3-5 Calculations (p 116 3.2)

• Natuurkunde
• Middelbare school
• havo, vwo
• Leerjaar 2

Calculations (p 116) 3.2

Energy

• During combustion energy is released in the form of heat and light.
• Your body also releases energy during the combustion of food. This is used to move and to heat up the body. 
  

Joule

Energy is measured in joule (J).

1 joule is a very small quantity so we rather work in kJ and MJ

1 kJ = 1 000 J

1MJ = 1 000 000 J

Energy

• Walking up one fight of stairs uses = 10 kJ
  
• Running 1 km = 200kJ
  
• 20% of energy used is by the brain
  
• 14 year old person uses about

        11 000 kJ per day. 

• Boys use a bit more.
  

Food

• On the label of food it is written how much energy is released during combustion.
  
• 100 gram penutbutter = 2 690 kJ
  
• Energy in food is also  

       indicated in the unit 
       kilocalories (kcal)
        1kcal = 4,2 kJ

Heat of combustion

• 1kg dry wood releses 16 MJ heat
  
• 1kg coal releases 29 MJ heat/ energy
  
• Heat of combustion - 

        The amount of energy that is released when buring 1kg

        of fuel.

• Liquids are measured in liters and gasses in m3
  

Natural gas

Hydrogen gas

Petrol

Diesel

Dry wood

Coal

Type of fuel                          Ammount.                           Energy                 HEAT of combus.

Calculations using the table

How much energy is released when you burn 20 kg of dry wood?

(from the table you see the heat of combustion = 16 MJ)

Therefore it will be 16 x 20 = 320 MJ

Amount	1kg	20 kg
Heat	16 MJ	?

Energy in food

• This is mostly indicated per 100 gram (kJ/100g) and liquids in kJ/100ml
  
• Nutritional value=  heat of combustion of food
  
• Of all the food you eat the body uses:

           Carbohydrates, fats and protein

• 1g protein/ carbohydrate = 17kJ
  
• 1g fat = 38 kJ
  

Nutritional value is the same as heat of combustion.

1. true
2. false

1kcal equals 4,2 kJ

1. true
2. false

The body uses carbohydrates, fats and hydrocarbons in foods.

1. true
2. false

The heat of combustion of a fat is higher that that of a hydrocarbon.

1. true
2. false

One liter of petrol has more energy than one liter of diesel.

1. true
2. false

Convert:
0,85 MJ =

1. 0,000 85 kJ
2. 0,85 kJ
3. 850 kJ
4. 850 000kJ

Convert:
250 000 J =

1. 250 000 MJ
2. 250 MJ
3. 0,25 MJ

730 J =

1. 730 000 kJ
2. 730 kJ
3. 0,73 kJ
4. 0,000 73 kJ

Now do the following questions

29, 31, 32, 33 (blz 117)

Of question 31 insert a picture on the next slide.

Insert a photo of your answer to question 31

How did the lesson go?