Day 161: Circuits ILD Continued

Advanced Physics:

The students started the hour by completing a formative assessment on determining the equivalent resistance of a circuit containing eight resistors.  Here it is:

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It really was formative… nearly every student (well, maybe except 3 or 4 in each class) totally nailed it… no need to re-assess or re-teach. For those that need it, I posted some practice on Schoology.   Love formative assessment.

The remaining time was spent working on the Circuits ILD. We completed the circuit that asked the students to predict the size of the reading on 3 voltmeters across three resistors in series (R1=R2>R3).  Most groups predicted ΔV1>ΔV2>ΔV3, because of the order they appeared in the circuit.  A bit disappointing because we have worked on drawing electric potential diagrams.  Once the groups saw the results, they quickly realized that the location had nothing to do with the value… I’ll have to find a way next year to try to address this as we practice with the EPD’s… maybe just asking about changing the location of bulbs in Bulb Boy.

We also looked at parallel circuits:

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This one was for the same two resistors that were wired in series in an earlier circuit.  I like this photo because it shows good the data is… just wish I had the circuit a bit more scrunched in so you could compare the brightness like the students did in class.  Speaking of students, here are two shots of the students preparing predictions:

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Tomorrow we will work with three resistors in parallel and add the voltmeters back in.  I still have not given them the ‘official junction rule’ yet… they have only worked with two parallel circuits.  They have formed some ideas as to how parallel circuits behave and tomorrow we will do one more. or maybe two before we discuss and I let them throw the rule out to me.

General Physics:

They learned about resistances in series and parallel  and completed the same sample problem the advanced class did.  I like it when the generals do the same thing as the advanced… it gives them some confidence.

 

 

Day 160: Circuits ILD and WB the Mother Part 2

Advanced Physics:

After discussing the schematic and electric potential diagram assessment, we started a an activity that will last two or more days.  It is a Circuits ILD. An ILD is an Interactive Lecture Demo. The instructional goals of the activity are to establish the relationships for current in series and parallel circuits. You know … current is constant in a series circuit (It=I1=I2=I3…) and the ‘junction rule’ for parallel circuits (It=I1+I2+I3..).

ILD’s are not really a new concept. I think this topic lends itself very well to an ILD.  The Adv Phys Circuits ILD is a way for me to help the students learn the concepts without resorting to a lecture or a purely inquiry investigation. I have also found it really helps bring some common misconceptions about how current behaves to light (no pun intended this time). For example, take a look at circuit #1, many student think the current is the same because the resistors are the same size OR that the reading on A1 is the largest because it is the first one and the current has not “gone through” any resistors yet, making A3 the smallest because it has gone through two resistors.  To check the first belief, circuit #2 adds a resistor and changes the value.

A really important aspect of the ILD (in my mind) is to actually check the values the students are predicting so they can see the results first hand.

So, here are some specifics about the set up and how I run it in my classes.  The resistors in the ILD are bundles of christmas tree lights. Why these lights?  Well, they are cheap and plentiful and they provide a visual for the students as we check them. The source is simply a 9v battery. Here is a picture of one of the circuits set up and ready to check:

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In class, I put the students into groups of three based on the results of some recent assessments. I put a green, a yellow and a red (see this post of the info on these colors) if possible.  I hand out each circuit one at a time and have the groups discuss and make predictions on a big WB. When all the groups are ready, we hang the boards, discuss a bit then actually check the values as shown above.   I do not explain the ‘why’ this is the answer until after a few circuits have been worked through.  I want the students to come up with some ideas to base the predictions on, then allow them to check it with the next circuit.  If I sense that things are going south or the students are getting really frustrated because they are not catching on, then I might provide some small group ‘hints’ as I am walking around.  Some years we even turn it into a competition.. wich group gets the most predictions correct.  More tomorrow about the remaining circuits.

General Physics:

Today was the WB’ing of the Ohm’s Law experiment. We followed the same basic process with this group that we did with the advanced group.  There was one slight change… to see if the material makes a difference, I had the students use the DMM to check the resistance of the ‘sister spool’ from a different set of spools.  The set we used in the experiment was Ni-Cr, and the other (older) set is NS.

Day 159: Determining Equivalent Reisistance and The Mother of Ohm’s Law Labs take 2

Advanced Physics:

So it is essentially the last day of AP testing  that will decimate my morning classes.  Today the students took a schematic diagram and electric potential difference assessment:  Here it is:

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I think it is important for students to be able to create and interpret schematic diagrams… somewhat like force diagrams.

