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Energy cycle.

posted Jan 31, 2015, 8:15 PM by   [ updated Sep 18, 2019, 7:19 PM by Upali Salpadoru ]

We  can run,                                                       jump, dance, and work.  

To do all this you need energy. 

From where do you get energy?

Work can be done only if there is 


Fig.1 Sun is the primary source of Energy.

What is Energy ? 

It is difficult to define. Normally it is said 

“ For work to be done there must be energy”.

It is somewhat like if you wish to buy things, you must have money. 

In this article you can learn the sources of energy, forms of energy and a few routes of energy flow.

Work and Energy are measured by the same unit; Joule. It is similar to the cash you have and the money you spent are counted in the same currency; $   or  £

 Energy and matter can neither be created nor destroyed.

  How does the sun get energy?
  This is due to a FUSION type of a Nuclear Reaction. 
                   Hydrogen changes to Helium releasing energy losing mass.
                   Matter changes to energy. 
A small portion of it, that comes to the Earth is responsible for over 90% of what happens in our planet.

Measuring energy.

 This girl has pushed her trolley a distance of 1 meter to left.
  Then she would have done 1 Joule of WORK spending 1 Joule of ENERGY.

 Work =  Force x Displacement.


It is a well known fact that  our body gets energy from the food we eat. 

From where do we get food?     Is it from the "super market" ?
All animals, including man gets food from PLANTS.


Fig.3  We are indebted to plants for all our food.

Respiration is the chemical process that releases the energy from food.  

As respiration is a chemical change we have to conclude that the energy we get is ‘chemical energy’.

 ANIMAL FOOD         
100 g of Food         Energy / kilojoules 100 g of Food          Energy / kilojoules
             Beef                             1000
  Banana                               318
          Apples                    175  Bread                                 990
          Butter                     3120 
  Cereal                                1600
          Cheese                    1720  Gram                                 1400
          Egg                         600
  Green Vegetables                100
          Mutton                    1880   
  Onions                               100
   Sugar                                 1600

When a chemical change takes place energy is either released or absorbed. 

The two most important chemical changes for the living are Photosynthesis and Respiration

During photosynthesis radiant energy from the sun is absorbed by  the plants. Carbon dioxide and water becomes glucose.


In respiration this process is reversed. Food is combined with oxygen to supply energy for all living beings.

Coal is much used to get energy. Coal and Petroleum are formed by the fossilization of plants and animals. They also harbour chemical energy derived from photosynthesis. Large scale of these fossil fuels increase the Carbon dioxide ratio in the atmosphere which is greatly responsible for the global warming.

Fig.5  Coal harbours chemical energy derived from photosynthesis.


This energy comes in the form of waves called ‘Electro Magnetic Waves’. They can also be considered as particles. Both theories are now accepted.
The radiation coming from the sun is sometimes called Solar Radiation.  Some of the important features of these are as follows:

  •     Constant speed.  300,000km/s.(in vacuum)
  •     Travel without the help of molecules. (Radiation)
  •     We do not have organs to sense most of these.     [The eye is sensitive to a small   range  called light. The skin is sensitive to Infra red. Some times called heat  radiation.]
  •     Some waves carry an immense amount of energy and cause severe damage to living tissues.     [These are often used to kill cancer cells, and for sterilization]
  • These waves differ due to their frequency.   (Frequency and Wave length are related)
          Formore on  Electro Magnetic waves click  here. Then search according to alphabetical order.

 Types of Radiant Energy and their wave length               

 Name From 
 1. Radio waves.
 104m10 m
 2 .T.V.Waves
 10m  10 –1 m
 3. Micro waves
 10 –1 m  10 –3 m
 4. Infra red waves 
 10 –3 m 10 –6 m
 5. Visible light  10 –6 m 10 –8 m
 6. Ultra violet light  10 –8 m 10 –10 m
 7. X’ rays    10 –10 m  10 –12 m
  8. Gamma rays
  10 –12 m   10 –15 m

    We are quite familiar with electricity. Electricity is nothing that comes from outside. It is the movement of some fundamental particles present in all atoms. They are called electrons.
All electrons go round the nuclei of atoms. Under certain conditions, they move in a stream as shown by dark arrows. This is an electric current. Substances which can pass electrons in this manner are called conductors. All metals are good conductors.

