Week 3: Electric Circuits & Power
Grade 8 Science | Rosche
The Phenomenon: The Dimming Lights Mystery
Anchoring Context & Focus Question
Why do the lights dim when you turn on a hairdryer?
Before We Begin: Activate Your Prior Knowledge
Think back to Week 2: You built electromagnets and discovered that electric current creates magnetic fields. You even designed motors that convert electrical energy into motion. Now ask yourself: Where does the current come from? What controls how much current flows? And what happens when too many devices share the same circuit? Keep these questions in mind as you investigate today.
You're getting ready in the morning. The lights are on. You plug in a hairdryer, flip the switch — and the lights dim noticeably. Why?
- A single household circuit typically carries 15 or 20 amps at 120 volts.
- A hairdryer alone draws 10-12 amps — over half the circuit's capacity!
- When you add devices to a circuit, total current increases — but the voltage supply stays the same.
- The wires themselves have resistance — more current means more voltage "lost" in the wires, leaving less for the lights.
St. Louis Connection
Ameren Missouri, headquartered in downtown St. Louis, manages the electrical grid serving 1.2 million customers across the region. During summer heat waves, St. Louis homes draw so much power for air conditioning that circuits approach overload — causing rolling brownouts in some neighborhoods. Engineers must carefully balance voltage, resistance, and power across thousands of miles of wire to keep the lights on.
Why This Matters to YOU
Every device you use — your phone charger, gaming console, microwave — draws power from a circuit. Understanding Ohm's Law and power calculations isn't just physics class — it's how electricians design safe wiring, how engineers prevent fires, and why your house has circuit breakers. Electrical engineering is one of the fastest-growing STEM careers, with a median salary over $100,000.
Focus Question: Why do the lights dim when you turn on a hairdryer — and how can circuits be designed to prevent overload?
By the end of this lesson, you will be able to:
- Apply Ohm's Law (V = IR) to calculate voltage, current, and resistance.
- Calculate power consumption using P = V x I.
- Compare series and parallel circuits and explain their real-world uses.
- Design a circuit that safely handles multiple devices without overload.
NGSS 3D Standards
This Week's Standards
MS-PS2-3: Ask questions about data to determine the factors that affect the strength of electric and magnetic forces.
Spiral Standards (Review)
- 6-8.PS3.A.1: Kinetic energy and speed relationships (MAP Achievement Season — RETEACH)
- MS-PS2-3 + C6 W1-W2: Magnetic fields & electromagnets (within-cycle)
Cognate Strategy: Many science words look similar in English and Spanish — use your Spanish to learn science!
| Term | Spanish | Definition |
|---|---|---|
| voltage | voltaje | Electrical pressure that pushes charge through a circuit (measured in Volts) |
| resistance | resistencia | Opposition to current flow in a circuit (measured in Ohms) |
| circuit | circuito | Complete loop through which electric current flows |
| power | potencia | Rate of energy transfer (P = V × I, measured in Watts) |
| series circuit | circuito en serie | Circuit where components are connected in a single path |
| parallel circuit | circuito en paralelo | Circuit where components have multiple paths for current |
| overload | sobrecarga | When a circuit draws more current than it can safely handle |
Hook – The Dimming Lights Mystery
Observe circuit behavior when load changes.
The Challenge
What You'll Do
- Examine a household circuit with multiple devices plugged in
- Predict what happens to brightness when a high-power device is added
- Connect to Week 2: How does the current that powers an electromagnet relate to circuit load?
- Answer diagnostic questions about voltage, current, and resistance
The Mystery Data
A student measures voltage across a lamp on a 120V household circuit as devices are added:
| Devices on Circuit | Total Current (A) | Voltage at Lamp (V) | Lamp Brightness |
|---|---|---|---|
| Lamp only | 0.8 A | 119 V | Full brightness |
| Lamp + Phone charger | 1.2 A | 118 V | No visible change |
| Lamp + Charger + Hairdryer | 12.5 A | 108 V | Noticeably dimmer |
| Lamp + Charger + Dryer + Heater | 22 A | — | CIRCUIT BREAKER TRIPS! |
Key Question: The voltage supply is always 120V, so why does the lamp only get 108V when the hairdryer is on? Where did the other 12 volts go?
Stop & Think
Before you open the form below, consider: The wires connecting your outlet to the breaker box have a small resistance. Using V = IR, what happens to the voltage "lost" in the wires when current increases from 1A to 12A?
Worked Example and Simulation – Calculating Circuit Power and Load
The Problem
Scenario: A student has a 120V household circuit with a 20-amp breaker. She wants to plug in a desk lamp (100W), a gaming console (200W), and a space heater (1500W). Will the circuit handle all three devices, or will the breaker trip? Calculate the total current and power to find out.