We then completed a group practice problem solving for the equivalent resistance.  Here it is:

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One of my goals here is not to make all my students experts, but to continue to practice their higher order thinking skills and problem solving skills.  The next step will be to insert some meters (both ammeters and voltmeters) and ask the students to solve for the values the meters will show.  The remaining time was small group work time with me answering questions along the way.

General Physics:

This crew also completed an assessment on drawing schematic diagrams (no EPD’s for this group).  Here si the one they took:

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After this was completed they gathered data for the Ohm’s Law experiment using the spools described in the last few posts. Tomorrow we will WB it.

Day 158: R in S and P

Advanced Physics:

That stands for Resistances in Series and Parallel.  This is usually one of my favorite experiments.  It is easy to gather the data and the results are always spot on. The experiment graphically develops the equations for resistance sin series (Rt=R1+R2+R3…) and parallel (1/Rt = 1/R1 + 1/R2 + ….) But… this year, thee was just no time to do it.  Here is the way we usually completed it.  It is patterned after an article i read in The Science Teacher a number of years ago.

We use a set of about a dozen fixed resistors with an ohmmeter and a Pasco Circuit board.  Very simply, the students measure the resistance value of each individual resistor, the pick one to hold constant. They wire the remaining resistors (one at a time) in series and measure the total resistance. They plot total resistance vs. resistor 2.  Here is a picture and some sample data:

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And the parallel…

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But, as mentioned, not time left to do it this year, so we just developed it qualitatively with the spools:

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After a schematic circuit diagram assessment, we will practice simplifying a complex circuit, .  More on that tomorrow.

General Physics:

We finished the Mystery Boxes activity and then practiced a bit for a schematic diagram assessment tomorrow also.  We then had a short pre-lab discussion for the Ohm’s Law experiment.

Day 157: WB’ing The Mother of all Ohms Law Labs and This one is a Mystery

Advanced Physics:

Today, we WB’ed the Mother of All Ohm’s Law experiments.  It’s such a simple experiment that is so incredibly rich.  Here is some sample data:

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As one can see, the two spools only have a different gauge (which we know relates to the cross-sectional area).  There are also spools in the set that have the same gauge but different lengths.  Here are the concepts we developed as we WB’ed this experiment:

1.  Resistance (of course): It is the slope of the linear graph. We build it from looking at the units on the slope; volts/1.0 amp. So it is the electrical potential difference that gives a current of 1.0A for each spool. We discuss how with the two spools there is ‘something’ opposing the flow of charge, especially for the thinner, #30 gauge wire. This is seen because a greater potential difference is needed for the same current when compared to the #28 gauge spool. The ‘something’ that is opposing the flow of charge is defined as electrical resistance.

2.  Ohm’s Law — The EOL for the linear graph.

3.  Using a DMM — We debate which group was correct with the slope value (each spool was done independently by two groups).  Eventually a student with some background will suggest measuring it with an ohmmeter. So we learn how to use the DMM.

4. The relationship between the thickness of the wire and the resistance (slope).  This is easily seen from the data. It shows a thinner wire will have more resistance that a shorter wire (of the same material and length).

5.  The relationship between the length of the wire and the resistance.  Again easily seen from the class data.  The longer the wire the greater the resistance.  I wish I had snapped a picture of the board that displayed the 80cm and 160 cm spools, almost exactly 1/2 the resistance for the 80cm spool.

6.  Using the above two relationships, it is a short transition to how the material affects the resistance.  Through discussion, we develop the concept of resistivity.  One could use the spools to experimentally develop this concept, if one had the time.

7.  Electrical Power– I ask the student to predict the shape of the graph of potential difference vs current graph if I had assigned each group a given potential difference that could not be changed, and they measured the current through the various spools.  The students do not have much trouble predicting a horizontal graph (well the ‘could not be changed’ gives it away).  The area trapped is shown (with units) to be the electrical power, P=ΔV*I. We then use some substitution to write the other common models for electrical power.

And there you have it… the mother of all Ohm’s Law experiments.

General Physics:

We started by using the Light Bar demo discussed in a previous post, but then move on to an activity that provides more practice on determining circuit types and schematic diagrams based on observations (data).  Here are two short clips from the activity:

The Mystery Boxes were constructed by a colleague that used to teach the Advanced Physics course. He made two sets of seven boxes. I really, really like them!  I have also used them as an assessment… pick a box, gather some data, draw the schematic and the EPD.

