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 (see Figure 1).

Figure 1

  • 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
Capacitor Inductor
Blocks Direct Current Blocks Alternating Current
Passes Alternating Current Passes Direct Current
Voltage in Capacitor cannot change instantly Current in an Inductor cannot change instantly
Quick Voltage change produces large Current Quick Current change produces large Voltage
Stores Energy in Electric Field Stores Energy in Magnetic Field
Current leads Voltage Voltage leads Current


  • Because of this, the Capacitor and Inductor are often called Dual Components.
  • Table 1 compares the properties of capacitors and inductors.

How are they made?
In order to understand how inductors are made, we have to change our water pipe analogy slightly to include the effect of magnetic fields.
Consider two pipes filled with water and small magnets attached to the walls of the pipes with rubber bands as shown in Figure 2.

Figure 2

  • The moving magnets, due to the original current, pull the magnets in the second pipe and force a small current to flow in the same direction as the original current.
  • When the rubber bands are fully stretched, the induced current will stop, even though the initial DC current is still flowing.
  • If the original current is an AC current however, it will induce a continuous AC current in the second pipe because the magnets will move back and forth, pulling the magnets in the second pipe back and forth.

Consider the two coiled pipes shown in Figure 3.

Figure 3

  • When the pipe is stretched out (increased length) as in coil 1, the adjacent turns have little affect on each other.
  • In coil 2 (decreased length) the magnets in each turn of the pipe are linking and the amount of “apparent mass” in the pipe seems to increase. In an inductor, pushing the coiled wire closer together causes the inductance of the coil to also increase, and stretching the coil out will lower the inductance of the coil.
  • In other words, the inductance of a coil is indirectly proportional to its length.
  • If the diameter of the coil is increased, it will take more hose to form a loop, and the amount of water will therefore increase.
  • More water means a larger “apparent mass”. Inductance will also increase in a coil if the cross sectional area increases. Inductance is directly proportional to area.

Consider the affect of adding more turns to coiled pipe.

  • The amount of material to push (mass) is increased and the amount of linkage is increased due to more magnets available.
  • This causes the “apparent mass” to increase at a greater rate than would be expected. When making an inductor, the actual inductance is directly proportional to the square of the number of turns.
  • The final factor to consider when making a coil is the core material at the center of the coil.
  • If our pipe wrapped around a material that contained many magnets, they would also link to the magnets in the pipe.
  • This would increase the “apparent mass” of the water in the pipe.
  • The tiny magnets in the core would rotate as shown in Figure 4 and force the water to keep moving in the same direction.

Figure 4

  • Placing an iron core at the center of an inductor will directly increase the inductance by an amount equal to the permeability of the core material.

INDUCTANCE, How is it calculated?
Reviewing  how  coils  are  made  will  show  the following:

  1. Inductance of a coil is indirectly proportional to the length of the coil.
  2. Inductance is directly proportional to the cross sectional area.
  3. Inductance is proportional to the square of the number of turns.
  4. Inductance is directly proportional to the permeability of the core material.

From   the   above   information   the   formula   for inductance of a simple iron core would be:

L = Inductance in microhenrys
N = Number of turns
μ= Permeability of core material
A = Cross-sectional area of coil, in square inches
l = Length of coil in inches
This formula is good only for solid core coils with length greater than diameter.

  • Faraday’s Law states that any time a conductor moves through a magnetic field (Figure 5) a voltage is generated. Because of this principle, it is possible to attach a magnet (or coil) to a rotating device and produce large amounts of electrical power (the Hoover Dam for example).

Figure 5

  • Lenz’ Law states that the induced currents in a conductor passing through a magnetic field will produce a magnetic field that will oppose the motion between the magnet and the conductor. To produce a large amount of electrical power, a large mechanical force is required (conservation of power).


  • 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.

Figure 6

  • The schematic symbols for coils and transformers are shown in Figure 6.
  • These symbols are the most commonly used to represent fixed coils, variable coils, and transformers.


  • The Q (figure of merit) of a coil is the ratio of the inductive reactance to the internal series resistance of the coil.
  • Since the reactance and resistance can both change with frequency, Q must be measured at the desired frequency.
  • Anything that will raise the inductance without raising the series resistance will increase the Q of the coil; for example, using an iron core.
  • Lowering the series resistance without lowering the inductance will also raise the Q, more turns of larger wire for example.
  • Q is important when the inductor is used in a resonant circuit to block or select desired frequencies.
  • The higher the Q, the tighter the selection of frequencies become.


  1. The Inductor prevents current from making any sudden changes by producing large opposing voltages.
  2. Magnetic coupling can be used to transform voltages and currents, but power must remain the same.
  3. Coils and transformers can be used to select frequencies.

1.Bunty B. Bommera
2.Dakshata U. Kamble

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