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The number of magnetic field lines passing through a given area is known as Magnetic flux. It is denoted by the symbol, $\mathrm{\phi }$ and its unit is $$weber$$ ($\mathit{Wb}$).
Field lines crossing in a particular area

Magnetic flux density:
Magnetic flux density is defined as the number of magnetic field lines crossing a unit area in a direction that is normal to the direction of the field lines. $\mathit{Wb}/{m}^{2}$ is the unit of measurement for magnetic flux density.
Properties of magnetic lines of force:
• Magnetic lines of force are continuous, closed curves that run through the length of a magnet's body.
• Magnetic lines of force run from the north pole to the south pole.
• Magnetic lines of force never cross one another.
• Magnetic lines of force will be highest at the poles and lowest at the equator.
• The magnetic field direction is determined by the tangent drawn at any point on the curved line.
North and south pole of a Bar-magnet
Reference:
https://commons.wikimedia.org/wiki/File:Flux_compression_1.png
https://pixabay.com/vectors/bar-magnet-magnetic-physics-poles-35424/