Day 156: The Mother of All Ohm’s Law Experiments

Advanced Physics:

Nice title as I write this on Mother’s Day!  The experiment described below really is the mother of all Ohm’s Law experiments.  It is done in the modeling style and develops no less that five different concepts.  The very simple purpose is to determine the relationship between the electric potential difference and current in a simple circuit.  The circuit consists of a power supply, an ammeter, a voltmeter and a spool of wire:

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The spool of wire is the ‘load’ in the circuit.  One HUGE SAFETY ITEM…. do not leave the circuit connected for more than a few seconds as the meters are read.  The spools will melt.

The spools are in various lengths (80cm, 120cm, 160cm, and 200 cm,) with all the same gauge and material.  There is also a spool that is 200cm and a different gauge.

More tomorrow on the concepts developed as we have the post-lab discussion.

General Physics:

This class had the post lab discussion on the Bulb Boy Activity.

 

 

Day 155: Retirement…

No, not mine ( I can’t even begin to fathom it yet).  Two come to mind this week.  The first is a long-time colleague from another high school.  This colleague has also been one of the regulars at our Phox Valley Share sessions over the last bunch of years.  She works at parochial school in town and will be very difficult to replace… OK, not the right word, but you know what I mean. She teaches biology, chemistry, and physics — at a PAROCHIAL school.  She continues to work very hard and still has her students working hard too… very cool.

The second is one of my older sisters ( I have four).  This one has been teaching for a total of 27 years.  The last 20 have been at a high school in the southern part of the state (just outside of Milwaukee).  My brother (a district level administrator in the one of the bigger districts in the state) and I surprised her by showing up at the dinner and ceremony her district held to honor its retirees and 25 year employees.  My sister was speechless, which I can tell DOES NOT HAPPEN very often!  She just sort of stared at us for the first 20 seconds… then started to cry.  It was a very nice meal and ceremony. It was amazing to hear about the things my sister has done for her district and school.

The power of relationships keeps popping into my mind as I reflect on these two retirements. The professional relationship with my colleague and the family relationship with my sister remind me that when there are solid relationships in place, amazing things can happen.  Easily transferred to the classroom setting… strong relationships with students can help them achieve amazing things.  I know I have not established the relationship with every student,  something for me to continue to work on.

Advanced Physics:

We started this class by looking at how the electric potential difference changes around a circuit.  I used these two circuits along with a Vernier voltage probe to build the idea of an electric potential diagram (an EPD).

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To draw the EPD, the end of each wire is labeled with a letter:

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The potential is measured by placing the black lead of the voltage probe on the wife labeled ‘F’. and the red robe starts on the wire labeled ‘A’.  Collect the potential difference values as the red lead is moved to each of the labeled positions.  This provides the values for the EPD.  In the end, here is what the EPD’s will look like:

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I like these diagrams because they show that the only time the electric potential difference drops is across a bulb, NOT along a wire, especially these short wires.  This type of diagram  is similar to the color coding for electric potential used with CASTLE. The only issue I have had with that process is choosing appropriate colors when the circuit is not a nice, clean one with the obvious colors.

Toward the end of the hour, I have the classes a situation to draw both the schematic and the EPD for.  Here is a clip of the demo that uses a bath room light bar I re-wired.

General Physics:

This group spent today completing the Bulb Boy activity.

 

Day 154: I know this is supposed to be about my classes, but ……

And, it will be, just not right away.  Lat night was the last of the five share sessions for the Phox Valley Physics and Physical Science Share Group.  As always, it was awesome professional development. There were some great pedagogical discussions  (g = -9.8N/kg was 9.8N/kg, and if it is OK for kids to interchange using N/kg and m/s/s as they see fit), some awesome ideas (using a Hot Wheels Loop-the-Loop to have the students predict the minimum velocity the care needs to make it around); and of course LOADS of freebies to give away as door prizes (gift certificates form Vernier, books from Vernier, goodies from AAPT… the list goes on).  I also gave  Periscope a test run.  It was pretty awesome.  One of our former members living on the opposite side of the state could watch the presentations.  Periscope even allows me to see how many views there were!  I can see a number of applications for it even during class.

OK, now for what I did in class yesterday:

Advanced Physics:

We had the post-activity discussion for the Bulb Boy activity.  As I explained in a previous post, I do not use the terms ‘series’ and ‘parallel’ at the beginning of the activity, I prefer to let the activity and the observations create a need for them.  Here is how I went about that.  I asked the students to prepare WB’s on a given circuit they had set up… just their sketch of what it looked like and the potential values they had measured.  We the first group was presenting, I asked a member to trace the path the charge follows.  We watched as the member traced FROM the positive terminal (awesome) to the negative, up through a side of the bulb and out the bottom of the bulb.  This gave us a chance to talk about conventional current.  Each group did that even the ones were there was more than one path (we just used different colors).  We also compared the brightness of the bulbs and noticed that the bulbs on cituits with only one path got dimmer as more got were added, but there appeared to be no change in brightness as more bulbs/paths were added to the circuits that already had multiple paths.  We even set up the actual circuits to check this and some of the potential difference values.

