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  1. When the magnetic flux produced by an inductor links another inductor, these inductors are said to be coupled.
  2. When two coils are placed very close to each other, the magnetic flux caused by current in one coil links with the other coil and induces some voltage in the second coil. This phenomenon is known as mutual inductance.
  3. For two coils of inductances L1 and L2, the mutual inductance is given as, Description: 4367.png, where, k is the coefficient of coupling.
  4. To denote the correct voltage polarity of the mutual inductance of two colis, dot convention is used. As per dot convention, if the directions of currents through the two coils are in the same sense with respect to the dotted terminals, the sign of the mutually induced emf is the same as the self-induced emf.
  5. Energy stored in two mutually coupled coils is, Description: 4373.png.
  6. The equivalent inductances of two coupled coils of self-inductances L1 and L2 when connected in series is, Description: 4379.png
  7. The equivalent inductances of two coupled coils of self-inductances L1 and L2 when connected in parallel is, Description: 4397.png
  8. A transformer is a four-terminal device comprising of two (or more) magnetically coupled coils, called primary and secondary.
  9. A transformer is said to be linear if the coils are wound on a magnetically linear material (such as, air, plastic, Bakelite, wood, etc) for which the magnetic permeability is a constant. A linear transformer can be represented by an equivalent T or p network.
  10. A transformer is said to be ideal; if its primary and secondary coils are lossless (i.e. R1 = R2 = 0), have very large reactances compared to any connected impedance (i.e. L1, L2, M → ∞) and their coupling is perfect, (i.e. k = 1).
  11. When a capacitor is introduced in the primary and/or in the secondary circuit of a transformer, the circuit becomes selective and the transformer is termed as a tuned coupled circuit or a tuned transformer. There are two types of tuned circuits, single-tuned circuit and double-tuned circuit.

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