Electricity & Megnetism

Magnetic flux

To analyze the phenomenon of magnetic induction in more detail, Faraday used a concept that he had created: the lines of force , which we know today as field lines .

Although the number of field lines is obviously infinite, he agreed that, to represent a magnetic field, the density of the lines would be proportional to the field strength. The calculation of this density of lines is based on checking how many lines there are for each square meter of a given flat surface perpendicular to these lines, as shown in the figure below.

In this way, we can say that the number of lines that cross a flat surface, of area A, placed perpendicularly to a magnetic field, is proportional to the product of the magnetic field and the surface area, ( B . A ). This product was called the flux of B (or magnetic flux ) through the surface, represented by ϕ. So we have:


ϕ – magnetic flux
B – magnetic field
A – flat surface area

According to the figure below, we have a loop of area A immersed in a uniform magnetic field. The angle formed between the field B and the vector n normal to the plane of the loop is θ. Thus, to calculate the magnetic flux B through the loop we have to take the angle into account. So we have to:

In the SI (International System of Units) the flow unit is called weber (Wb).

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