Day 34 – Capacitors and Charge

Engineering Physics – As a re-introduction to dc circuits, I had students play with capacitors for a bit using some of my CASTLE materials. The highlight, as I expected, was when we discussed the conclusions from the above activity. About 1/2 of this class took enriched physics last year and wrote the AP Physics 1 exam. However, pretty much all students said that the charge originated in the battery. I then started to question their ideas and assumptions?

• What is the middle of a capacitor? An insulator they tell me.
• Can charge move through a capacitor? No, they tell me.

…

Hmmm…. lots of strained faces at this point. Then one student puts his hand up. “Are you saying that the battery pushes the charges into the capacitor and that the pressure in the capacitor pushes the charges on the other side, and that charges are everywhere in the wires?”

At this point I’m jumping up and down. “No! I didn’t say that! You did!”
Huge win for today.

Day 119 – Origin of Charges

Science 9 – Students used compasses and charging/discharging circuits to see where current flowed in a circuit.  This is a crucial lab to show that charges don’t originate in batteries.  Evidence for this is that charge flows on both sides of the capacitor and charges can’t get past the insulator in the capacitor.  As well, there is current when the circuit discharges without a battery.

This is such a great lab to do but it requires the right classroom culture.  Students have to want to find answers to the questions being asked of them.

Day 117 – Capacitors in Circuits

Science 9 – The above video is me going over a lab that the students had already performed.  Sometimes I go over them again quickly to make sure we all have the same consensus on what happened.

Using capacitors in high school science in BC generally is not done. However, we are using them not to learn about time variant current characteristics, but to analyze the nature of charges in a circuit.

The insulator in the capacitor breaks our original model of complete circuits. We get a working circuit, yet we know that charges aren’t crossing through the capacitor. As well, a discharging circuit will light a bulb, so charges can’t originate from the battery. In fact, as shown in the video we must be able to reason that charges are everywhere in the circuit because the compass rotates in all parts of the circuit no matter where the capacitor is.

There is a lot more to say about this topic, but that’s the general idea. These are great activities for the students because it challenges them to reason through logical inferences. Fantastic!

Day 115 – Soldering Capacitors

Science 9 – I made a video for students so they could try out soldering. The end goal here is for students to solder capacitors back to back. I want the students to do this because easily purchased large capacitors are polarized. If a student hooks up a polarized capacitor backwards, it can get fried. By soldering them back to back, we get a non-polar capacitor. The capacitance drops according to 1/Ct = 1/C + 1/C but this drop is ok. And, students enjoy learning a new hands-on skill.