Does Faraday's law work both ways, and if so why don't we use d.c for a generator? Faraday's law in the integral form can be stated as $V = -d\Phi/dt$, where the right-hand side represents the rate of change of the magnetic flux and the left the voltage difference. In other words, a changing magnetic flux generates a voltage. 
My question is if a constant voltage would generate a changing magnetic flux.
If it does, then why is a.c. used in transformers? Even for d.c. currents to flow, a voltage must be present, so if there was a d.c. current in the primary, then there would be  a voltage, and so, by Faraday's law, a changing magnetic flux. Also, if a.c is required, then what equation says that only a changing current generates a magnetic field?
Finally, does Ampère's law work both ways? That is, does a circulating magnetic field produce a current flow?
Thanks for the answers.
 A: The fact of the matter is that a DC current induces a constant magnetic field, except for the instant at which it commences and ceases. If a square wave, for example, were applied to the primary of a transformer, the secondary would produce a series of spikes of alternating polarity (as in an automotive ignition coil). Voltage can be transformed by a DC-to-DC converter, which works by generating an AC current, applying it to a transformer, and rectifying the output of the secondary, but the cost is higher than when using a transformer. (This is an oversimplification, but the details are not the question here.) Transformers and induction motors require an AC current. Take the case of a high voltage (up to 1,000,000 volts) long distance transmission line. It operates at the high voltage to reduce losses, but the voltage is obviously unsuitable for either generation or local distribution, thus requiring transformers to change the voltage. It is much more economically feasible to use an AC system from one end to the other. (Nowadays, since the advent of cost effective and reliable high-power semiconductors, some high voltage transmission lines are operated at DC, requiring conversion at both ends.) So, the answer to your question about why AC is used: it's cheaper.
For an interesting take on the matter, look up the battle between Edison Electric Light, which advocated DC distribution and Westinghouse Electric Company, which chose AC.
A: The key thing about transformers is their primary winding's magnetic field only induces currents in the secondary winding(s) during that time while that field is in motion, same as an alternator or generator.
The field can be put in motion by machinery as in the alternator or generator, or the field can be put in motion (as in the stationary transformer primary winding) by varying the primary winding input voltage, causing the field to move or pulsate (rise\fall,expand\collapse, etc), whilst the winding is stationary.
Of course if the transformer primary winding requires constant input voltage change, using an AC voltage source is a natural fit.  If you had a DC source, you'd have to build something to turn it into AC (or at least pulses of dc) before sending it into the transformer.
I have never seen a coil with a constant current produce changing flux, unless that coil (or another in the vacinity) was mounted on moving machinery.
