# Day 143: Da Da Da Dut da daaa— Charge! and Not so Hot Wheels

As explained yesterday, today was about practicing with the charge model. We were still WB’ing the practice sheet, but that went hand-in-hand with drawing a series of pictures for conduction and induction, and each student having an opportunity to charge an electroscope with each method:

General Physics:

Well, I really like the Hot Wheels activity for energy conservation… after all, who does not love to play with the little cars?  The issue for some today was that the kinetic energy at the bottom of the ramp, had a greater value than the gravitational energy at the op of ramp… not so Hot.  So is the error with the determination of the change in height or the velocity?

I think I figured out the issue.  I was using  a single photo gate with ‘One Gate Timing’ to get the instantaneous velocity of the car at the final height.  I’ve used a single photo gate successfully over and over and over and over …. it’s one of my favorite uses of the photogate. When I run the experiment, no issues, but when some of my generals did it, they must have measured the ‘flag’ incorrectly.  It is a narrow flag to give a better value for the instantaneous velocity, but even a millimeter off makes a difference when the energy values are so small to start with.  Tomorrow I will try a different photogate, one with no flag needed… a Bee-Spy photogate.  I am pretty sure the cars will fit through.  Stay tuned…

# Day 141: Harry Potter Visits NHS (or an intro to electrostatics)

Yeah, I know it is a pretty big jump…. from the momentum model (including impulse) to electrostatics. I’ve never been really pleased, but have never taken the jump to put momentum in somewhere else.  I guess just more to ponder.  Anyway, today we white-boarded ‘The 7 Charge Questions’.  It is a worksheet I out together to build the essential aspects of charge. It’s nothing fancy mind you.  It just gets across the info I want the students to understand about charge (symbol,units), how objects get charged (friction, conduction, induction), how objects get uncharged (grounding), that there are only 2 types of charge (+ and -), how charged objects interact (likes repel, opposites attract, and that implies a force is exerted between the objects, that charge is mobile and can be transferred (both + and -), oh yeah, and what polarization is.  All of this helps us form a model of what and how it behaves.

It’s interesting that many students just want to define charge as being when thee are more or less electrons.  But then I ask them what an electron is.  They politely regurgitate ‘a negatively charged particle’.  But wait… charge is when there is more or less charged particles?  Then they see there is a bit more to it.

After discussing for a while, we finally do a demonstration to bring many of the aspects of the charge model together:

What you do not see is the way I introduced it… I did the demo first, dressed with Harry Potter glasses and a cape.  The Harry Potter Halloween theme was playing on repeat the entire time.

VIDEO DISCLAIMER: The young lady in the last clip is from two years ago.. it’s just a nice longer clip and she is an awesome daughter (err, I mean  student).

For the next class, the students were given a practice sheet. Again, nothing too incredible, but they do have to draw a series (at least three) diagrams to illustrate the process of induction and the process of conduction.  In the next class meeting, they will also have an opportunity to demonstrate mastery by charging an electroscope using induction or conduction.

General Physics:

This class period was spent white-boarding a series of COE problems.  I also used it as an opportunity to do two things. (1) Ask some conceptual questions about the given situation (if the mass had been twice as large…), and (2) show them the power of substituting equations FIRST before putting values in.  For example:

In this case, Eelas = Eg.  So rather than solving for elastic energy, then setting it equal to the gravitational, then using that value to solve for the height., simply substitute in the equations:  1/2kΔx^2 = mgh, now solve for h.  Using this approach makes it soooooo much easier to answer the conceptual questions like what is the mass had been twice as large….