Lesson 13 — Energy calculations

• Energy
• Middelbare school
• havo, vwo
• Leerjaar 1

Energy calculations

What did we cover previous lesson?

•  The law of conservation of energy

Demonstration

• Tennis ball
  
• Basket ball
  
• Bouncy ball
  

This lesson

• Demonstration
• Recap
• New formula's
• Practice examples

Learning goals

• Explain what is meant by efficiency
• Name the formula for efficiency
• Explain what is meant by power
• Name the formula for power
• Apply both formula's in calculation exercises

Recap

• Energy cannot be created or destroyed
  

• In every process you loose some amount of energy

• Useful energy

• wasted energy

• Efficiency
  

80 J/s of electrical energy.

20 J/s of light energy

A construction lamp

60 J/s of thermal energy

Efficiency=(​totalenergy​​usefulenergy​​)⋅100

%

Efficiency=(​totalenergy​​usefulenergy​​)⋅100

%

=(​E​t​​​​E​u​​​​)⋅100

%

η

• η: efficiency (%)
  
• Eu: Energy useful (J)
  
• Et: energy total (J)
  

80 J/s of electrical energy.

20 J/s of light energy

A construction lamp

60 J/s of thermal energy

Practice examples

Chemical

energy – J/s

Electric energy 

250 J/s

Light energy

45 J/s

Heat energy

– J

Practice examples

Chemical

energy 250 J/s

Electric energy 

250 J/s

Light energy

45 J/s

Heath energy

205 J

Practice examples

Chemical

energy 250 J/s

Electric energy 

250 J/s

Light energy

45 J/s

Heath energy

205 J

Useful 

Wasted

Practice example – 1

Make ETDs for the following objects. Which energies are useful? Which are wasted? 

• Fan (Ventilator) 
  
• Car 
  
• Television 
  

Practice example – 2

a) A petrol car engine transforms 1000 J of chemical potential energy into 300 J of kinetic energy and 700 J of wasted energy. calculate the efficiency of this engine.

b) A wind turbine transforms 500 J of kinetic energy from the wind into 150 J of kinetic energy. Calculate the efficiency of this wind turbine.

c) A kettle produces 1817 J/s in heat, and 483 J/s in sound and other by products. Calculate the efficiency of this kettle.

Practice example – 3

A log of wood on average contains 5400000000 J of chemical energy. When burned It uses 600 kJ/s. It turns 120 kJ into light, and 30 kJ into sound 

a) make an energy transfer diagram for burning a log of wood. Include the amount of energy used/produced per second for each energy type.

b) How much kJ of heat does it produce every second?

c) What is the efficiency of burning this log of wood?

d) How long can one log burn? Give your answer in hours.

Power=​time​​Energy​​

P=​t​​E​​

Power=​time​​Energy​​

P=​t​​E​​

Power=​time​​Energy​​

• P: power — watt  (W – J/s)
  
• E: Energy — Joules (J)
  
• t: time — seconds (s)

Practice example – 4

a) A kettle is filled with water and turned on. It takes 3 minutes to boil the water, and the kettle is supplied with 324,000 J of energy in this time. What is the power of the kettle?

b) A solar panel under the right conditions generates 2000 J per hour. Calculate the power of such a solar panel.

c) A 250 W television and a 400 W television are both used for two hours. How much more energy does the 400 W television require to operate for this time than the 250 W television?

Homework

paragraph 2.10: The workbook exercise

Learning goals

• Explain what is meant by efficiency
• Name the formula for efficiency
• Explain what is meant by power
• Name the formula for power
• Apply both formula's in calculation exercises