Energy In, Energy Out: Mastering System Efficiency
Target Audience: High School (Age 15) | Subject: Physics / Physical Science | Duration: 60 Minutes
📋 Materials Needed
- Scientific Calculator (or smartphone calculator app)
- Graph paper or blank paper
- Colored pencils or highlighters (at least 3 colors)
- Ruler
- Bouncy Ball (tennis ball, superball, or basketball) & Tape Measure / Meter Stick
- Device with internet access (for optional PhET simulation: Energy Skate Park or Energy Forms and Changes)
- Printed or Digital "System Energy Audit" Worksheet (provided in lesson body)
🎯 Objectives & Success Criteria
Learning Objectives:
- Apply the Law of Conservation of Energy to account for all energy entering and leaving a system.
- Calculate the percentage efficiency of various real-world technological systems.
- Construct and interpret a Sankey Diagram to visually represent energy transfers and losses.
Success Criteria:
- "I can explain why energy isn't 'destroyed' when a phone gets hot while gaming."
- "I can accurately calculate efficiency using $ \text{Efficiency} = \left(\frac{\text{Useful Energy Out}}{\text{Total Energy In}}\right) \times 100 $."
- "I can draw a balanced Sankey diagram where Input Width = Useful Output + Wasted Output."
The Hot Smartphone Mystery
Instructor Talking Points: "Have you ever been in the middle of a gaming session or streaming a high-definition video on your phone, and noticed the back of the device getting super hot? You didn't buy a pocket warmer—you bought a smartphone! So why is it producing heat?"
"Here's the deal: every system—from your phone, to an electric skateboard, to a roller coaster—runs on energy. The Law of Conservation of Energy states that energy cannot be created or destroyed. It can only change forms. That means 100% of the electrical energy coming out of your phone battery must go somewhere. But not all of it goes toward what you actually want it to do!"
- Energy Input ($E_{in}$): Total energy supplied to a system (e.g., electrical energy from a battery).
- Useful Output ($E_{useful}$): Energy transformed into the intended work (e.g., light and sound from the screen/speakers).
- Dissipated (Wasted) Output ($E_{wasted}$): Energy transformed into unintended forms, usually thermal or acoustic (e.g., excess heat).
1. I DO: Modeling System Calculations & Sankey Diagrams
Let's look at the mathematical rule governing all systems:
$\text{Efficiency (\%)} = \left( \frac{\text{Useful Energy Output}}{\text{Total Energy Input}} \right) \times 100$
Example Problem: An old incandescent lightbulb uses 100 Joules (J) of electrical energy every second. It converts 10 J into useful light energy, while 90 J is wasted as heat radiating into the room.
- Efficiency Calculation: $ \left(\frac{10\text{ J}}{100\text{ J}}\right) \times 100 = 10\% \text{ Efficient} $
Visualizing Energy Flow: The Sankey Diagram
A Sankey Diagram is an arrow map where the width of the arrow represents the amount of energy.
========================> USEFUL OUTPUT: 10 J Light (Straight ahead, narrow arrow)
|
| WASTED OUTPUT: 90 J Heat (Curves downward, thick arrow)
v
2. WE DO: Hands-On Bouncy Ball Audit
Interactive Experiment: Let's test system efficiency right now using gravity and a ball!
- Set the Input: Hold your ball at a height of 100 cm (1 meter). At this height, the ball has 100% Gravitational Potential Energy ($GPE$). Let's call this 100 units of Input Energy.
- Release: Drop the ball without pushing it down. Observe the height of the first rebound bounce.
- Record the Output: Measure the rebound height in centimeters (e.g., if it bounces back to 65 cm, it retained 65 units of useful kinetic/potential energy).
- Analyze Together:
- Rebound Height = Useful Output Energy ($E_{useful}$)
- Missing Height ($100\text{ cm} - \text{Rebound Height}$) = Wasted Energy ($E_{wasted}$) transformed into sound ("thud") and thermal energy (friction/deformation heat).
- Calculate Efficiency: $ \text{Efficiency} = \left(\frac{\text{Rebound Height}}{100\text{ cm}}\right) \times 100 $
3. YOU DO: Independent Energy Audit Challenge
Choose ONE of the following real-world tech scenarios to analyze. Perform the calculations, draw a scaled Sankey diagram on your paper, and propose an engineering fix to reduce energy waste.
Option A: Gas-Powered Car vs. EV
A traditional gas engine receives 800 Joules of chemical energy from fuel. It transforms 200 Joules into motion (kinetic energy). The rest is lost as engine heat and exhaust noise.
- Calculate the efficiency %.
- Calculate wasted energy in Joules.
- Draw the Sankey diagram.
Option B: Gaming Console
A gaming console draws 150 Watts of electrical power. 105 Watts goes into rendering graphics and audio. The remaining 45 Watts is emitted as heat through the cooling fan.
- Calculate the efficiency %.
- Identify useful vs. wasted forms.
- Draw the Sankey diagram.
Option C: Modern LED Bulb
An eco-friendly LED light uses 50 Joules of electrical energy. It converts 42 Joules into visible light energy and dissipates 8 Joules as excess heat.
- Calculate the efficiency %.
- Compare to the incandescent bulb (from I DO).
- Draw the Sankey diagram.
The 60-Second "Energy Consultant" Pitch
Imagine you are an engineer hired to improve the system you chose in the "You Do" section. Present a 60-second verbal or written pitch explaining:
- What the system's current efficiency percentage is.
- Where the energy is being "wasted" (dissipated).
- One creative engineering design change to capture or reduce that waste (e.g., using heat pipes, regenerative braking, improved insulation, sound dampening).
Mind-Bending Big Picture Question:
Is it ever possible to create a 100% efficient machine on Earth where zero energy is wasted? Why or why not?
(Hint: Think about friction between moving parts and electrical resistance in wires!)
📌 Assessment & Differentiation Strategies
Assessment Methods:
- Formative: Observation during the bouncy ball activity and real-time correction of calculations.
- Summative: Evaluation of the independent scenario Sankey Diagram for accuracy (balanced arrow widths) and correct calculation of efficiency percentages.
Differentiation Options:
- Support (Scaffolding): Provide a pre-drawn arrow template grid for the Sankey diagram so the student only has to scale arrow widths.
- Extension (Advanced): Challenge the student to research "Regenerative Braking Systems" in hybrid/electric cars and calculate how recapturing kinetic energy increases overall efficiency.