Step-by-Step Solution
Step 1: Calculate current for each device using I = P / V
"Lamp: I = 100W / 120V = 0.83A. Console: I = 200W / 120V = 1.67A. Heater: I = 1500W / 120V = 12.5A."
Step 2: Add the currents (parallel circuit — currents add up)
"Total current = 0.83 + 1.67 + 12.5 = 15.0 A. The breaker is rated for 20A, so we're under the limit — barely!"
Step 3: Calculate total power consumed
"Total power = 100 + 200 + 1500 = 1800W. Check: P = V x I = 120V x 15A = 1800W. It matches!"
Now YOU Complete Steps 4-5:
Step 4: What if the student adds a second space heater (1500W)? Calculate the new total current. Will the 20A breaker trip?
Step 5: The student's roommate suggests putting the two heaters on different outlets. If both outlets are on the SAME circuit, does this help? Why or why not?
Fading Support: Steps 4 and 5 are YOUR turn. Use I = P/V and add currents to check the breaker limit. This prepares you for Station 1's Ohm's Law investigation.
Simulation: Ohms Law
PREDICT (before running the sim)
Look at the simulation controls. Before changing any variables, predict what will happen when you adjust them. Write your prediction down.
OBSERVE (while using the sim)
Change one variable at a time. Record what happens after each change. Use the data journal to capture at least 3 trials.
EXPLAIN (after collecting data)
Compare your observations with your prediction. Use scientific vocabulary to explain the patterns you found. What surprised you? What confirmed your thinking?
Station 1 – Ohm's Law Investigation
Explore relationships between voltage, current, and resistance.
Your Mission: Discover Ohm's Law
Circuit Simulation Data: Voltage vs. Current
| Voltage (V) | Resistance (ohms) | Current (A) | Pattern |
|---|---|---|---|
| 3 V | 10 ohms | 0.3 A | V / R = I |
| 6 V | 10 ohms | 0.6 A | Double V → double I |
| 6 V | 20 ohms | 0.3 A | Double R → half I |
| 12 V | 20 ohms | 0.6 A | V = I x R |
| 12 V | 5 ohms | 2.4 A | Low R → high I! |
Phenomenon Check-In
Think back to the dimming lights. The wires in your house have about 1 ohm of resistance. Using V = IR, how much voltage is "lost" in the wires when 12.5A flows (hairdryer on) vs. 0.8A (lamp only)?
Station 2 – Power & Energy Analysis
Calculate power consumption and circuit capacity.
Your Mission: Power & Energy Analysis
Common Household Device Power Ratings
| Device | Power (W) | Current at 120V (A) | Daily Use (hrs) | Energy/Day (Wh) |
|---|---|---|---|---|
| LED Light Bulb | 10 W | 0.08 A | 8 | 80 Wh |
| Phone Charger | 18 W | 0.15 A | 3 | 54 Wh |
| Gaming Console | 200 W | 1.67 A | 4 | 800 Wh |
| Microwave | 1000 W | 8.33 A | 0.5 | 500 Wh |
| Hairdryer | 1500 W | 12.5 A | 0.25 | 375 Wh |
| Space Heater | 1500 W | 12.5 A | 6 | 9000 Wh |
Key Insight: The hairdryer and space heater draw the same power (1500W), but the heater runs 24x longer — so it consumes far more energy over a day. Power tells you how fast; energy tells you how much.
Phenomenon Check-In
The hairdryer draws 1500W on a 120V circuit. That means it pulls I = 1500/120 = 12.5A of current. If the circuit breaker is rated for 20A, how many amps are left for other devices? What happens if you plug in another 1500W device on the same circuit?
Station 3 – Design a Safe Circuit
Apply circuit principles to prevent overload.
Design Challenge: Build a Safe Circuit
Your client needs a bedroom circuit that safely powers all of these devices on a single 20-amp, 120V circuit:
| Requirement | Details |
|---|---|
| Devices | 3 LED lights (10W each), 1 phone charger (18W), 1 laptop (65W), 1 TV (120W), 1 fan (75W) |
| Circuit type | Parallel (household standard) |
| Breaker limit | 20 amps maximum |
| Safety margin | Total current must stay below 80% of breaker (16A) for sustained use |
Constraint Table
| Constraint | Why It Matters |
|---|---|
| No single device > 12A | Prevents one device from hogging the circuit |
| Total sustained load < 80% | NEC code requires safety margin to prevent wire overheating |
| Must include GFCI protection | Required near water sources to prevent electrocution |
| Must include AFCI breaker | Detects arc faults (sparking) that cause house fires |
CER Scaffold: Is Your Design Safe?
Claim:
State whether your circuit design is safe or unsafe, and whether it meets the 80% rule.
