3. Inductors

Same as the coil used in the previous notes. The Inductor is a coiled conductor usually a copper wire. From the circuit below, an Inductor is connected across a DC voltage source. The current flows across the Inductor coil, and a Magnetic field is generated around the Coil.


This generated Magnetic field forces the current from changing in the circuit even when the voltage supply is shorted. This was discovered by “Emil Lenz” and is called Lenz Law. 

The direction of the Magnetic field and Current are as shown below, It is determined by the right Hand Rule where the thumb points the direction of the current and the folded fingers points the direction of the Magnetic Field. So inductors store current in the form on Magnetic Field. 

The Inductor maintains a constant flow of current through it. Generated Magnetic field forces this to happen. If a switch is introduced in the circuit as shown below.

From the circuit even when the Voltage source is OFF the current still flows in the circuit due to the inductor. For an ideal wire with “0” Resistance, the current flows forever in the loop. In a real case the duration of the current flow depends on the resistance of the wire and the inductance of the Inductor. If an LED is introduced in the circuit the LED glows momentarily even if the Voltage source is turned off. In a steady state condition the Inductor acts like a normal short cable.

No matter what the resistance on the cable is, the inductor will force to maintain the current, be it for a very short time. This results in generating very high voltages when the connected Resistor has a very high Resistance. This is the reason why the Inductors are used in high voltage generating circuits like “Boost converters”.

The inductance of an Inductor increases with increase in N- Number of turns or A- Area of the coil or Permeability of the core and decreases with increase in l- Average length of the coil.  

L : Inductance (Henry, H)

N: Number of turns

u:  Permeability of the core

l: Average length of the coil 

A: Area of the coil

As the Inductance (L) increases the Voltage (v) across will increase. The relation between L and v is as below, Voltage is the Inductance (L) times the rate of change of Current (di/dt).