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.
$$\phi=B_{||}A=BA\cos{\theta}$$
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.
Quantity |
Magnetic Flux |
Magnetic Flux Density/Magnetic Field Strength (B) |
Area (A) |
Unit(s) |
Weber (Wb) |
Tesla (T) or Weber per metre squared (Wbm-2) |
Metre squared (m2) |
Scalar or Vector? |
Vector |
Vector |
Scalar |
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 |
Solenoid |
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· 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