Mood Brightener: ...more from Stay Homas. (Confination II)
Your Conceptual Goals for this section: Charges
Experiments with Charge:
Having established that we have some understanding of the makeup of the physical world (Breadcrumbs: Chemistry) it is time to start with some basic experiences with static charge. What we are seeking to practice with these acivities is creating explanations for what we observe that are consistent with other mental models (frameworks) that we have about the world.
Magic Tape:
We will start with some activities using 'scotch tape'. If you decide to try this at home be aware that there are some kinds of tape that do not show the same behavior that we will observe here. I'm not entirely sure that I know what the differences actually are. The activities we will do are drawn from those which are descibed in this electrostatic activity guide. One caution I would have is that this guide tends to offer explanations based on a traditional physics understanding yet the real value to you as a student is to try and figure out how to explain what you see without external guidance.
Single Tape:
Exploring the behavior of a single tape we discover that it is repelled by similarly prepared tapes and attracted to almost everything. The tape can be neutralized (made to not respond) by touching it gently or breathing on it or dipping it in water. In this case I used a 'magic wand' made of braided copper wire. The tape can be made 'magic' again by sticking it to something and ripping it loose.
The summary of these experiences can be captured in two statements:
'like magic' objects appear to be repelled by each other.
no clear explanation why this 'magic' is attracted to all sorts of things.
Two Tapes: Top and Bottom
There is an unfortunate inconsistency in this demo but I wouldn't want to finesse it too much. When I pull up the two pieces of tape that are stacked there's no reason to expect it to act differently than a single piece of tape just thicker. I would have expected to see the two tapes repel each other initially. Wasn't worth doing the whole demo again and is an example that there are many factors at play when we do an experiment and we all can be unaware of hidden factors.
The result of this exploration is hopefully a clear experience with our basic qualitative expectation that 'like' charges will repel and 'unlike' charges attract. What is hard to demonstrate is whether there are just two kinds of charge or more than two? We will not explore this in any great depth but you might do well to think about how you might make an argument.
We did introduce Occam's Razor as a tool for choosing among possible explanations. When we are seeking to understand the world around us (one definition of science) then we will nearly always have multiple explanations for our observations. Occam's Razor (actually William's Razor) is an early version of KISS (keep it simple stupid). In this case we use it to explain why we accept that we only need two types of charge, and the absence of charge, to explain all the phenomena we see. We label these +, -, and neutral.
The summary of our experiments with two tapes might be
'like magic' objects still appear to be repelled by each other.
'top and bottom magic' are attracted to each other
neutral objects contain both 'top and bottom magic'
all forms of this 'magic' are attracted to all sorts of neutral things.
It is getting a little painful to keep refering to all of this as 'magic' but the challenge is that once we start calling it charge we all get a little too confident that we know what the word 'charge' means and that confidence is misplaced.
Insulators and Conductors:
While we're in the process of dividing things into two classes lets talk about insulators and conductors. One pragmatic way of defining these is based on whether it is safe to hold on to them and stick them in an outlet.
Conductors: Examples
We will solicit examples in class and see what you think. These will mostly be materials that have loosely bound outer electrons and occur in a particular part of the periodic table.
Insulators: Examples
Similarly we will explore examples of insulators and consider their chemical structure.
Conceptual Takeaway:
In the end the conceptually important thing is that solid conductors are ones where there are mobile electrons that are not bound in place. If the material holds it's electrons tightly it will be an insulator. ONLY the electrons move!
For liquids it's a different story. For liquids it depends whether material or contaminants in the liquid dissociate into ions. Water is an insulator but water with salt and minerals in it is pretty conductive. In a liquid both the positive ions and the negative ions can move (dragging around their nuclei as they go. There are also sometimes mobile electrons and protons as well. Liquids are complex....
Don't get me started on semi-conductors. Too many unprintable things I would say here so I won't...
Balloons and Electroscopes:
Electroscopes were created as a way to detect and then measure the charge on an object. We will use commercially available electroscopes for our experiments if we are in class but the video below will illustrate their basic function.
Experiments with the Electroscope:
[video demo here]
Balloons are a useful tool for generating charged objects (the balloon) because they are a little easier to work with than tape. We began by rubbing the balloon on our hair or clothing and bringing it near the sensing plate of the electroscope. We observed the deflection of the needle and that the needle would return to it's neutral position when the balloon was removed. We also noticed that if we brought a part of the balloon near the sensing plate that had not been charged that the needle would NOT deflect. This is consistent with our previous ideas that insulators do not permit charge to move.
We then explored, with pictures, why bringing a charged object near the sensing plate would lead to the observed motion of the needle. Our pictures indicated where there was excess + and - charge even though only the negative charges (electrons) are actually moving. Our pictures also explained why the needle returned to the neutral position when the balloon was removed. Be sure you are able to explain and illustrate this entire sequence of events.
Then we actually touched the balloon to the sensing plate and discovered that needle remained deflected even after the balloon had been removed. Again we discussed how our understanding of the movement of charges on a conductor could explain this.
[video demo induction]
Finally we explored a sequence of steps that started with the balloon and included Bruce touching the electroscope at one point and finally removing the balloon. The balloon had never touched the electroscope nor had any charged object (Bruce is very neutral!) touched the electroscope and yet it was charged. Hmmmm... Further experiments indicated the charge on the electroscope was the opposite of the charge on the balloon. We eventually worked out an explanation. This form of charging is called 'charging by induction'. Clear thinking about how charges move and distibute themselves is important for understanding this phenomena.
This video from Paul Anderson at Bozeman Science cover much of the ground in a little more directed sort of way.
Polarization:
There still remains that bothersome question of how/why charged objects are attracted to neutral objects. We have observed that charged objects (the tape and balloons) are attracted to neutral objects regardless of whether the neutral objects are insulators or conductors. There are important differences between how charge polarization takes place in insulators and conductors. Remember the freebody analysis we did for the behavior of the + and - charge in an insulator. Newton's 3rd Law suggests that the forces on the balloon are equally balanced and there is no excess force that can compensate for gravity. Apparently the charge distributions in atoms/molecules displace more than you would predict from a simple force analysis. I am looking for an appropriate discussion of polarizability that might be helpful but no luck so far:)
Here is a nice traditional discussion of polarization:
Assignment Breadcrumb Reading:
Occam and Razors
Who was Occam and what is his razor? Bb Test
Before Next Class:
Assignment HW: Bb Assignment
Pick up bits!
When you rub a plastic pen through your hair it will generate a static charge on the pen. With this charged pen you can now pick up little bits of (neutral, uncharged) paper (try it, it works!). Explain and illustrate why this happens. You can assume either sign of the unbalanced charge created on the pen although it tends to be negative due to the nature of plastics.
Assignment HW: Bb Assignment
Charge by Induction
You are given two (2) identical metal spheres on insulating stands. One sphere is already negatively charged while the other is neutral. Using only yourself (mind and body) and the spheres how can you give the neutral sphere a positive charge that is roughly equal in magnitude to the negative charge? Illustrate this process with multiple annotated sketches (think xkcd level illustration!).This is the process of charging by induction.
Looking Ahead:
Look ahead to the next Breadcrumb: Coulomb
Assignment Breadcrumb Reading: Bb Test
Coulomb's Law!
Two small spherical water drops have charges of -10 nC and a separation of 1.0 cm. What is the electrostatic force between them and how many excess electrons are there on each drop?
