Free / Grades 7-8, with decimal and unit support
Power and Energy Lab
How can I tell which model transfers more energy, and explain what my comparison leaves unknown?
4 flexible sessions / about 149 minutes including practice / power, interval accounting and evidence-based explanations
Suggested rhythm: two sessions a week for two weeks. First distinguish rate and total; then compare resistor conditions and defend a choice. Pause for unit or decimal 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
Multiply and divide decimals, convert milliamps to amperes, and distinguish voltage across a resistor from current through it. Review Watts and joules with a reader or calculator when useful.
Paper models only, not a building guide. No batteries, wires, meters, outlets or real components are needed. Do not build, connect, open or test electrical equipment. All readings are declared ideal model values, not component ratings, temperatures or real-device efficiency measurements. Use the browser and paper only; no actual waiting is required.
Materials
- 4 sheets per learner: Project journal
Print four pages or copy the headings into a notebook.
- 2 sheets per learner: First explanation and revised report
Keep both versions on clean scrap paper or in a notebook.
- 1 per learner: Pencil
Point, speak, dictate or use an accessible writing method.
- 1 shared: Browser and optional calculator
Read the transcript and paper evidence together; separate worksheet printing is optional.
Choose the support that fits
- More support: keep a unit key beside every calculation, do one interval at a time, and dictate the reasoning. Use a calculator for decimals without skipping the explanation.
- More challenge: make a different pair of durations with the same total energy, then state the assumptions needed for that comparison. Do not turn model values into building instructions.
- Access options: every diagram has a written equivalent. A private spoken explanation can replace drawing or handwriting. No equipment purchase, upload, recording or public performance is required.
What good evidence looks like
- Convert current to amperes before calculating watts.
- Use power and duration together to calculate joules.
- State what stays equal before comparing resistors.
- Add successive energies and times before calculating average power.
- Retain a first explanation and justify a revision with evidence and a limit.
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 rate and total different jobs
Goal: Convert current units and explain watts without treating them as a total energy.
Preparation / about 6 minutes of adult support: Have journal 1 and a first-explanation sheet ready. Keep the original explanation even if it contains a mistake; return to it in session 4.
A watt is one joule transferred each second. In a steady ideal resistor model, multiply voltage in volts by current in amperes. A separate worked model has 2 V across its resistor and 30 mA through it. Convert 30 mA to 0.03 A, then calculate 2 x 0.03 = 0.06 W. Each second transfers 0.06 J, but the total needs a duration. A calculator will not ask where the missing unit went. We must keep track of it.
Which number is a rate? What would you need before deciding the total energy? Would converting 30 mA to 0.03 A change the physical current?
- Watts and joules / about 6 minutes
- Watts: Energy Each Second / about 10 minutes
- Keep my first power explanation / about 15 minutes
Fun fact: A watt and a joule answer different questions: one joule every second is a power of one watt. The energy of a run also depends on its duration. Source
S3U / Power and Energy Lab / Journal 1 of 4
What does this reading tell me?
These are steady ideal resistor readings, not measurements to make. Find current in A and power in W. The duration is deliberately missing in row D. On a separate sheet, write your first response to: Higher watts always means more joules. Keep it for session 4.
| Declared model | Current (A) | Power (W) |
|---|---|---|
| A: 6 V, 25 mA | ||
| B: 6 V, 50 mA | ||
| C: 3 V, 50 mA | ||
| D: 6 V, 25 mA; no duration |
Use two rows to explain how equal power can come from different voltage and current values.
Does row D give total energy? State what is missing without inventing it.
Session 2 / about 35 minutes
Compare whole runs, not just watts
Goal: Calculate energy from rate and duration, and distinguish alternative runs from successive intervals.
Preparation / about 7 minutes of adult support: Use journal 2 and keep the first explanation nearby. Draw a separate box around each alternative run before adding anything.
For a constant-power interval, E = P x t. A separate worked comparison has 0.4 W for 5 seconds and 1 W for 2 seconds. Both transfer 2 J. Higher power alone did not settle the comparison. These are alternative runs, so adding them would describe neither run. If a new record instead says one happens and then the other, their energies add and their times add. The word then has important work to do.
Can a longer run use less energy? Which words say that intervals belong to one successive run rather than two alternatives?
- Joules: Energy Over an Interval / about 10 minutes
- Compare the energy of three runs / about 15 minutes
- Explain what then changes / about 10 minutes
Fun fact: Different power-duration pairs can transfer equal energy. Doubling constant power and halving the duration leaves their product unchanged. Source
S3U / Power and Energy Lab / Journal 2 of 4
Alternatives or one run?
