Week 3: Electric Circuits & Power
Grade 8 Science | Rosche | Kairos Academies
MS-PS2-3 Forces and Interactions
The Phenomenon: The Dimming Lights Mystery
Anchoring Context & Focus Question
Scenario: You're blow-drying your hair and notice the bathroom light flickers and dims. A few seconds later, the light comes back to full brightness.
Driving Question: Why do lights dim when you turn on a hairdryer?
W1 + W2 + W3 Connection
- W1: Magnetic fields exist around magnets; force decreases with distance (inverse relationship)
- W2: Moving charges create magnetic fields; changing fields create current (electromagnetic induction)
- W3 new concept: In circuits, multiple devices share the same current path โ when one device draws more, less is available for others
Learning Targets for This Week
- Connect magnetic field properties (W1) to electromagnetic principles (W2)
- Apply force-distance relationships to electromagnetic device design
- Explain how circuits share current and voltage
- Integrate W1+W2 concepts to explain real-world electrical systems
Vocabulary
Cognate Strategy: Many science words look similar in English and Spanish โ 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 ร 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 โ The Dimming Lights Mystery
Observe circuit behavior when load changes.
The Dimming Light Mystery
You're blow-drying your hair and notice the bathroom light flickers and dims. Seconds later it's bright again. What just happened?
Stop & Think โ Consider these before starting the form:
-
What changed when you turned on the hairdryer?
(Think about: current demand in the circuit) -
If both devices are on the same circuit, what might they
share?
(Think about: current, voltage โ what do parallel devices share?) -
How is this similar to what we learned about magnetic force and
distance?
(Think about: inverse relationships โ more of one thing means less of another)
Tiered Support โ Sentence Starters
- When I turn on the hairdryer, the circuit has to supply more _______ to both devices.
- Devices on the same circuit share _______, so when one uses more, others get _______.
- This is similar to magnetic force and distance because both show a(n) _______ relationship.
Worked Example โ Calculating Circuit
Power and Load
Scenario: A refrigerator (150W) and a lamp (60W) share a 120V household circuit. What happens when a hairdryer is plugged in?
Use the formula: Current (I) = Power (P) รท Voltage (V)
Step-by-Step Walkthrough
Step 1: Calculate current for each device
Refrigerator: I = P รท V = 150W รท 120V = 1.25 A
Lamp: I = P รท V = 60W รท 120V = 0.5 A
Step 2: Calculate total current (refrigerator + lamp)
Total = 1.25A + 0.5A = 1.75 A (well within a 15A circuit)
Step 3: Add the 1500W hairdryer โ what happens?
Hairdryer: I = 1500W รท 120V = 12.5 A
New total: 1.75A + 12.5A = 14.25 A
At startup, hairdryers surge briefly above rated current โ this
voltage drop dims the lamp.
Step 4: Connect back to W1 & W2
Increased current → stronger magnetic field in circuit wires
(W2: Oersted)
Changing current → induced voltage in nearby circuits (W2:
Faraday induction)
High current in parallel branches → less voltage available per
branch (W3 circuit sharing)
Common Mistake to Avoid
"Turning on more devices makes them all stronger."
WRONG โ devices compete for available current. When one draws more,
the others get less. This is why lights dim and why circuit breakers
exist.
Station 1 โ Ohm's Law Investigation
Explore relationships between voltage, current, and resistance.
Station 1: W1 + W2 Integration Review
Synthesize what you learned in Weeks 1 and 2 by completing the concept table below. For each row, write the real device example in your form.
Concept Synthesis Reference Table
| Concept | From Week | Real Device Example |
|---|---|---|
| Magnetic field around current-carrying wire | W2 (Oersted) | Motor, speaker coil |
| Force decreases with distance | W1 | Magnetic levitation, MRI machine |
| Induced current from changing magnetic field | W2 (Faraday) | Generator, transformer |
| Field strength depends on current amount | W1 + W2 | Electromagnet lifting power |
Tiered Support โ Sentence Starters for Connecting W1 & W2
- In Week 1, I learned that magnetic force _______ as distance _______. This connects to W2 because _______.
- Oersted discovered that a current-carrying wire creates a _______. This is useful for devices like _______ because _______.
- Faraday's law tells us that a changing _______ creates a _______. Without this principle, devices like _______ could not work.
- An electromagnet's strength depends on _______, which means you can control it by _______.
Station 2 โ Power & Energy
Analysis
Calculate power consumption and circuit capacity.
Station 2: Circuit Analysis Lab โ How Devices Share Current
Use the formula I = P รท V to calculate current draw for each appliance. Then determine if the circuit is safe.
Appliance Current Reference Table (120V circuit)
| Appliance | Power (W) | Voltage (V) | Current Draw I = P รท V |
|---|---|---|---|
| Lamp | 60 | 120 | 0.5 A |
| Refrigerator | 150 | 120 | 1.25 A |
| Microwave | 1200 | 120 | 10.0 A |
| Hairdryer | 1500 | 120 | 12.5 A |
Key Safety Concept: Circuit Breakers
Circuit breakers trip when total current exceeds 15โ20 A to prevent wire overheating and fire. If a hairdryer (12.5A) + microwave (10A) are on the same circuit, total = 22.5A โ the breaker trips! This is by design, not a malfunction.
Tiered Support โ Calculation Scaffold
To find current: I = P รท V
- P = power in Watts (W) โ found on the appliance label
- V = voltage in Volts (V) โ standard US household is 120V
- I = current in Amperes (A) โ what you calculate
- To check safety: add all the individual currents. If total > 15A, the circuit could overload.
Station 3 โ Design a Safe Circuit
Apply circuit principles to prevent overload.
Station 3: Smart Home System Design Challenge
Design a smart home electrical system that safely handles peak power demands for a 5-room home. Apply everything from W1, W2, and W3!
Design Constraints
- Service panel capacity: 200A total
- Usable capacity: 80% maximum = 160A (safety code requirement)
- Must power: 5 rooms
- Circuit breaker limit: Each circuit should not exceed 15โ20A
Typical Appliance Loads โ Reference for Your Design
| Room | Typical Loads | Approx. Current |
|---|---|---|
| Kitchen | Refrigerator, microwave, small appliances | 20โ40A peak |
| Bathroom | Hairdryer, lighting | 13โ15A peak |
| Living Room | TV, gaming, lamps | 5โ10A peak |
| Bedroom | Lamps, phone chargers, fan | 3โ6A peak |
| Laundry | Washer (dedicated 20A), dryer (240V, 30A) | 20โ30A peak |
W1+W2+W3 Connection for Your Design
- W1: Wires carrying high current have stronger magnetic fields โ keep high-current circuits away from sensitive electronics
- W2: Large current changes (like motors starting) induce brief voltage spikes in nearby circuits
- W3: Distribute loads across multiple circuits to prevent any single circuit from overloading
Tiered Support โ Design Steps
- List each room and its highest-demand appliances
- Calculate current for the biggest load in each room: I = P รท V
- Assign each room to its own circuit (separate breaker) so loads don't add together
- Add up total current across all circuits โ must stay below 160A
- Justify your design: why did you separate certain rooms?
Exit Ticket โ Circuit Systems
Integration
Synthesize understanding of circuits and power.
Enrichment & Extension
Optional content if you finish early or want to go deeper.
Week 3 Complete!
Next Week: Electric Motors & Mechanical Advantage โ how can a tiny motor lift a car?