Free / Grades 6-8, with ratio and decimal support
Circuit Detectives
How can a circuit model help me predict, test and explain a design?
4 flexible sessions / about 150 minutes including practice / electronics, mathematical models, explanation and design
Suggested rhythm: two sessions a week for two weeks. Week 1: quantities and resistance. Week 2: circuit paths and a paper design. Pause for division or unit practice when needed.
Reading, watching and paper activities are free. Parents and instructors can save a collection; scheduling it for a student requires Plus or Lifetime. No upload is required.
Before you begin
Divide whole numbers and decimals, multiply by 1,000, and read a complete path in a circuit picture. Review the difference between a quantity and its unit. An adult may read or calculate alongside you.
A diagram investigation, not a construction guide. No batteries, wires, meters, outlets or real components are needed. Do not connect, open or test electrical equipment. Never short a battery. All values are model assumptions, not instructions or safety ratings for real parts. Use paper and the browser only.
Materials
- 4 sheets per learner: Project journal
Print four pages or copy the headings into a notebook.
- 2 sheets per learner: First explanation and final design
Keep the first explanation on clean scrap paper; use another page for the final diagram.
- 1 per learner: Pencil
Point, dictate or use an accessible writing method.
- 1 shared: Browser and optional calculator
Read transcripts together. Use written calculations with adult support; separate worksheet printing is optional.
Choose the support that fits
- More support: keep a unit key visible: V is volts, A is amperes, mA is milliamperes and ohms measure resistance. Trace one path at a time and dictate an explanation.
- More challenge: enumerate every allowed resistor pair in the final design and explain why swapping branch labels changes the named readings but not their total.
- Access options: replace neat drawings with labeled connection lists and spoken traces. Use the supplied paper cases if game controls are unsuitable. No upload, equipment purchase or public performance is required.
What good evidence looks like
- Distinguish voltage from current and include the correct unit.
- State the ideal-model assumptions before calculating.
- Predict, then compare with a recorded or calculated result.
- Trace the common path and each branch before explaining an open switch.
- Show that every design requirement is met, and name a limit of the model.
These are discussion criteria, not a new automatic score. Existing lesson and worksheet records keep their own subjects. Checking off a planned task does not demonstrate mastery or add a second grade.
Session 1 / about 31 minutes
Give every number a job
Goal: Distinguish voltage from current by calculating and explaining their units.
Preparation / about 6 minutes of adult support: Have journal 1 and a first-explanation sheet ready. Read the units aloud and retain the first explanation for the last session.
Voltage describes energy per charge between two points; current describes charge per time through a point. In a separate example, 10 joules per 2 coulombs is 5 volts. Six coulombs passing in 3 seconds is 2 amperes. Those numbers answer different questions. A bare number without its unit has arrived at the wrong classroom without a timetable. Label it before comparing.
Ask which calculation describes a difference between two points and which describes a rate through one point. Why does doubling the observation time not automatically double the current?
- Electronics basics: voltage and current / about 6 minutes
- Electronics Basics: Voltage Is Not Current / about 10 minutes
- Keep my first circuit explanation / about 15 minutes
Fun fact: One ampere means one coulomb of charge passing per second. Milli means one thousandth, so 1 A and 1,000 mA name the same current. Source
S3U / Circuit Detectives / Journal 1 of 4
What does the number measure?
Calculate each idealized case. Label voltage with V and current with A. These are supplied quantities, not a request to measure electricity. Keep a first explanation on a separate page.
| Given quantities | Calculation and result | Voltage or current? |
|---|---|---|
| 6 J of energy per 2 C of charge | ||
| 12 J of energy per 2 C of charge | ||
| 4 C pass in 2 s | ||
| 4 C pass in 4 s |
Compare the last two rows. What stayed the same, and why did the rate change?
Would a label of 3 with no unit tell you whether it is voltage or current? Explain.
Session 2 / about 34 minutes
Hold the voltage, change the resistance
Goal: Use I = V / R for the stated ideal resistors and convert amperes to milliamperes.
Preparation / about 7 minutes of adult support: Have journal 2 ready. Write the unit conversion beside each result; use a calculator or supported division if needed.
For these ideal resistors, divide volts by ohms to find amperes, then multiply by 1,000 for milliamperes. A separate 9 V example with 300 ohms gives 0.03 A, or 30 mA. Compare one change at a time. Keep the source voltage and closed path fixed when changing resistance. The unit conversion changes the label and number, not the circuit.
Ask why 0.03 A is not 0.03 mA. What must stay fixed before the claim "more resistance means less current" fits this comparison?
