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Spoken words are in blue

Scene: It is a windy summer's day. The branches on the trees are bending threateningly. Two children are trying unsuccessfully to play badminton - the wind is stalling the shuttle. Resigned, the children are going home. Their father is working on something in his small workshop.

Both children, together: Can we help you? We can't play badminton as it is too windy.

Father: Do know what the wind is?

Child1: The wind is fast moving air, isn't it?

Child 2: The wind is the effect of unequal heating of sea and land by the Sun - we've just done this in school.

Father: You are both right and your definitions are quite exact. Let’s have a look in this book.

Father goes to the shelf and takes down the big science book. He is opening the book and reads:
“Wind is the horizontal motion of the air past a given point. Winds begin with differences in air pressures. Pressure that's higher at one place than another sets up a force pushing from the high toward the low pressure. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated. Meteorologists refer to the force that starts the wind flowing as the "pressure gradient force." High and low pressure are relative. There's no set number that divides high and low pressure. Wind is used to describe the prevailing direction from which the wind is blowing with the speed given usually in miles per hour or knots.”

Father: You see, I prefer a definition based on the difference of heat between sea and solid ground because the different temperatures are creating air pressure differences. Looking for historical information in the book. So we have succeeded in defining the wind. Let’s also have a look at the advantages of windy weather, not only the bad effects. Do you know that wind power has perhaps the longest history of any of today's mainstream or alternative energy sources. Nobody knows for sure when man started using the wind's power to grind flour or pump water, but it is thought that the first windmill appeared in the Persian region. From there, this windmill technology spread back to northern Europe. Windmills crafted by the Dutch were used primarily to pump water.

Child 1: So the wind can be useful then?

Father: Yes and in lots of different ways - windmills were definitely not the first structures to harness the wind. This award belongs to the sailboat. More than likely, founded in small scale (small canoe with an animal skin as a sail) the sailboat became the only way to cross large areas of water. The sailboat evolved into large ships moving great distances by using only wind as a source of power. The old papyrus books inform us that more than 5,000 years ago, the Egyptians used the wind to sail ships on the Nile. Later, people built the first turbines to grind grain. These machines looked like paddle wheels and were used in Persia as early as 200 B.C. By the 14th century, the Dutch had taken the lead in improving the design of windmills. They invented propeller type blades and used wind power to drain the marshes and lakes of the Rhone River delta. In America, early European settlers used windmills to grind wheat and corn, to pump water, and to cut wood at sawmills. By the early twentieth century, small windmills were used for pumping water and electric power generation in Europe, the United States, Africa, and elsewhere. In addition to thousands of small wind electric generators, a few larger systems were built in North America and Europe. Modern usage of windmills to produce electricity was brought about mostly because of the oil crisis of 1973.
Today, the latest technologies allowed people to develop huge wind turbines of 6 megawatt-installed capacity.

Father takes another book from the shelf. It is a book about the most important 70 inventions of the ancient world, and shows it to the children. Then he takes another book about windmills and wind turbines. The books show all types of wind turbines demonstrating their characteristics with many pictures allowing better understanding the principles.

Child 2: I didn't know there were so many different shapes of wind turbines.

Father: Yes - all invented by people. There are two main types of wind turbines, those that use wind energy to pump water, and those that use it to produce electricity to power homes. Electricity-producing wind turbines come in many shapes and sizes, and range from those that can produce a few tens of Watts for small power demands like keeping boat batteries topped up, right up to huge multi-megawatt machines that can run hundreds of homes.
They basically consist of a set of blades connected to a dynamo or alternator, either directly or via a gearbox, which produces power as it is turned by the spinning blades. By far the most common type is the "horizontal axis" turbine, which has blades like an aircraft propeller, though "vertical axis" machines are available in some countries. These have the advantage of not having to turn into the wind as the wind direction changes, though they are often less efficient than horizontal axis types.
Do you remember the paper toy that was rotating in the wind? The same principle is moving the big wind turbines of several megawatts power.

Child 2: can we make a wind turbine that produces electricity?

Father: I was just about to propose the same thing! We can make a micro turbine in several ways, starting from the materials we have in our house.
Luck! We have a bicycle wheel that we can transform in less than 2 hours into a real wind turbine producing electricity. We can build a wooden rotor, or a plastic one, even a metal one. Today for example we can make the wooden rotor using the materials and tools we have in front of us.
The Father gets all the tools and equipment together and camera focuses on these:
1. Plywood, ½ cm thick
2. List, 4 cm, ½ cm thick
3. Plane, saw
4. Sandpaper
5. Electric drill
6. Metal strips and screws
7. Bicycle dynamo

For full instructions to build this turbine go to energy experiments on this Inforse website

The camera follows the work of everyone. Father is doing the drawing and explaining what everyone needs to do.
Child 1 is drawing two propellers on the plywood so he can work to the outline.
Child 1 saws out the propellers and sands the edges.

Father: The blades are looking good.

Child 2 - Saws a groove in each end of the list. The groove should be diagonal from corner to corner. He needs to drill a hole through the centre of the list.

Father: Place the blades in the grooves and mount to the bicycle dynamo.

Meantime the Father has prepared and tested the bicycle dynamo and now he is making the fixtures for the installation outside.
The team is ready to mount the windmill blades and the dynamo onto a round stick and try the windmill outside.

The wind is still blowing. The wind turbine once mounted is spinning very fast.

Father: We can check the voltage by connecting the ends of wire coming from the dynamo to this shown on camera voltmeter. Can you read the number on the voltmeter?

Child 2: It is showing 7 Volts.

Father: connecting a small lamp to the wires and it shines. Great! We have succeeded in building a real wind turbine producing clean electricity.

Both children: That's amazing and not difficult either. I'm going to tell my teacher about our wind turbine.