Day 67: Capacitors in Series and Parallel in RC Circuits

Today I was off-campus for professional development related to our district’s Digital Learning Initiative. Given that we ran short on time yesterday, I gave them a more detailed than normal purpose for today’s lab: “Determine the effect on the circuit of adding a second capacitor in series or in parallel (half the groups add in series; half in parallel) to an RC circuit. Specifically, graphically and mathematically model the relationship between voltage across the capacitor(s) and time as the capacitor(s) discharge. Extract the time constant from this mathematical model and determine how the equivalent capacitance changes with the addition of the second capacitor.” I left them a demo circuit as well.

Day 67: Capacitors in Series and Parallel in RC Circuits

No frantic emails or messages were sent to me during class, which is a good sign. They left whiteboards summarizing their conclusions in the classroom; I’ll take a look tomorrow during our institute day and see how it went!

  ##circuits ##paradigmlab  

Day 66: RC Circuits

Today, I introduced RC circuits by displaying the voltage across the capacitor as it charges and discharges. I share that when first-year physics students describe a graphical relationship as “exponential” they usually mean “quadratic.” Today, however, we can use that label correctly! The curve fit for an RC circuit is always fantastic.

Day 66: RC Circuits

Unfortunately, We ran short on time today, and students don’t have a strong understanding of what the time constant really means. I won’t be in the classroom tomorrow; so, we’ll see how they do on the activity which is to determine how adding a second capacitor in series or parallel to the first affects the behavior of the RC circuit and how we can model the equivalent capacitance.

I still need to finalize the lab practicum for this unit, but I want something involving RC circuits, time constants, and flashing LEDs!

  ##circuits  

Day 53: Bike Wheel Direct Measurement Video

Day 53: Bike Wheel Direct Measurement Video

AP Physics C: I had to leave early today, so AP Physics students practiced their rotational motion problem solving using this direct measurement video. Click picture above to view video (QuickTime).

Here’s the lab handout. (It is focused on rotational kinematics and energy because we haven’t done torque and Newton’s Second Law for Rotation yet.): LAB DMV Bike Wheel Energy 2015 (word)

NGSS Science and Engineering Practices:
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking

Day 53: Bike Wheel Direct Measurement Video

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Day 65: Constant Current Source Circuit Activity

I wanted my AP Physics 2 students to review the behavior of resistors in series and in parallel but from a new perspective. Inspired by the article “Experimenting with constant current and voltage sources” in The Physics Teacher, I provided students with solar panels instead of batteries and had them investigate how the voltage across and the current through the source changes when a second resistor is added in series or in parallel. I told them upfront that the solar cells were constant current devices. I should have left that unsaid and let them come to that conclusion on their own. Regardless, I hope that this introduction to solar cells will provide a hook when discussing the photoelectric effect next semester in our quantum unit.

Day 65: Constant Current Source Circuit Activity

The solar panels only function as constant current sources for a range of voltages and resistive loads. I need to investigate the best combination of light intensity and resistors to improve the outcome of this lab. Those groups investigating the effect of adding a resistor in parallel had good results, but the groups investigating the effect of adding a resistor in series did not.

  ##circuits ##setbacks ##paradigmlab  

Day 64: Circuits Peer Instruction

Even after a previous physics course with a fantastic focus on conceptual understanding of circuits, my AP Physics 2 students still struggle when reasoning through some scenarios. Today, I posed the following question:

Day 64: Circuits Peer Instruction

Students agreed that bulb A would again be brighter. They were also able to justify their answer by stating that the current through the bulbs is the same and the resistance of the bulbs is different. What threw them was when I asked which bulb had the greater resistance.

  ##circuits ##peerinstruction