Just knowing how to calculate the magnetic flux does not solve our induction problems, because for it to exist, it is necessary to have **variation** in the magnetic flux.

Knowing that the magnetic flux is calculated by:

As the equation shows us, the flow depends on three quantities, **B**, **THE**, and **θ**. Therefore, for **Φ** vary It is necessary that at least one of the three quantities vary, as we shall see below.

## Flow variation due to magnetic induction vector variation

Imagine a tube capable of conducting induction lines generated by a magnet inside, for example. If at one point in the pipe there is a reduction in the area of its cross section, all lines passing through an area **THE** will have to go through an area **THE'**, smaller than the previous one. The only way for all induction lines to pass, that is, to keep the flow through this smaller area is if the induction vector increases, which leads us to conclude that the induction lines must be closer to each other in the parts. where the area is smaller. Since the cross-sections in the tube mentioned are parallel to each other, this statement can be expressed by:

So if we think of any magnet, it will have a stronger magnetic field near its poles, since the induction lines are more concentrated at these points. So one way to make it **Φ** vary is to zoom in or out on the surface of the magnetic source, varying

## Flow variation due to area variation

Another way to vary **Φ** is using a uniform magnetic field and a surface area **THE**.

Since the uniform magnetic field is well delimited, it is possible to vary the magnetic induction flux by moving the surface perpendicular to the field between the under and outside of its influence. Thus, the effective area through which there is magnetic flux varies.

## Flow variation due to angle variation θ

Besides the two forms mentioned above, it is still possible to vary **Φ **causing the angle between the surface normal line and the vector to vary . A practical and possibly the most widely used way to generate magnetic induction is to make the surface through which the flow passes rotate, causing **θ** vary.