Re: how does an amp make a guitar louder

All amplifiers use an input signal to control how much electrical power is 
fed from its power supply to its output.
In the case of a guitar amp, the input is the small voltage from the 
guitar's pickup or microphone, the output is fed to a loudspeaker, and the 
power comes ultimately from the mains socket. 
The larger amount of power fed to the speaker produces the increase in 
volume. 
One mechanical analogy is the tap on a fire hose - you use a small force 
(your hand) to control a larger force - that of the emerging water jet.