My Science School

Container ships do not sink because of buoyancy. Ships made of steel are constructed in a very special way so that they do not sink. The weight of water displaced by the hull of the ship must be greater than the weight of the ship (along with the load it is carrying). As a result, the water below the ship pushes it upwards with a force known as buoyancy. We also experience buoyancy when we swim and feel lighter in water than on land. Floating tires increase the buoyancy further. In contrast, submarines must overcome the buoyancy in order to sink. For this, special tanks are flooded with water. 

Water has huge power. We can feel this on our bodies when we cross a sea or river. The power we get from water is called hydropower. Hydropower has been one of the first forces used by human beings to simplify their work. For example, tasks such as turning wheat into flour, processing wood to make paper, making textile, and turning hard metals into various interesting shapes, which were difficult to do earlier, have been made easy by using hydropower. Dams are made by erecting huge walls in water to generate electricity. Even today there are ferries and boats without engines that use the force of flowing water. Even tides in the oceans can be used to produce energy. 

A watermill uses a large wooden or metal wheel to generate power. It consists of many small blades, over which the flowing water of a stream runs; this wheel is called the ‘water wheel’. The flowing water rotates the wheel. This rotational movement produces electricity when connected to a generator. Although mills were used to generate hydropower in earlier times in all countries, people in the underdeveloped countries use them even today. Electricity produced in such a manner is pollution free and does not harm our environment. 

Dam walls are mostly made of a material called concrete. These dams retain a large amount of water. The retained water puts very high pressure on the lowermost layers of water. If the water in these lowermost layers is diverted, it flows at a very high speed, and hence a high energy is produced with the help of a device called turbine. In the turbine, the energy is used to generate electricity.  Continue reading "How is electricity generated by using a dam? "

High and low tides come alternately, that is, one after the other. In high tides, the level of water rises and covers a part of the coast, and in low tides it flows away again. This energy of the tides is used to run turbines. Special water turbines are used in tidal power plants. Water - rising in case of high tides and receding in case of low tides - can flow through these turbines in both the directions. Electricity is generated in tidal power plants due to the rotary movement of the turbines. These power plants need a tidal hub - the difference in the height of water between high and low tides - of more than 5 m to be able to work economically. Such a power plant is in the French Bretagne, in St. Malo. Here, the tidal hub is at 12-15 m. 

Ferries without engines are driven by the flow of water. They are attached to a wire cable, which is spanned across the river by means of two short ropes. When the ferryman shortens one of the two short ropes with the help of a device called ‘winch’, the ferry is tilted at an angle to the current, and the force of the current propels the ferry across the river.

 

At room temperature, there is always some air dissolved in water. The colder the water, the more air is dissolved. Air becomes less soluble in water as the temperature rises. When the water boils the air is released in the form of bubbles. Two thousand years ago, an attempt was made to use the power of steam with Heron’s ball, but the great breakthrough came only with the steam engines. The steam engine improved by James Watt is the most well-known. A little later the steam engines were used to drive locomotives and paddle steamers - the modern steam turbines followed later. 

 

The Heron’s ball is a bellied vessel, which is partly filled with water and sealed with a cork. A pipe which is open on both the sides goes inside the vessel through a hole in the cork. When the air pressure inside the vessel becomes higher than the atmospheric pressure, the water pushes out through the pipe. The internal air pressure can be increased in different ways: by connecting two vessels, by blowing air in the vessel through another pipe, or by heating the water. The water vapour that is formed needs more space and pushes water out from the pipe; the water actually sprays out. In principle, the Heron’s ball is the first steam engine. 

James Watt is often called the inventor of the steam engine, but in reality he improved upon the working steam engine of Thomas Newcomen and got his design patented in 1769. In the steam engine of James Watt, hot steam is passed inside the cylinder not just from the side but alternatively from top and bottom. The expansion force of the steam causes a piston in the cylinder to rise and fall. Watt converted this up and down movement into the rotational movement of a flywheel. From there, the force generated by steam could be transferred to machines - to sewing machines or weaving looms. 

Paddle steamers are ships that are driven by a large paddle wheel present on the side or at the rear of the ship. The wheels get their energy from a steam engine, which was fired with coal in earlier times. The ship is pushed forward when the blades of the wheel dip in water and shovel it backwards. Such paddle steamers are seen mainly on River Mississippi, which is very shallow at places. A flat, box-shaped hull is ideal for the river steamers because the blades need to dip in water partially, making it ideal for the Mississippi.e after the other. 

 

In a steam turbine, hot water vapours flow from jets onto a rotor having several propeller-type shovel blades. There is a high pressure in front of the turbine, while a low pressure is maintained at the back. As a result, the steam shoots at high speed through the turbine transferring its energy to the rotor. The movement of the turbine is converted into electrical energy, through generators. Steam turbines are used mainly in coal-fired engines and nuclear power plants. The turbines of large power plants are up to 6o m long and weigh several hundred tons. Several rotors are connected in them one after the other. 

In a pressure cooker, the food is cooked at a higher temperature than in normal vessels. Pressure cookers are closed and air-sealed vessels that do not permit air or liquids to escape below a preset pressure. Upon heating, water evaporates inside the cooker and the steam pressure starts building up. The water then starts boiling at this higher pressure. This lets the potatoes cook faster. Pressure must be released before opening the cooker by letting it cool down or by opening a drain valve carefully. 

Burning is a very fast chemical reaction in which a fuel combines with oxygen, releasing heat and light. Each substance has its own ignition temperature. Thus, some substances which have a low ignition temperature catch fire easily, while others with a high ignition temperature catch fire with difficulty. The size of the surface is also important, which reacts with the oxygen in air. A big log of wood cannot be ignited with a single matchstick, but fine wood shavings can be ignited very easily. A flame arises only when a substance burns in the gaseous state. In case of wood, the oil vapours in the wood burn, which give the typical smell of the wood?

The secret of the candle lies in the wick. A matchstick ignites the wick. The heat of the flame melts the wax of the candle. The liquid wax rises up in the wick, and more it evaporates, more higher it rises. The vapour burns. Wax is made up of carbon and hydrogen. Both elements react with oxygen during burning, whereby water vapour, soot, and waste gases like carbon monoxide and carbon dioxide are produced. This also gives the colours to the candle flame - yellow light from carbon and the blue light from water vapours. 

A fire starts with the ignition of a substance by a source that provides the necessary heat to the substance. This source can be a matchstick, an electrical spark, or the effect of a burning lens, such as a magnifying glass. The required temperature for ignition depends on the material. Paper burns at temperatures starting from 190°C, wood at 280°C, and cotton at 450°C. Therefore, a pair of jeans does not catch fire quickly, while a piece of paper catches fire very quickly. Once a substance starts burning, its intensity depends on the supply of oxygen. For this reason, windows and doors of a building should always be kept closed in case of a fire.