Hello friends! In this lesson, I will explain you the structure and working principle of the coils which is one of the basic circuit devices of electricity and electronics. Coil is also called as inductor. Unit of the coil is Henry and is denoted by the letter L. In daily life, they are used as toroid, resistance and surface mounting types. Coil is the circuit device that is formed by winding a conductive wire over and over. Coils are composed of winding and core. In coil windings, varnished insulated copper wire is used. This feature allows the coil to store electrical energy as a magnetic field. It is mostly made of a ferromagnetic core such as , iron which has very good magnetic permeability to increase the strength of the magnetic field in inductors. So let's see how the coil works now. To understand the working principle of the coil better, first of all we need to have look at the Ampere’s Law. According to this law, a magnetic field is formed around the conductor that flows a certain current. The intensity of this magnetic field is directly proportional to the intensity of the current. If the current flowing through the conductor is DC, the magnetic field is constant, and if the flowing current is AC, the direction of the magnetic field constantly changes. In daily life, many permanent magnets are produced and used that attract materials made of elements such as iron, nickel and cobalt. Permanent magnets are produced in Stick, U, Ring, Round and etc. shapes. By taking the advantage of the magnetic field created by these magnets, a wide varitey of devices are produced. But sometimes it is needed to create very strong magnetic fields. Therefore, producing a very strong magnet creates devices that are both costly and large in size. Instead, we produce electromagnets having magnet properties by using coils. So, how do electromagnets work? Let’s have a look together. As you can see in the figure, when voltage is applied to a coil , a magnetic field is formed around the coil . Now, this coil acts like a permanent magnet with an N and S pole. If the voltage value is increased, the magnitude of the magnetic field also increases. The magnet that consists of this coil is called an electromagnet. Although we cannot see the magnetic field lines around the magnet , we can see them by experimenting with iron powder. If iron powder is scattered around a magnet, this powder is collected on magnetic field lines that move from the N pole of the magnet to the S pole. As seen here, just as a magnet with a fixed magnetic field attracts the paper clips, it attracts the paper clips around it when voltage is applied to the coil structure that we have created with copper windings around a nail. In general, the symbol of the coils in circuit is shown as follows. But according to the type of core used in the coils, their symbols differ slightly as seen on the right, Well, how to create a coil? now let's have a look. There is a core inside the coil. This core is made of a ferromagnetic material with very good magnetic permeability, such as iron and steel. An insulated copper wire is wrapped around this core, which passes the current very well. The coil is basically formed in this way. There is also a coil inside the transformers that we use to increase and decrease the voltage value of alternating current. The voltage value is increased and decreased according to the ratio of the windings at the input and output. When we look at the internal structure of the transformer we see here, the voltage value of the sinusoidal wave at the input is decreased at the output since the winding number at the input is higher than output. The cylindrical part on the laptop charger cable can be sugessted as an example to coil-used devices. It is even possible to find coils in many devices such as keyboard, monitor and printer cables. The cylinders on these devices called ferrite beads, are basically a coil and serve to reduce high frequency noise. During Electricity Transmission, the transmission is exposed to external noise. Ferrite bead ensures the proper charging and operation of electronic devices by preventing the noisy frequency values coming from outside. Speakers are also one of the coil-used devices. Thanks to the magnet and coil structure inside, the sound waves are amplified and become audible. In Tesla coil, a magnetic field is formed around the coil connected to the power source, and thanks to this magnetic field, the lamp brought close to the coil is illuminated wirelessly. In addition, the coils have a wide range of applications in signal processing and filters in analog circuits. In a Resistor-Coil circuit, it acts as a low-pass filter as the impedance of the coil increases with frequency. In the Coil-Capacitor circuit, coil is used as a band-pass filter with capacitor. Thanks to the filter circuits, it is ensured that the desired frequency value is used or not.