# 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.

Magnetic flux (in Webers, Wb) is given by:

$$\phi=B_{||}A=BA\cos{\theta}$$

where:

• B is the magnetic field strength in Teslas (T) or Wb m–2.
• A is the area of the conductor through which magnetic field lines project in metres squared (m2)
• \theta is the angle between the magnetic field lines and the normal area of the area

From this equation, 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.

### Changes in Magnetic Flux

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

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

In the diagram above, a rectangular coil is moved out of a uniform magnetic field (directed into the page). As a result, the coil experiences a decrease in flux.

Previous section: Interaction Between Two Parallel Current-carrying Conductors

Next section: Faraday's Law of Induction