What is an ore?
Metals seldom occur in their pure form in nature. They occur mostly in the form of ores, which means they are mixed with rocks. Through the process of smelting, the metal is then melted ‘out’ of the rocky mass, for instance, iron from iron ore. Even in 6000 BC there were furnaces for smelting copper ore. Metals are used mainly in the form of alloys which is a mixture of two metals. For instance, iron is often changed into steel by adding carbon. Depending upon how much carbon is mixed, the steel is either soft and can be moulded or is hard and can be used in making knife blades.
Diamonds are very expensive because they occur at relatively few places in the world - like Russia and South Africa - and are very difficult to extract. They are made of carbon and are the hardest substance known to us, which is the result of the high pressure at their place of origin deep below the Earth’s surface. They are naturally thrown up on the Earth’s surface through volcanic eruptions. For this reason they are mined in the vents of the extinct volcanoes. Being very hard they have great industrial use for grinding tools. Diamonds can also be made artificially by creating the same conditions as found under the surface of the Earth, but these artificially made diamonds are not as valuable as natural diamonds.
Windmills are machines that are driven by wind. Their blades have slightly diagonal surfaces. If the wind hits them from front, they deflect it to the side and in this way convert the wind pressure in a rotary movement. Initially, millers had to set the blades themselves in such a way that the wind came onto them from the front. The rotary movement of the blades gets transferred to the milling stones inside the mill through cogwheels. Windmills were traditionally used to mill corn to make flour and grate spices. They also acted as drive for oil presses and sawmills.
When compared with the two-blade rotors, the three-blade rotors are more stable and quieter. The disadvantage of the two-blade rotors is that they need a special ‘wobble’ motor to prevent the severe collision when the rotor passes the tower. When the rotor blade is on top, the wind presses against it with full force and bends it. At the same time, the opposite rotor blade is in front of the tower. Thus, it cannot counteract the high pressure at the upper blade which leads to a strong jerk which is compensated by the wobble motor. This problem does not occur in three-blade rotors because the blades are never exactly opposite to one another. In addition, the three blades form a kind of disc for the flowing wind and are hardly prone to faults.
When we sail at ‘high wind’, we travel - as far as possible - against the wind. The wind, thus, comes diagonally from the front. The propulsion takes place when the wind sweeps quickly past the sails, which are possibly set up in the middle of the boat. In contrast, if the boat travels ‘before the wind’; the sails are diagonal to the boat. The wind comes from behind, blows into the sails, and pushes the boat forwards.
Wind creates special shapes in nature. These include the elongated dunes of the sandy deserts, which are always reshaped by the wind and the treetops, which adapt their appearance to the constantly blowing wind. With its constant force the wind also erodes the surface of the rocks and hollows them out gradually. This is known as wind erosion. This gives rise to rocky arches, troughs, and spires. Wind erosion also takes place in the unprotected fields, where the fertile soil cover is blown away. In this way, the wind contributes to the spread of deserts in the dry regions of the Earth.
Aircrafts need a combination of procedures to fly in the air. They need a propulsion system – a propeller or a jet engine. They also need a lift, which comes from their wings. While flying, the forward thrust causes the air to flow at great speeds along the wings and their slightly titled position give rise to a low pressure above the aircraft and high pressure below it. This is how the aircraft is pushed upwards. The pressure difference thus generates buoyancy, which is greater than the weight of the aircraft, which pulls it down. The aircraft will fall if the buoyancy is less.
Air at a temperature of around 90°C is filled into the hot-air balloon. The hot air keeps the balloon in the air because its density is lower than that of the surrounding cold air. Before starting the balloon, a gas burner is ignited below the opening of the huge balloon cover and the flame heats up the air in the balloon. When the balloon rises to the desired height, the burner is switched off; if one wants to rise further, it is started again. The balloon must have a minimum size so that it can overcome the force of gravitational attraction of the Earth. Thus, hot-air balloons are able to float in air and look so beautiful in the sky.
A propeller in an aircraft scoops air, just like it scoops water in a ship. As in a wind mill, the blades of a propeller are arranged in the form of a star around a rotating shaft. They are shaped and aligned in such a way that the air falls diagonally on them and flows along them. This creates a strong air flow that gives the aircraft the required thrust. This principle is used not just by the smaller aircrafts but also by hovercrafts, which can travel over meadows or frozen water.
The sun is the source of all energy either in the form of fossil fuels or wood. After all, these substances have been made from the energy of the sun and have stored its energy. However, the heat of the sun can be used directly in the form of solar energy. Solar cells and giant solar plants are used for generating electricity. It has been estimated that the solar energy reaching the Earth’s surface in one year is double than we will ever receive from all the Earth’s resources of coal, oil, natural gas, and uranium together.
The easiest way to capture the heat energy of the sun is to use it to heat water. In countries with regular supply of abundant sunlight, the roofs of the houses have tanks with water that gets heated by the sunlight. ‘Solar houses’ can now be seen in various countries across the world.
The major part of all solar cells is made of silicone, which is a semi-conductor. Semi-conductors are substances that conduct electricity only under the action of light or heat. When light particles fall on a solar cell, they bounce off electrons from the silicone layer. These electrons wander to the opposite layer, when an electrical field is applied. The upper and the lower sides of the cell thereby get electrically charged and generate an electrical voltage. This is tapped via electrode and produces - similar to a battery - direct current. This current can be used directly or fed directly to the electricity grid after being converted into alternating current.
