electricity
In an alternating current, how are frequency, voltage, amperage, and watts related?
For instance, imagining the power as a sine wave, what is amperage if voltage is the amplitude? Is there a better analogy than a sine wave?
EDIT: One of the things I specifically wanted to know is whether frequency and voltage are related? A 60 Hz power signal seems to only be transmitted at 120 or 240 volts – does it have to be a multiple?
One way to interpret the statement "electricity is another form of magnetism" (or vice versa) is through special relativity. If I look at a classical stationary electric charge, it sets up a purely electric field - the Coulomb field, which is responsible for generating the phenomena of static electricity. However, if I now look at this same electric charge, but from the standpoint of a reference frame which is moving with respect to it, what I now see from my new point of view is a moving electric charge, in other words an electric current. An electric current sets up a magnetic field (Ampere's law).
Thus we find the same physical source generating a field which looks like either a magnetic field or an electric field depending upon how you look at it. This transformation between electric and magnetic fields is perfectly described by the Lorentz transformations of special relativity.
Returning to the specifics of your question, permanent magnets generate their magnetic field through two main mechanisms - firstly there is the orbital motion of electrons around the nucleus. Since the electrons are charged, this is equivalent to an electric current and sets up a magnetic field. Secondly, there is the spin of the electrons themselves - this again creates a magnetic field (although it is tempting to think of the electron as a little spinning charged object of finite size, this would be incorrect, the proper description being a quantum mechanical one). The net effect is that the atoms behave as tiny magnets.
In ferromagnetic materials the motion has the right "collective" properties such that the atoms, which behave like a tiny magnets, are able to align their magnet's directions (in local units called magnetic domains) to provide a large magnetic field. However (1) the orbital motion of the electric charge in the atoms is cyclical, and (2) the spin of the electron doesn't move the charge from one place to another whereas what you'd need for a battery is a movement of charge which results in separation of positive and negative charge, to make available at the terminals. Thus permanent magnets can't function as batteries.
There is no relationship between the density of a metal and its electrical resistivity.
There is a big database of material properties called MatWeb which is recommend as a legitimate source of data by UCSD's and Stanford's library systems, Rose-Hulman, etc.
I took data from around 60 different metals and graphed them:
As you can see there is no empirical relationship. From a theory perspective, density has to do with atomic packing and resistivity has to do with electronic structure.
I will admit, however, that gaseous copper is an extremely poor conductor.
Best Answer
He didn't mean 'forget' it. He just meant that it's not really relevant. It's easy to understand the various relationships in DC. When you go to AC they are all the same AT ANY GIVEN MOMENT ON THE WAVE. It all boils down to V=IR and VI=W, RI^2=W, V^2/R=W. With DC it's just a constant. With AC they change with time. If you pick any moment in that time and apply the above, you will have your answer.