Electricity Visualized! Teaching Circuits in AP1 with ideas from the CASTLE Curriculum.

A student puts their own spin on the Modeling version of a CASTLE activity.

CASTLE stands for Capacitor-Aided System for Teaching and Learning Electricity.

It is a conceptual curriculum available for download from Pasco Scientific (go to https://www.pasco.com/prodCompare/castle-kit/index.cfm and scroll down to find the downloads). The curriculum is a basically complete set of activities, demonstrations, student classwork, homework, and quizzes.

A teacher guide is available by contacting me (or Pasco Scientific).

It takes a lot of time for students to complete the whole curriculum, but there are benefits. The curriculum is student-guided and inquiry-based, and emphasizes deep conceptual understanding (there are virtually no calculations and equations). Because of time constraints, my students, even my students in “on-level” physics where there is less time pressure, don’t do all of the activities. I am presenting some of the useful ideas here, so that you can see the benefit of the curriculum and perhaps make use of some of the ideas even though you don’t use the whole thing (which would be ideal, but won’t work for everyone).

Any of these activities that use a capacitor and bulbs to visualize what is happening in a circuit will not work with ordinary capacitors. As far as I know, Pasco.com is the only source for the capacitors that are utilized in the CASTLE curriculum. But, many of the activities below do not require the CASTLE capacitors

1.2 Activity:  “Using the compass to investigate a closed loop.”

A compass is placed under one of the wires in a circuit. 

  • The deflection of the needle indicates something is happening in wires.
  • Reversing the battery reverses the deflection – whatever is happening has a direction.
  • Moving the compass to different parts of the circuit shows the same deflection – indicates that whatever is happening in the wires happens everywhere. Charge is everywhere in the circuit. The battery provides energy that makes charge move/causes charge to do work, but is not the source of charge (See also Activity 8.1).

1.11 Activity: “Lighting a bulb with a single cell”

Students are given a miniature bulb, a D cell and a single wire. They are challenged to light the bulb using only those materials. Students are given a “dissected” light bulb to examine. 

  • An incandescent bulb must provide a single conducting path for charge to move through the bulb, including the filament
  • Charge must enter and leave through conducting parts of the bulb
  • Insulating parts of the bulb separate the conducting parts to provide the single path

2.3 Activity “Additional Symbols for Circuit Diagrams”

Try drawing “starbursts” to represent bulb brightness:

And drawing “Arrowtails” to represent current magnitude and directions:

2.6 Activity “Examining filaments under magnification” and 2.8 “Comparing the resistance of a bulb to a wire”

This is done after students observe that adding resistors or bulbs in series to a circuit decreases the brightness of a bulb. They examine the filaments of incandescent bulbs using magnifying lenses. The filament wires are much thinner than the support wires and connecting wires (alligator clip leads). Adding a long wire to a circuit does not appreciably change the brightness. Connecting a wire in parallel with a bulb causes the bulb to go out, as nearly all of the flow stays in the wire. Students deduce that most of the resistance is in the bulb.

  • Most of the resistance is in the bulb
  • The filament of the bulb is much thinner than other parts of the circuit
  • Thinness appears to equate with increased resistance
  • In a series circuit, a long bulb is brighter than a long bulb – since current is the same in the series circuit, the brightness of the bulb must depend on the resistance of each bulb. The filament of the round bulb must have less resistance than the filament of the long/the filament of the round bulb is thicker than the filament of the long bulb a thin filament has high resistance (see pictures below)
Round bulb (image by Charles Mamolo)
Long Bulb (image by Charles Mamolo)

2.7 Activity “Detecting the resistance of straws to air flow”


In this activity different sized straws are used as an analogy for conductors in a circuit. Air is a compressible fluid that serves as the analogy for electric charge in the circuit. Coffee stirrer straws, ordinary drinking straws, bubble-tea straws, and paper towel tubes are all useful for this activity. Students blow through the straws and observe that resistance to airflow decreases with diameter. Putting like straws together with tape illustrates the series and parallel relationship with resistance. 

  • Resistance to flow decreases with increased diameter
  • Resistance to flow increases with increased length
  • Resistance to flow decreases when “resistors” are arranged in parallel, increases when they are arranged in series.

2.8 Activity “Comparing the Resistance of Wire with a Filament”

Students insert a long wire into their circuit and observe that the compass needle deflects the same as before. Then they insert a second bulb in series into their circuit, and observe less deflection. This leads to the conclusion that wire has negligible resistance and bulbs have significant resistance. 

