The Payoff, April 2013

On Exploring RC Circuits and trying to figure out why the capacitor charges faster than it discharges

Student 1: “Is the charge time always the same as the discharge time?”

Me: “According to this model, it is, if the resistance and capacitance haven’t changed.”

Student 2: “I’ve got data where the charge time was short and the discharge time was long.”

Me: “Why would a reasonable teacher say something that contradicts your data?”

Student 3, excitedly: “What circuit was it?  Was there anything else in the circuit?”

Student 1: “I can’t remember what it was called — it had a resistor, a capacitor, and a diode.”

Student 2: “That’s it then!  The diode — it’s changing its resistance!”

Student 1: “Yes — it goes from acting like a short to acting like an open.  Thanks for bringing that up [Classmate’s Name] — I just answered a HUGE question from that lab!”

Student services counsellor who sat in for a day

“You’re challenging my whole idea about science.”

While exploring why capacitors act like more and more resistance as they charge

“Maybe the negative side of the cap is filling up with electrons, which means less capacitance.  According to the ‘tau model’, charge time = 5 * R * C.  So if the charge time never changes, and the capacitance is going down, then the resistance must be going up.”

[I’m excited about this because, although it shows a misunderstanding of the definition of capacitance, the student is tying together a lot of new ideas.  They are also using proportional reasoning and making sense of the story behind a formula. I need a better way to help students feel proud of things like this…]

Student critique of a Wikipedia page

“There’s some great begging the question, right there!”

Student analyzing the mistake in their thinking about a resistor-diode circuit

“I didn’t think of current not flowing at all during the negative alternation of the source.  This would mean that the direction of current through the resistor does not technically change.  I thought that if current was flowing through the resistor, it would change direction even if there is a very small amount of current flowing.  I did do a good job about thinking of the electrons already in the wires.”

One student’s feedback on another student’s paper

“I understand fully what you are trying to explain!”

On figuring out why a diode works

“If you make the connection to a wire, it’s like how copper atoms…”

“If it wasn’t doped, wouldn’t current flow in both directions?”

Students discussing a shake-to-charge flashlight they are designing

“In our rechargeable flashlight, if you put the switch in parallel with the diode, when it’s closed it will just short it out…”

Student who gave a recruiting presentation at a high school

“The day was a great step up for me that I never ever thought possible.  To be able to go back to the high school where I am pretty sure most had given up hope on me and see and hear them tell me how proud they are of me for where I am today is a feeling I will never forget.”

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