- Electronics basics: use resistance / about 6 minutes
- Electronics Basics: Use Ohm's Law Carefully / about 10 minutes
- Compare fixed-voltage model currents / about 18 minutes
Fun fact: An open switch breaks the conducting path. In this steady-current model its current is zero, even though the source can still have a voltage between its terminals. Source
S3U / Circuit Detectives / Journal 2 of 4
One change, a fair comparison
Use an ideal 12 V source and one ideal resistor. Before calculating, predict how current changes as resistance increases. Wires and closed switches have zero resistance in this model; there is no heating or source limit.
| Path and resistance | Current (A) | Current (mA) |
|---|---|---|
| Closed; 200 ohms | ||
| Closed; 400 ohms | ||
| Closed; 600 ohms | ||
| Open switch; 200 ohms |
State your prediction, then use two results to explain whether the calculation supports it.
A learner labels 12 / 400 = 0.03 mA. Repair the unit and give the matching value in mA.
Session 3 / about 45 minutes
One route or two?
Goal: Predict which currents stop when a series path or one parallel branch opens.
Preparation / about 8 minutes of adult support: Open Circuit Bench for its recorded comparisons. Have journal 3 ready; its 12 V paper cases deliberately differ from the game's fixed 6 V source.
In series, both resistors lie on one path, so opening that path stops current through both. Parallel branches each connect across the source. In the ideal-source model, opening one branch leaves another complete branch operating. Total source current is the sum of branch currents. Current is not a snack that the first resistor eats. Predict a change before toggling a switch, then compare with a record.
Ask what part of the path both branches share. Would a break there have the same effect as opening only branch B? Keep that question separate from brightness or real equipment.
- Circuit Bench / about 20 minutes
- Trace the 12 V paper cases / about 15 minutes
- Explain a break from its evidence / about 10 minutes
Fun fact: Two unequal parallel resistors can carry unequal currents while having the same voltage across them. Equal voltage does not mean equal current. Source
S3U / Circuit Detectives / Journal 3 of 4
Find the path before the number
Use ideal 12 V, resistor A = 200 ohms and B = 400 ohms. In series the one path passes through A then B. In parallel each has its own complete branch and switch across the source. All other connections are good. Record currents in mA.
| Model and switch state | A (mA) | B (mA) | Total (mA) |
|---|---|---|---|
| Series; both closed | |||
| Series; B open | |||
| Parallel; both closed | |||
| Parallel; B open |
A parallel case has A = 60 mA and B = 0. Of a break in A, B or the common source path, which fits? Assume exactly one break and good resistors.
What would opening the common source path do? Why are these limited model cases not a complete diagnosis of a real device?
Session 4 / about 40 minutes
A design that earns its explanation
Goal: Design two unequal parallel currents within a stated total-current limit, and test every switch state.
Preparation / about 9 minutes of adult support: Bring the first explanation, all journals and a fresh design sheet. Keep the design entirely on paper; no physical circuit is authorized by these numbers.
Turn a design brief into checks. Draw the common source path, both resistor branches and each branch switch. Choose values, calculate, and test both-on, A-only, B-only and both-off states. If a requirement fails, keep the first attempt and explain your revision. A tidy picture without a check is only a confident picture. Name what the model leaves out before calling your design finished.
Ask whether a design can meet the total limit yet fail the unequal-current requirement. Which result would change if the branch labels were swapped?
- Draw my model circuit design / about 15 minutes
- Test all four switch states / about 15 minutes
- Revise my first circuit explanation / about 10 minutes
Fun fact: For an ideal fixed-voltage source, adding another complete resistor branch increases the total current. A real source has limits that this paper model does not include. Source
S3U / Circuit Detectives / Journal 4 of 4
My design and its limits
Design two parallel resistor branches on paper with an ideal 6 V source. Choose each resistor from 100, 200 or 300 ohms. With both switches closed, currents must be unequal and total no more than 80 mA. Both branches must conduct. Draw it on a separate sheet; this is not a building plan. My choices: A = ____ ohms; B = ____ ohms.
| Switch state | A (mA) | B (mA) | Total (mA) |
|---|---|---|---|
| Both closed | |||
| A closed; B open | |||
| A open; B closed | |||
| Both open |
State the chosen resistances, check every requirement and explain any change to a first design.
Revisit your first explanation. Name an improvement or justified original idea, then one real-world effect this model leaves out.
What changed in your explanation?
Show your evidence, explain one revision, and choose a question to investigate next. You can keep everything on paper.
Optional: record actual offline learning in Learning Records. Keep reported minutes separate from website time. For an assigned pack, use your existing daily tasks; this page does not award additional completion credit.