INDUCTORS, What do they do?

             The electronic component known as the inductor is best described as electrical momentum. In our water pipe analogy the inductor would be equivalent to a very long hose that is wrapped around itself many times . If the hose is very long it will contain many gallons of water. When pressure is applied to one end of the hose, the thousands of gallons of water would not start to move instantly. It would take time to get the water moving due to inertia (a body at rest wants to stay at rest). After a while the water would start to move and pick up speed. The speed would increase until the friction of the hose applied to the amount of pressure being applied to the water. If you try to instantly stop the water from moving by holding the plunger, the momentum (a body in motion wants to stay in motion) of the water would cause a large negative pressure (Suction) that would pull the plunger from your hands.
           Since Inductors are made by coiling a wire, they are often called Coils. In practice the names Inductor and Coil are used interchangeably. From the above analogy, it is obvious that a coiled hose will pass Direct Current (DC), since the water flow increases to equal the resistance in the coiled hose after an
elapsed period of time. If the pressure on the plunger is alternated (pushed, then pulled) fast enough, the water in the coil will never start moving and the Alternating Current (AC) will be blocked. The nature of a Coil in electronics follows the same principles as the coiled hose analogy. A coil of wire will pass DC and block AC. Recall that the nature of a Capacitor blocked DC and passed AC, the exact opposite of a coil. Because of this, the Capacitor and Inductor are often called Dual Components.
         The Inductor prevents current from making any sudden changes by producing large opposing voltages. Magnetic coupling can be used to transform voltages and currents, but power must remain the same. Coils and transformers can be used to select frequencies.

INDUCTANCE SYMBOLS AND MARKINGS
       Most inductors are custom made to meet the requirements of the purchaser. They are marked to match the specification of the buyer and therefore carry no standard markings. The schematic symbols for coils and transformers are shown in fig . These symbols are the most commonly used to represent fixed coils, variable coils, and transformers.