Fig.6    Movement of electrons from atoms in a

                                                 Fig.7 Lightening is a vast source of untapped electrical energy.

 Question Answer
 Most of the Hydroschemes and Wind forms give their output in Kilo Watts or Giga Watts.  Watts measure Power.
How are they related to the Energy they produce?
 POWER is the unit of using Energy.
Watt is the unit of power named to honour James Watt (1736 - 1819 )
One Watt is the power of using 1 Joule in one second.

 Energy = Power x Time   Joules = Watts X Seconds.
    Practice problem.
Jenny's mum uses a kettle of 2. kW to boil the water.  How much energy does it take in 5 seconds.
1 W power means using i Joule In 1 second
2kW power means using 2000 J in 1 s.
Therefore In 5 seconds it will use .....5X 2000 J....= 10000. J              


This is the energy stored up in the nucleus of atoms. There are three methods by which this energy may be released.

  1. Natural decay of very large nuclei emitting alpha. Beta and gamma rays. This is called radioactivity.
  2. Nuclear Fission:- Breaking up of a large nucleus to form smaller nuclei. This happens in the Atom bomb.
  3. Nuclear Fusion:- Two nuclei join together to form a large nucleus. This is what happens in the sun and the hydrogen bomb.

Fig.8  In places like this Atomic energy is converted to Electrical energy. 


This energy is always connected with matter. When a force acts on a mass, work will be done. That amount of work has been bought by spending a certain amount of energy. The energy spent has to exist somewhere in some form. Most of this energy will exist in one of these forms:


Kinetic means motion. So this means energy present in a body due to its virtue of motion. Wind and water falls are good sources of  kinetic energy.

Fig. 9. These wind turbines convert Kinetic energy to Electrical energy.

If you kick a ball, you will be giving a lot of energy to it. Given the particulars, it is possible to find the magnitude of this
This is the formula to find out Kinetic energy in a body .

HALF the Mass multiplied by the square of Velocity ( Speed in that direction)

written as :  Kinetic Energy = 1/2 MV2


Very often it is very difficult to find out whether an object has potential energy. Energy

may be stored up in different ways. [According to some sources even chemical energy is also

potential energy]

Easiest to understand is the energy that may be stored up in springs.
1.    Energy is usually stored up in springs by the following ways.:-

  i.    Bent plates.
  ii.    Wound springs.
  iii.    Stretched springs.
  iv.    Compressed springs.

2.    Gravitational Potential Energy.

The Earth also acts like an invisible spring. when we lift an object from the ground.The
moment we relase the object it will be pulled back to the ground.
Therefore any object taken away from the surface of the Earth will gain potential energy.
This is what we call Gravitational Potential Energy.

Fig. 10.  A boulder capable of coming down.

Consider  a big boulder at the dge of a cliff.It looks quite harmless, but a slight tremor
can cause much damage to men and material. It posesses potential energy by virtue of its height

The gravitational Potential Energy is calculated by multiplying the height by the product of mass and gravitational attraction . which is g taken as 10N/kg.
Formula  PE (g) =   mgh

Gravitational Potential Energy of the boulder should be

                                                                                      =  100 X 650 X 10  J

                                                                                      = 650,000 J



This is really the energy present due to the movements and vibrations of the molecules in matter. This is also called 'Internal Energy '  or Thermal energy. Usually when a substace gains thermal energy, the temperature rises. This is measured by multiplying the mass by the product of the temperature change and a factor called Specific Heat Capacity. which depends on the substance.



Here the reference is for the matter waves such as that found in the ocean and the air. Earth Quakes produce powerful waves on land called 'Seismic waves.

