Page "Ferrite bead" Paragraph 5

A pure inductor does not dissipate energy ; it merely absorbs energy from the circuit and returns it at a later time.

A ferrite bead, by design, filters out the high frequency noise in the circuit by dissipating it as heat.

The ferrite bead is effectively an inductor with a very small Q factor.

When electrical noise within the target frequency range travels in the signal cable a back-emf is induced in the ferrite bead because of its high inductance.

The material used to construct the ferrite bead however, becomes highly resistive at the design frequency range and the induced current inside the bead is dissipated as heat instead of inducing an opposing current back in the signal cable.

It is for this reason that the specific circuit characteristics as well as the frequency band of the noise need to be taken into account when the ferrite bead is installed as a noise filter.

Different bead materials have different properties with respect to frequency and the manufacturer's literature will show the frequency ranges where dissipation is highest.