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 134 – Ohm’s Law

Science 9 – Students used the PhET DC Circuit Construction Kit to explore the relationship between voltage and current.  I had them use the simulator because it’s difficult to get good data using our ammeters and voltmeters.  I found that it was too easy for kids to make a mistake, and then they get data that doesn’t reveal any patterns.

Even though in grade 9 the kids are not especially great at deriving equations from a linear relationship, they are able to see a pattern and relationship using a data table, and can use words to describe this relationship.

For example, with the table below once they’ve collected data for current using different voltages, they will see that the voltage is always 100x the current.  From this, most kids decide to write something like “resistance times current is voltage”

Resistor Size = 100 Ohm
Current	Voltage

Day 131 – Circuit Voting Peer Instruction

Science 9 – We did lots of Peer Instruction in class today, here are a few of the questions.  Students were able to make progress going from one question to the next, but as always is the case in my classes, it was difficult to get the kids to walk around the room and talk to people that were not their friends. At one point I had them walking around but they stayed in their groups…

Day 129 – Measuring Voltage, Current

Science 9 – The students finally started to take measurements on current and voltage today. They were glad to be rid of the compasses!  Reminder for next year if I teach science 9: buy new liquid filled compasses are are sensitive but not so jittery.

Students did a good job of reading the different scales on the voltmeters and ammeters. Of course, no one in the world uses these types of ammeters and voltmeters, everyone has and uses a digital multimeter.  But that’s what you get with public ed in BC.  No money to buy suitable equipment.

Day 127 – Circuit Analysis Tool

Science 9 – Here is a tool I showed the students to use for analyzing circuits. They color code wires in a circuit according to high (purple) and low (blue) pressure.  A wire has equal pressure at all points.  In order for a bulb to light, they know there must be a pressure difference across the bulb, because charges move from high pressure to low pressure.  Tracing and colour coding the wires lets the students see which bulbs have a pressure difference and which ones don’t. We then quickly checked the circuits in real life to see that they agreed.

Once a person gets more comfortable with looking at circuits and schematics, they can easily identify a short. However, this is a great bridge between what they they’re learning (pressure differences) and what they can do (predict bulbs lighting).

Day 125 – Water Analogy

Science 9 – Sometimes I worry if the students are not seeing the forest through the trees by working through all of the lab ideas.  Today I gave the students this as part of some practice.  Without having to read about the water analogy for circuits, all students were able to do this correctly.  Ok, it’s not the most difficult thing in the world but it gives me confidence that they are learning something.

Day 123 – Bulbs in Parallel

Science 9 – Students continued with their circuits. Today’s results were really surprising to everyone.  By adding another bulb in parallel, the bulbs didn’t go dimmer!  This was a shock to everyone.  I tried to help them to understand what was happening by using an analogy of blowing through one small straw vs blowing through three small straws at once.  Unfortunately I didn’t actually have the straws and many kids thought three small straws would be harder to blow through.

Note to self: have a box of stir stick straws in class next year.

Day 121 – Bulbs in Series

Science 9 – Students looked at how adding more bulbs in series affects the current of the circuit.  They figured this out quite well.  They then were given a reading and a worksheet to apply their new knowledge.

We went through the reading quickly and I pointed out to the students the parts where they should focus their attention.  I think some type of homework/reading quiz may be needed in the future to ensure that students review their readings.

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!