered into student mode. In student mode we try and put ourselves into the mindset of our students. What would they know going into the class? What questions would they have? What would they be thinking? It is much harder to do than many would realize.
Laura rubbed a balloon on her hair and then stuck the balloon to the ceiling. We had to draw a force diagram for the forces on the balloon. Our original diagram looked like this:
We knew the balloon wasn't accelerating so the forces had to be balanced. But many others had a third force with the ceiling pushing down. We tried standing on chairs and looking to see if the balloon was deformed at the top but we couldn't really tell. Then Laura pulled the balloon down about 4 inches and let it go and it jumped back up to the ceiling!! Woah! So after that we discussed that the force upwards must be bigger than the force of the Earth pulling down so we made the change and added yet another ceiling force this time pushing down so that the forces were balanced.
The end result looked like this:
But how can the ceiling pull up and also push down? Since the balloon was pulled up without the ceiling touching it we concluded it must be some type of non-contact force. Some of my fellow students and I decided it must be some sort of static electricity force from the rubbing of the balloon. From prior knowledge we know this is most likely caused by the transfer of electrons from Laura's hair to the balloon or from the balloon to Laura's hair but we couldn't figure out a way to tell outside of just guessing. We also had some additional observations that as of yet are unexplained. The balloon only stuck to ceiling if the exact are Laura rubbed was placed near the ceiling. If we pointed the opposite side to the ceiling it did not stick. So it appears whatever is transferring stays in one location. Lastly, we didn't do anything to the ceiling so I'm not sure why're balloon stick to it and when we tried a balloon that was not rubbed it would not stick no matter what.
Hoping to answer some of these questions over the course of the unit, Laura gave us a tool to use that might help us. We did an activity with Scotch tape. We had to layer 3 pieces of tape on the table like this:
We made two of these set-ups but always left the last piece on the table in case we needed to reset the other tapes. The 2 pieces on top were labeled top and bottom. When we pulled off the two top pieces and held them together they repelled each other! The closer they got together the more intensely they seemed to repel. Then we stuck the tapes back down on the stack. The second time we pulled the top and bottom tapes off the stack at the same time. Then we separated the tapes in the air. The top tapes still repelled each other but they attracted the bottom tapes. When two bottom tapes were brought together they repelled as well. (Pro-tip: when you first pull the tapes off of the table run your fingers down them before separating the two tapes. This makes them work better) Then a curious thing happened. Don had given us some strips of paper and Al foil. Both of those were attracted to the top AND the bottom tapes. This seemed weird because the paper and foil did not attract each other at all, so it seemed as if they had no charge.
After some discussion we decided that like charges seem to repel each other since tops repel tops and bottoms repel bottoms. And then opposite charged objects attract each other but so do a charged object and a neutral object. So the only way we can really know the charge of something is if they repel. If this happens we can 100% know the charges are the same.
To try and figure out what was happening in the neutral objects attracting we zoomed into the tapes on a micro level and imagined the plus and minus charges of atoms. The pluses are in the nucleus and can't move, but the minus charges are zooming around the outside. SO when a plus charged object comes near a neutral one, the pluses in the charged object push on the electrons which depending on the material can move a little to a lot.
You can see in the image from a pHet sim we used. The minus charges in the wall bend away from the charged balloon leaving more pluses behind which can't move. So the surface of the wall is more positively charged and so the opposite charged balloon sticks to it. (This sim has a slight issue. In our experiences the charged side of the balloon has to face the wall in order for this effect to work. The opposite side from where we charged the balloon will not stick. Charges don't easily move on an insulator which the rubber of the balloon apparently is. Conductors like metal can allow electrons to move freely.) The opposite effect happens if you bring a positive charged object toward a neutral object. In the picture if the balloon had a positive charge on it the electrons would move toward the balloon leaving a net negative charge on the surface of the wall and they would attract. We named this phenomenon Polarization.
I have more yet to discuss but this post is getting very long so I will make a part 2!
|pHet Simulation Balloons and Static Electricity|
But we still don't know what charge the tapes are. Is top a + charge? A - charge? Well, apparently, it is well-known that a glass rod rubbed with silk develops a + charge. So we used that to determine charges of things that we didn't know. Results are listed in the next picture:
R stands for repel and A attract. Remember that only repelling tells us anything for sure so looking at the chart, top tape and glass are the same charge (positive), and bottom tape and plastic are the same charge (negative). The silk didn't really seem to react to anything, but since the glass became positive, the silk had to have become negative. This is due to the idea of conservation of charge. If the glass is becoming positively charged that means that negative charges are being removed from it. They have to go somewhere and the only logical place would be on the item rubbing against it. So the rubber balloon became negative (repelled the bottom tape in testing) which means the fur or hair (human fur) must take on a positive charge.
We then played with Fun Fly sticks which through testing we discovered develop a positive charge on the end when you push a button. No clue how it works. We also used foam boards with rabbit fur to charge them and pie plates with a styrofoam cup to create an insulated handle. Last we had a pith ball hanging from a thread on an apparatus that Don called an electrophorus. A pith ball is a very light ball about the size of a marble. It has a silvery coating on it and is fairly easy to charge and discharge.
When you hold a charged object near the pith ball it attracts, but as soon as it touches the object it leaps away and then is repelled by it. We concluded that the pith ball must have had charges transferred to or from it through the contact. It didn't have a charge (attracted by polarization) then it did after it touched (same as the charged object that touched it.) To discharge the pith ball you touch it with your finger. We called this grounding. Evidently you can do this all day and not build up a charge yourself. We apparently have LOTS of electrons that we can donate or accept without changing our overall charge. Whereas the pith ball has very little mass and so the same number of electrons transferred is a large percentage of its overal charge.
We then did a weird thing with the pie plate. We charged a foam board and brought the pan close to, but not touching the foam. Then we touched the side of the pie pan and got a small shock. We removed the pan away from the foam and, upon testing, the pie pan had a positive charge! Then we touched the pan and again it gave a little shock and went back to being neutral. You could do this over and over and the foam didn't seem to ever lose charge. We drew out some representations of what we thought were happening to the charges and after discussion came up with this idea:
In the top picture, the pith ball is neutral (equal + and -). The positively charged fun fly stick is brought near it and then we touch the pith ball. Since the ball is near a positive charged object the - charges in the ball are attracted and move toward the fly stick side of the ball. This leaves a positively charged area on the backside of the ball. As my finger got near to the pith ball the same thing happens again in my finger. The - charges are attracted to the tip of my finger. When I touch the ball some of those - charges move onto the ball because they are attracted by the positive charges near the surface and repelled by the negatives in the tip of my finger. When I remove my finger and the fun fly stick, the ball now has an excess of - charges so is negatively charged. You can do this in reverse if you bring the balloon (negative charge) near the pith ball. The charges leave the ball and go into my finger instead of the other way around.
I have more yet to discuss but this post is getting very long so I will make a part 2!