Matter waves which can be perceived by the ears are called  SOUND WAVES. 

  Practice questions.

Answers for questions 1 to 4.
     A-  Wind energy.    B-  Energy from tides.   C- Chemical energy.  D- Geo thermal energy.
1.0    Which can be the form of energy  that does not start from the sun?
2.0     Food substances posses this energy.
3.0     A good source of renewable energy.
4.0     Not yet fully tapped.
                     Answers for questions 5 to 7.     
A - Newton  B- Joule,  C - Watt D- Mass. What is the proper unit for the purpose given?
 5.0   To compare work done.
 6.0   To measure force of gravity.
 7.0   To describe the power of a micro wave cooker.
       Questions from 8 to 10.
           What kind of energy change can be made by these.
   Select the Answers  from  A-  Nuclear to Radiant.   B- Kinetic to chemical.  C- Chemical to kinetic,
                                          D- Chemical to vibrational.
 8.0  A speeding motor car.    9.0 A child singing.      10.0    A star.   
Highlight to get Answeers :-    1.- D,  2. - C     3- A    4- B ,  5- B,  6. A,  7- C. 8- C   9- D,  10- A,

         Multiple Choice Questions.

   Multiple choice questions.


2-1 This reading gives an idea of the average kinetic energy of the molecules in a substance.

2-2 The SI. unit for the quantity of heat energy.

2-3 What type of energy would a cloud possess?

2-4 The SI unit for the rate of using energy ?

2-5 Energy gained by water when rising up in a plant stem.

Answers for 1 to 5.

A-Potential     B Joule.      C- Watt        D-Temperature

2-6 What can be considered as the primary source of energy?

            A. Earth .B. Plants   C. Sun   D.   Food

       2-7 Which out of these may possess energy that did not originate in the sun?

            A-   Tornado  B- Volcano  C- Waterfall  D- Thunder and lightening.

       2-8  What is the greenhouse gas formed when fossil fuels are   used to   get energy?

      A. Carbon dioxide    B.Water vapour. C. Nitrogen   D. Ozone.  

       2-9  ‘Calorie” is a word often used by nutritionists. What is the SI unit for this?

             A- Celsius.   B-Fahrenheit  C- Kelvin   D- Joule.

       2-10.Which of these can be considered as renewable sources of energy?

             1- Hydropower,  2- Wind.  3- Sea waves  4- Biogas.     

           A- All of them    B- None of them  C- Only 1 and 2.   D- 4 only.

3x10 = 30 marks.


Give the energy made use of and the main form of energy released.

Device / Action

Starting Energy

Final energy formed or released.

Green plant exposed to sun.

Turning a bicycle dynamo .

Flashing a torch light.

Wood burner.

Stopping a moving car.

2x10 = 20 marks.



3-1 A Waterfall has a height of 60m and water drops at the rate of 500kg for every second.What is the maximum amount of energy that can be obtained from that in a second?    

3-2 Michael has eaten 200g of bread with 100g egg 5g of butter. He had a cup of tea with 3 g of sugar. If only 25% of the energy can be used by him How much work can he do?

Energy values for 100g. are:






Energy / 100g

990 J


600 J


3-3  A car of mass 1,200kg. Is moving at a velocity of 100 m per second. How much kinetic energy does it have?

3.4 Wind turbines at the Tararua wind farm, in New Zealand has a capacity 3 MW .

One megawatt (MW) = 1,000 kilowatts = 1,000,000 watts. If an average house needs 500 W at a time how many houses can this serve?

5x4=20 marks


4.1 These bulbs use electric energy to produce light energy. Which one use more energy in a unit time?bulbs.jpg

4,2 Which one will give more light at a time?

4.3 In what ways would the two bulbs lose energy?

  1. Fluorescent bulb-......

  2. Filament bulb-...........

5x4 =20 marks

4.4 Describe how the two bulbs operate.


  For answers click: Answer page -Ph.