Run A is constant 0.6 W for 2 s. Run B is constant 0.2 W for 6 s. Run C is constant 0.1 W for 8 s. These three runs are alternatives. A different record D says 0.6 W for 2 s, then 0.2 W for 6 s, with no gap or overlap. Calculate each row separately.
| Record | Total duration (s) | Energy (J) |
|---|---|---|
| Alternative A | ||
| Alternative B | ||
| Alternative C | ||
| Successive record D |
Which alternatives have equal energy? Which runs longest but transfers the least? Use products and units, not just a bigger-number rule.
Why can D add energies while comparing A and B does not automatically create a combined run?
Session 3 / about 38 minutes
Ask what stays equal
Goal: Compare resistor powers under equal-current and equal-voltage conditions, without treating either condition as universal.
Preparation / about 8 minutes of adult support: Have journal 3 ready. Separate the two model headings and say the equal quantity aloud before choosing an equation.
Within an ideal series pair the current is equal, so P = I x I x R. Within equal-voltage parallel branches, P = V x V / R. In a separate worked model with 0.01 A, 150 ohms gives 0.015 W and 300 ohms gives 0.03 W. At equal 3 V instead, those resistances give 0.06 W and 0.03 W. The larger resistor did not change its mind. We changed the condition. Neither calculation supplies a temperature or a safe component rating.
Which equation matches each heading? Why is bigger resistance means less power incomplete without its equal-voltage condition?
- Compare resistor power / about 6 minutes
- Resistor Power: What Stays Equal? / about 12 minutes
- Defend two resistor comparisons / about 20 minutes
Fun fact: For ideal resistors, doubling resistance doubles power at equal current, but halves power at equal voltage. Name the equal quantity first. Source
S3U / Power and Energy Lab / Journal 3 of 4
One claim needs its condition
Model S is a series pair carrying the same 0.02 A through 150-ohm and 300-ohm resistors. Model P is a different parallel pair with the same 6 V across 150-ohm and 300-ohm resistors. Values are steady and ideal. Compare within each pair, not across an unstated physical change.
| Model and resistor | Equal quantity | Power (W) |
|---|---|---|
| S: 150 ohms | ||
| S: 300 ohms | ||
| P: 150 ohms | ||
| P: 300 ohms |
Which resistor has more power in S, and which in P? Repair: A larger resistance always dissipates less power.
Do these power values establish which real resistor is hotter? Name a missing kind of physical information.
Session 4 / about 45 minutes
Audit, choose and revise
Goal: Check an unequal-interval notebook and defend a choice using a stated energy criterion, while keeping performance and safety claims separate.
Preparation / about 9 minutes of adult support: Bring all journals, the first explanation and a fresh report sheet. Keep crossed-out ideas readable so the final explanation can show what changed.
Audit each interval before trusting an average. A separate example runs at 0.5 W for 2 s and 0.25 W for 6 s. It transfers 1 J + 1.5 J = 2.5 J in 8 s, so average power is 0.3125 W. Adding powers or averaging them without duration weights gives the wrong full-run value. A lower energy total can meet an energy criterion without proving a useful real device works better. A model report should say what it can justify, and stop there.
What does the energy limit test, and what does it not test? Which journal result challenges or refines your first explanation?
- Audit the choice notebook / about 20 minutes
- Revise my power and energy explanation / about 15 minutes
- Explain my choice and its limits / about 10 minutes
Fun fact: Average power is total energy divided by total duration. Unequal-duration intervals generally cannot use the simple unweighted mean of their powers. Source
S3U / Power and Energy Lab / Journal 4 of 4
My model choice and its limits
The sole criterion is no more than 2.8 J transferred over one 8-second run. Option X runs at 0.6 W for 2 s, then 0.2 W for 6 s. Option Y is constant 0.4 W for 8 s. No gap or overlap occurs. A draft notebook incorrectly lists X energy as 0.8 J and its average power as 0.4 W. No useful output, real temperature or component ratings are provided.
| Check | My calculation or explanation |
|---|---|
| X: energy of each interval and total | |
| X: average power over 8 s | |
| Y: total energy and average power | |
| Which option meets the stated criterion? |
Repair both draft entries and explain each mistake. State why meeting this energy criterion alone does not establish real efficiency, usefulness or safety.
Use a separate report sheet to compare your first explanation with your final one. Cite evidence from two sessions and name one remaining unknown; keep both versions.
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.