EEG: why do slower waves have a higher amplitude?

[Lewj]

I'm actually not sure where I read this, but my understanding is that a characteristic of slow waves, is that they also have a higher amplitude than shorter wavelengths measured by EEG.

Does anyone know why? Is it something to do with electrical charge having less dissipation when of a lower frequency? Or something else?

And why are there 'waves' any way? Intuitively, I would have thought that measuring electric current/voltage(?) on the scalp would just give you a jumble of different electrical activity, or perhaps a more or less stable voltage level, or both.

Does anyone know a good book or Website where I can find these answers?