Magnetic Flux and Magnetic Flux Density

This is part of the HSC Physics course under the topic Electromagnetic Induction.

HSC Physics Syllabus

  • describe how magnetic flux can change, with reference to the relationship `\phi =B_(||)A=BAcos\theta` (ACSPH083, ACSPH107, ACSPH109)

Magnetic Flux and Flux Density

This video explores what magnetic flux is and how it changes with reference to the relationship `\phi =B_(||)A=BAcos\theta`.


What is Magnetic Flux and Flux Density?

Magnetic flux is a measurement of the total number of magnetic field lines passing through a given area. Flux density is a measurement of the density of magnetic field lines. It is another name for the magnetic field strength B. So, Magnetic flux in a given area equals the flux density multiplied by the area.




where `\theta` is the angle between the magnetic field lines and the normal of the surface. From this, we deduce that:

  • when the surface is parallel to the magnetic field lines, its normal is perpendicular to the magnetic field (`\theta=90°`), thus the magnetic flux is zero.
  • when the surface is perpendicular to the magnetic field lines, its normal is parallel to the magnetic field (`\theta=0°`), thus the magnetic flux is maximum. 

Any changes to the area, magnetic field strength and angle `\theta` results in a change in magnetic flux passing through the given area. 

For example, a change in magnetic flux occurs when the area moved to a location with differing magnetic flux, either higher or lower.


Magnetic Flux

Magnetic Flux Density/Magnetic Field Strength (B)

Area (A)


Weber (Wb)

Tesla (T) or Weber per metre squared (Wbm-2)

Metre squared (m2)

Scalar or Vector?





Currents in Straight Conductors and Solenoids

The direction and orientation of magnetic field accompanying currents in straight conductors and solenoids can be both determined by the right-hand grip rule


Straight Conductor


·       Magnetic flux density decreases further away from the straight conductor

·       Constant magnetic flux density in the core of the solenoid. Flux density is reduced outside the solenoid


·       Direction determined by right-hand grip rule. Thumb points in the direction from south to north pole. Currents flow in the direction of curled fingers.


·       Effectively used to create electromagnets where the magnitude of the magnetic field B is proportional to the size of the current I


Previous section: Interaction Between Two Parallel Current-carrying Conductors

Next section: Faraday's Law of Induction