4.2 Activity “Exploring air as an analogy”

Two linked hypodermic syringes (minus needles) show how pressure equalizes in a circuit. Potential difference (voltage) in a circuit acts like pressure differences in the syringes. 

  • When circuit elements are connected to a source of electric pressure (a battery, for instance), the pressure quickly equalizes
  • Although the elements may be different, the pressure equalizes if it can. 

4.7 Activity: How below-normal air pressure behaves (Air Capacitor)

In this activity, students work with an air capacitor (shown above in schematic and below in a photograph).

Blowing air in or out causes the balloon membrane to move, displacing air and/or changing the pressure within the capacitor. This is analogous to the flow of charge into and out of a capacitor. Air/Charge is already present in the capacitor, and is displaced from one side or the other, rather than moving through the capacitor. The balloon membrane acts like a dielectric, insulating one side from the other. 

4.9 Commentary “Color Coding for Electric Pressures in a Circuit”

In this section, students use colored pencils to represent levels of electric potential in a circuit by color-coding schematics of the circuit. Once students understand what is expected, it is very quick to determine which bulbs in a circuit will be the brightest, or identify any bulbs that will not light (because both ends are at the same “electric pressure”). Try color-coding this circuit: 

See below for a CASTLE-style representation of an RC circuit charging. 

8.1 Activity “Circuit with a Conducting Island”

Build the circuit, predict what will happen, and connect it. The bulbs light and then go out. Charge that lights the bulbs must not have come from the battery, since the capacitor has insulators that don’t let charge through. Charge everywhere in the circuit moves until the electric pressures are all equalized. Energy is stored in the capacitors when they charge, but not a net charge. 


There are twelve sections in CASTLE. The later sections delve into advanced topics, such as electric fields, semiconductors, magnetism and induction, and electromagnetic signal propagation and detection. All without making significant use of mathematics and equations. There are great teaching ideas in those sections, I just chose to highlight some sections that may be more broadly applicable.

You can download the curriculum from Pasco.com. Or leave me a message below /send me an email and I can share the entire curriculum, including a few things that are not available on the Pasco website.

Pasco Scientific is a convenient source of good-quality materials for the CASTLE kit used by students in the curriculum. You may be able to find everything in the kit cheaper elsewhere, but the capacitors. I have never seen those specific capacitors (High Capacitance, relatively high Voltage, and non-polar) anywhere else. 

About marcreif

I live and teach high school physics in the town I was born in, Fayetteville, Arkansas. My professional interests include modeling instruction and Advanced Placement courses. I also work as a College Board Workshop Consultant, which means I lead Pre-AP and AP Science Teacher workshops. Lately I've also been leading a fair amount of student review sessions for the National Math and Science Initiative. I have a website for students (fysicsfool.info) and another for AP Summer Institute participants (apsifool.info). I tweet infrequently (@marcreif).
This entry was posted in Uncategorized and tagged , . Bookmark the permalink.

11 Responses to Electricity Visualized! Teaching Circuits in AP1 with ideas from the CASTLE Curriculum.

  1. Pingback: Links for January Chattanooga Workshop – Teach. Brian. Teach.

  2. Naade, Noble. B. says:

    Please I need the teacher guide or manual and the complete CASTLE project kit.

    • marcreif says:

      Please send me an email from a teacher account and a link to the school website and I will share.

    • marcreif says:

      Are you a teacher? Send me an email with a link to your teacher page.

      • Noble Naade says:

        I contacted my school ITC and I was told we don’t have that now. But if you want to confirm my position as a teacher, you can check my profile on researchgate.

      • Naade, Noble says:

        Good morning,
        In response to your previous messages, My school doesn’t have such a link or an account. But if you want to confirm if I am a teacher you can visit my profile on researchgate.
        Someone got the teacher’s guide but I couldn’t access the files again. I sent a mail to Tom Rush one of the Pasco sales rep, concerning the kit. I am Yet to hear from him. I need it to teach my students in a bid to reduce their misconceptions in electricity.

  3. Adam Holman says:

    Hi Marc! Do you know if the teacher edition has been updated since 2009? I have the ’09 teacher guides from back when I taught physics but just downloaded the student guides and it appears they are from 2016. I can’t find your email address anywhere to message you 🙂 Great website and thanks for any info!

    • marcreif says:

      I do have a version from 2014 – not sure how much different it is from earlier editions. I do not have a 2016 Teacher Guide. I can share via dropbox or GDrive.

  4. Mirabel says:

    Hi Marc! I was wondering if you could possibly share the teacher guide. I have the 09’ and the student guides are from 2014, i was wondering if you could possibly send it along via google drive. Mine is mirabelstreu@gmail.com thank you so much

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.