Lesson 11

• Physics
• Middelbare school
• vwo
• Leerjaar 3

Chapter 3 — Energy — Efficiency

This lesson

• Efficiency
• Taking a look at the homework
• Making a summary
  

Learning goals

• You can explain the concepts of efficiency, useful energy and wasted energy in your own worlds.
  

• You can read energy flow diagrams.
  

• You can name, rearrange and apply the formula for efficiency in terms of energy and power
  

• You can explain the difference between power and energy
  

Recap

• What is heat transfer?
  
• In what three ways does it take place?
  

Recap

let's explain and come up with examples examples of:

• Convection
• Conduction
• Radiation

Efficiency

Useful energy

Wasted energy

η=​E​tot​​​​E​used​​​​⋅100÷

η=​P​tot​​​​P​used​​​​⋅100÷

• η: Efficiency (no unit)
  
• E_used: Useful energy output (Joules)
  
• E_tot: Total energy input (Joules)
  

• η: Efficiency (no unit)
  
• P_used: Useful power (Watts)
  
• P_tot: Total power (Watts)
  

Exercise 1

Someone uses their laptop for 40 minutes. Their laptop has a power of 250 watt. In this time the laptop loses energy through things like heat and generating sound. In these 40 minutes the laptop loses 216000 joules to this. Calculate the efficiency of this laptop.

Exercise 2

When the sun is shining brightly, the incoming irradiation at an alpine hut is 1000 W/m2. A solar panel with a surface area of 1.8 m2 supplies a maximum power of 288 W.

Calculate the efficiency of this solar panel.

Exercise 3

A motorcycle has an efficiency of 27%. The motor cycle uses 7,4 liter of petrol to drive about 110 kilometer. There is 31500000 joules in one liter of petrol. Calculate how much energy in joules went into making the motor move.

The homework

• Section 3 exercise 8, 9
  
• Section 4 exercise 1, 2, 3
  

Learning goals

• You can explain the concepts of efficiency, useful energy and wasted energy in your own worlds.
  

• You can read energy flow diagrams.
  

• You can name, rearrange and apply the formula for efficiency in terms of energy and power
  

• You can explain the difference between power and energy