Introduction of Magnetic Circuit

Introduction of Magnetic Circuit

Following quantities analyze a magnetic circuit;

  • Magnetic field (Magnetic lines of force)
  • Magnetic flux
  • Magnetic flux density
  • Magnetomotive force (MMF)
  • Magnetic field intensity

Magnetic Field (Magnetic Lines of Force)

Definition: The region around the magnet in which the effect of magnetism can be experienced is called a Magnetic Field.

A magnet’s magnetic field can be tested by using a compass needle or by bringing iron parts near to the magnet.

Imaginary lines are drawn around the magnet to represent the magnetic field. These imaginary lines are known as Magnetic Lines of Force.

The magnetic line of force for the bar magnet is shown in the figure below.

Magnetic Field (Magnetic Lines of Force)
Magnetic Field (Magnetic Lines of Force)

Properties of Magnetic Lines of Force

The properties of magnetic lines of force are listed below.

  • It is a close path.
  • The direction of a magnetic line of force outside of the magnet is from north pole to south pole.
  • The direction of a magnetic line of force inside of magnet is from south pole to north pole.
  • These lines of force do not intersect with each other.
  • The direction of a magnetic field is obtained by drawing a tangent at any point on a magnetic field.
  • All the magnetic lines are equal in strength.

Magnetic Flux

Definition: In a magnetic field, the total number of lines of force is called Magnetic flux.

ะค denotes Magnetic Flux.

SI unit of magnetic flux is weber (Wb).

    \[ 1 \; Wb = 10<sup>8</sup> lines \; of \; force \]

Magnetic Flux Density

Consider a magnet shown in the figure.

Magnetic Flux Density
Magnetic Flux Density

As shown in the figure, a plane is drawn perpendicular to lines of force. And consider the area of this plane is A.

Definition: The flux per unit area measured in a plane perpendicular to the flux is called Magnetic Flux Density.

The flux density is denoted by B.

    \[ B = \frac{Total \; Flux}{Unit \, Area} = \frac{\phi}{A} Wb/m^2 \, or \, Tesla \]

The unit of magnetic flux density is Wb/m2 or Tesla.

Magnetomotive Force (MMF)

Consider a magnetic material on which a coil is wound, as shown in the figure below.

Magnetomotive Force (MMF)
Magnetomotive Force (MMF)

The number of turns wound on a magnetic material is N, and the current that passes through the coil is I ampere. The magnetic field is produced around the magnetic material, as shown in doted imaginary lines in the above figure.

Definition: In a magnetic circuit, the force required to flow magnetic flux is called a magnetomotive force (MMF).

Magnetomotive force is also defined as the product of a number of turns (N) wound on magnetic material, and the current (I) passes through the coil.

    \[ MMF = NI \]

F denotes MMF But, it is referred to as MMF only in most cases.

The unit of magnetomotive force is ampere-turns (AT).

Magnetic Field Intensity

Magnetic field intensity is also referred to as Magnetic Field Strength or Magnetic Intensity.

Definition: Magnetic field intensity is defined as the force experienced by a unit north pole placed at any point in a magnetic field.

It is denoted by H.

    \[ H = \frac{Ampere-Turns}{Length} = \frac{NI}{l} AT/m \]

The unit of magnetic field intensity is AT/m or newton/meter (N/m).

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