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Day 161: Modified Bike Light Generator Presentation

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Conceptual Physics: Students gave their presentations for their modified bike light generator project.

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Day 151: Build Phase (Modified Bike Light Circuit)

IMAG2201Conceptual Physics: Groups started building their modified bike light circuits based on their designs from yesterday. (Nobody said it had to look pretty!)

College-Prep Physics: Reflected upon the video we saw yesterday showing wave interference in the pond (areas of rippling water and still water).

  • What do you observe?
  • Come up with 3 possible explanations for what we observe.
  • Shared explanations.
  • Show students long slinky and send pulses along it.
  • Use the slinky to design an experiment to test the “canceling out” explanation.

AP Physics C: Began reading Tom Stoppard’s Arcadia, an amazing play about love, physics, and mathematics.

Day 150: Design Phase (Modified Bike Light Circuit)

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IMAG2187-1Conceptual Physics: Students began drafting the initial design for their modified bike light generator circuits. It’s really a combination of 3 circuits:

  • A generator-battery circuit that recharges the battery while pedaling.
  • A generator-bulb circuit that lights the headlight and taillight while pedaling.
  • A battery-bulb circuit that lights the headlight and taillight when not pedaling.

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College-Prep Physics: Double slit with laser pointer demo to begin wave model of light. Knowing how light behaves will help use decode the light we observe from stars and galaxies. Used this slide that has multiple slits and gratings on it:

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It’s really old. Here’s what it says at the top (click to embiggen):

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Also showed this AWESOME movie of interference in a pond:

AP Physics C: Today was the AP exam. One student showed up to class this morning, even though the exam didn’t start until noon.

Day 148: Clicker Review Game

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Conceptual Physics:
 We played a review game using clickers from Eduware. We chose Eduware clickers because they have test banks of old Regents Exam questions and come with several different games (Jeopardy, Racing, and Tug-of-War). That’s really the only good things about the system. Using it as a traditional clicker system with Powerpoint is awkward, as is generating questions on the fly. Even though our school dropped the giving the Regents Exam in physics, I still use the software and clickers from time to time.

College-Prep Physics: Students determined the relationship between angular size and size/distance ratio. Handout: A Handy Measuring Tool Part 2 Angular Size

AP Physics C: Mechanics multiple choice review.

Day 147: Watts in the Box?

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Conceptual Physics: Students attempted to figure out the wiring inside 8 different mystery circuit boxes by unscrewing the 4 light bulbs for each box. (Well, really 7 because one box needs repair.) I built them a number of years ago with a lot of help from my dad. If you’re not very handy, you can rewire bar light fixtures purchased from a hardware or lighting store. (See: Mystery Circuit Box and Make a Mystery Circuit with a Bar Light Fixture.) I used 15-watt bulbs that have a standard base (clear glass whenever possible). They are a bit more expensive, but they never get too hot to touch. Students enjoyed solving these circuit puzzles!

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College-Prep Physics: After graphing the data from yesterday’s activity, we saw that when objects have the same apparent size, they also have the same size/distance ratio. We used this concept to create a “handy tool” for determine the size or distance to unknown objects. Students were challenged to determine the distance to the doors at the end of the hallway and to determine the height of the flagpole. In the picture above, the width of my pinky finger held at arms length is the same apparent size as the height of the doorway at the end of the hall. How far away is the doorway? What assumptions are you making? Handout: A Handy Measuring Tool 2013

AP Physics C: No class. All students were taking the AP Calculus exam today.

Day 144: What is the Bike Light Circuit?

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Conceptual Physics: So far we’ve looked at what’s inside the bike light generator, how it produces an alternating current, and how a diode can be used to convert alternating current into direct current in order to recharge a battery. But we have not yet looked at the actual bike light circuit — how are the generator, headlight, and taillight wired together?

What’s confusing is that there’s just one wire that runs from the generator to the headlight (gray wire, pictured above) and another one that runs from the generator to the taillight (black wire, pictured above). This contradicts what we know about complete circuits. So where is the return wire from the light back to the generator?

To figure this out, we looked a disassembled headlight and taillight. I asked the students how we could wire one of the lights to a battery pack so we could “find” the return wire:

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As you can see in the picture, the metal light mounting bracket is the return. On the bike, the light bracket is connected to the bike frame. The bike frame is “return wire” that connects back to the generator. The generator is also connected to the bike frame via a mounting bracket. Then we sketched the complete current-carrying path for the headlight and taillight:

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We also noticed that when the headlight was disconnected from the generator, the taillight still lit up, and vice-versa. Students were then challenged to design and test (using PhET) a circuit with a battery and two bulbs that behaved the same way.

College-Prep Physics: Work day to tie up loose ends on WebAssign and lab work.

AP Physics C: Faraday’s Law problems.

Day 137: What Is A Diode?

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Conceptual Physics: Here a bike light generator is connected to a voltage probe in Logger Pro. The red graph shows what we saw yesterday — the voltage cycles positive and negative (AC). The blue graph shows what happens when a diode is added to the circuit. Diodes will be important later when we use the bike generator as a battery charger.

College-Prep Physics: Based on students’ work to the M&M (light collection) lab and the properties of light activity from prior days, I decided to do a review of mathematical relationships. The handout is here: MATH RELATIONSHIPS