Evidence:
Show your calculations: total power = sum of all device wattages. Total current = total power / 120V. Compare to the 16A safety limit (80% of 20A).
Reasoning:
Explain WHY circuits have breakers and safety margins. What would happen without them? Connect to the phenomenon: overloaded circuits cause voltage drops (dimming lights) and can overheat wires, causing fires.
Real-World Connection: Protecting Your Home
Circuit Breakers
Automatically trip (open the circuit) when current exceeds the rated amperage. This prevents wires from overheating. After the cause is fixed, you can reset the breaker — no replacement needed.
GFCI Outlets
Ground Fault Circuit Interrupters detect when current leaks to ground (like through water or a person). They trip in 1/40 of a second — fast enough to prevent electrocution. Required in kitchens, bathrooms, and outdoors.
AFCI Breakers
Arc Fault Circuit Interrupters detect dangerous electrical arcs (sparking) caused by damaged wires. House fires from faulty wiring kill hundreds of Americans each year — AFCIs prevent most of these fires.
Part 3 — Misconception Check
Targeted check of the most common misconceptions from this cycle. (20 points, ~20 min)
Stop & Think Before You Begin
Review your work from today's stations. You investigated Ohm's Law (V = IR), calculated power (P = VI), and designed a safe circuit. Now show what you've learned — and connect it to what you already know from Weeks 1 and 2!
Exit Ticket Question Guide
Questions 1-2: NEW Content (This Week)
- Q1: Apply Ohm's Law to calculate current or voltage in a circuit scenario
- Q2: Use P = VI to determine power consumption and compare devices
Questions 3-4: SPIRAL Review (MAP Achievement Season — PS3.A.1)
- Q3 (MAP PS3.A.1): Kinetic energy and the squared relationship — how does doubling speed affect KE?
- Q4 (MAP PS3.A.1 Bridge): Connect squared relationships across kinetic energy (KE = ½mv²) and electrical power (P = I²R)
Question 5: [INTEGRATION + SEP-6]
Construct an explanation about circuit overload: Why does the squared relationship in P = I²R make even small current increases dangerous? Connect this to kinetic energy (KE = ½mv²) and to electromagnets from Weeks 1-2. Use SEP-6 (Constructing Explanations) to build a complete chain of reasoning.
Enrichment & Extension
Optional content if you finish early or want to go deeper.
Scientist Spotlight: Lewis Latimer (1848–1928)
Lewis Howard Latimer was an African American inventor, engineer, and draftsman whose work transformed the electrical industry. Born in Chelsea, Massachusetts, to parents who had escaped slavery, Latimer taught himself mechanical drawing while serving in the U.S. Navy during the Civil War.
Latimer's most important contribution was developing an improved method for manufacturing carbon filaments for incandescent light bulbs. His process made filaments last significantly longer — turning Edison's invention from a laboratory curiosity into a practical product that could light homes and streets. Without Latimer's filament improvement, early light bulbs burned out in hours rather than days.
He worked alongside both Alexander Graham Bell (drafting the patent drawings for the telephone) and Thomas Edison, eventually joining Edison's team as one of the original "Edison Pioneers." Latimer also supervised the installation of electric lighting systems in New York City, Philadelphia, Montreal, and London.
Connection to this week: Latimer understood that the carbon filament's resistance was the key to producing light. Using V = IR and P = VI, engineers could calculate exactly how much current a filament needed to glow white-hot without burning out — the same circuit analysis skills you practiced today.
Environmental Justice: Energy Poverty in St. Louis
Not everyone in St. Louis has equal access to safe, reliable, and affordable electricity. In some underserved neighborhoods — particularly in North St. Louis and parts of East St. Louis — residents face energy poverty: the inability to adequately meet household energy needs. Families may spend 15–20% of their income on electricity bills, compared to 3–4% for wealthier households.
Older homes in these communities often have outdated wiring that cannot safely handle modern electrical loads. Undersized circuits (15A instead of 20A), missing GFCI protection, and deteriorating insulation increase the risk of electrical fires. According to the National Fire Protection Association, homes in low-income neighborhoods experience electrical fires at nearly twice the national average rate.
The grid infrastructure itself is uneven. During St. Louis heat waves, rolling brownouts disproportionately affect historically redlined neighborhoods where power lines and transformers have received less investment. Residents who cannot afford air conditioning face dangerous heat exposure — a direct health equity issue.
What can be done? Organizations like the Missouri Division of Energy and Ameren Missouri's low-income weatherization programs help upgrade wiring and insulation. Understanding circuits — the very skills you built today — is the foundation for advocating for safer, more equitable electrical infrastructure in every neighborhood.
Week 3 Complete!
Next Week: Electric Motors & Mechanical Advantage — how can a tiny motor lift a car?