After this, we summarized everything by creating a list of characteristics we had observed. At the very end of this discussion, I suggested that we give a name to the type of circuit that displays each set of characteristics…. Series and Parallel.

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We also agreed that a defined set of symbols might make the circuit drawings easier to follow… enter the schematic circuit symbols!

General Physics:

In this class we practiced using the voltmeter to measure potential differences and we made the simplest possible circuit  with one cell, one wore and one bulb (as discussed/referenced in  Minds of Our Own).   Bulb Boy for this group with the next class meeting.

Day 153: Bulb Boy and Polarity Shift

Advanced Physics:

My three sections of physics were pretty small today thank to the AP Calc test.  About half of each class was missing.  Today was spent working through the circuits in an activity I call Bulb Boy.  The activity consists of 5 different circuits (well 6 if you count the single bulb, single cell, single wire ask I wrote about yesterday).  Here are the guts of the handout.  Screen Shot 2015-05-05 at 7.35.00 PM

 

You’ll notice I do not use the terms series and parallel.  Basically, there are two reasons.  First of all, if I use those terms, I have to define them first, RATHER than letting the activity and post-activity discussion define them (recall concept first, then name).  I also find that just because some students know the names of the terms does not mean they understand them or can use the equipment to physically set them up.

Here are just a few shots of some of the set-ups:

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The post-activity discussion for this is really rich.  More on that tomorrow.

General Physics:

We went through the 7 Charge Questions they were asked to explore last night.  I’ve written about them before here.  This group also had an opportunity to try the Harry Potter Demo.  With this group, we do not focus too much on each the details of the charge, but more on the interactions and how the force(s) are involved.  Because we are a bit short of time (aren’t we all), we are giving a differentiated authentic assessment (yep, a mouthful).  We decided to use the game ‘Polarity Shift’ from The Universe and More website.  There are some pretty nice teacher materials provided at the website that we obtained the questions from.  We made use of them and provided a chance for the students to decide how much they wanted to work for. Here is a sample of the questions and how it was differentiated.

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SIDEBAR: The Universe and More website is a goldmine of computer based learning experiences. We also use the Graphing Game on occasion. I really encourage you to visit the site, it would be a wise investment of your time.

Day 152: No Clever Title today

Today was a strange and tough start to the week.  A tragedy occurred last night evening (around 7:30PM) at a very popular outdoor site in our area. I’m thinking about what has come to be known as the Trestle Trail Bridge Shooting. I will not go into the details, but will mention that four people were killed and a fifth is in critical condition. While the family affected did not attend our schools, the children did attend a parochial school in our city and the Trestle Trail connects our community and it’s neighbor.  Nearly everyone in the community knows the area and has been there.  We did not talk about it in my classes, but it was there in many minds including mine.  My thoughts and prayers go out to the families involved.

Advanced Physics:

We started the class by discussing how to use an analogue (panel) meter t measure electrical potential difference (or voltage). We just measured the potential difference across individual cells, then the cells ‘end-to-end’, two cells ‘side-by-side’, and finally in a cell holder (it was interesting to watch the students struggle to figure out how to use only two cells in the holder and to get he maximum potential difference).  Here are a few photos:

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If you look closely at the meter, you will see I have added a label, electrical potential difference, in J/C, to remind the student exactly what the ‘voltage’ and volt mean.

From there we built the simplest circuit… one cell, one wire, and one bulb.  I ask  hem to get it to light two different ways and that simply switching the cell ends does not count.  This activity is one of the more famous ‘electrical tasks’, even discussed in the documentary Minds Of Our Own.  It is interesting to me that several of our engineering and manufacturing academy students will say almost immediately that it cannot be done.. two wire need to be used, but the other students will just dig in and give it a try.  I like this task because it introduces circuits and how a simple bulb is wired, that the bulb does not ‘care which direction the charge moves through it.  Tomorrow more circuits, with electrical potential difference readings taken while the circuit is functioning.

General Physics:

We discussed the energy test they took on Friday and then had a brief introduction to the next unit, electrical concepts here too.  They were sent home with the task of answering the 7 Charge questions.