I have to disagree with Sean and say Ohms law, and its derivatives, tells you a lot about
An amp and speakers (of course mnf. specs can be doctored). Ohm's Law for DC circuits is the fundamental relationship between voltage, current, and resistance. It is very relevant. It is usually stated as: E = I*R, or V=I*R, where E or V = voltage (in volts. E stands for "electromotive force" which is the same thing as voltage), and I = current (in amps), and R = resistance (in ohms). The equation can be manipulated to find any one of the three if the other two are known. For instance, if you know the voltage across a resistor, and the current through it, you can calculate the resistance by rearranging the equation to solve for R as follows: R = E/I. Likewise, if you know the resistance and the voltage drop across it, you can calculate the current through the resistor as I = E/R.
A related equation is used to calculate power in a circuit: P = E*I, where P = power (in watts), E = voltage (in volts), and I = current (in amps). For example, if you measure 20V RMS and 2.5A into a load, the power delivered to the load is: P = 20*2.5 = 50W. This equation can also be rearranged to solve for the other two quantities as follows: P = E*I, E = P/I, and I = P/E. You can also combine the power equation with the first Ohm's law equation to derive a set of new equations. Since E = I*R, you can substitute I*R for E in the power equation to obtain: P = (I*R)*I, or P = I2R. You can also find P if you know only E and R by substituting I=E/R into the power equation to obtain: P = E*(E/R), or P = E2/R. These two equations can also be rearranged to solve for any one of the three variables if the other two are known. For example, if you have an amplifier putting out 50W into an 8 ohm load, the voltage across the load will be: E = sqrt(P*R) = sqrt(50*8) = 20V RMS.
There is a form of Ohm's law for AC circuits too. In AC you must deal with capacitance(C) and inductance(L) in addition to resistance(R). Together, any combination of the three is impedance (Z). If you simply substitute Z for R in the Ohms law formula they are accurate for AC too. E=IZ, I=E/Z, Z=E/I are the AC Ohms law equations. You can tell a great deal about how an amp will drive a speaker with these equations all based on Ohms law.
Finding Z is a little more difficult because it involves current out of phase with voltage and it involves complex numbers, vectors, etc
I think I agree with Scots statements above but it is a little confusing on a point that is often confused. He says:
Imagine a river. The amount of water that is moving downstream is analogous to the voltage -- i.e., it's a measure of the size or quantity of the flow (say, 2500 cubic feet per minute). The current, or force, behind the water (usually due to gravity) is the other measure of actual or potential energy.
This seems to refer to Voltage as water that is moving and the force, behind the water at the same time. Well
.. is voltage like the stuff that is moving(water) or the force moving it(gravity)? Not good to confuse the two. The amount of water moving downhill seems analogous to Power not Voltage and from the equations above you see that these two are not the same. No amount of gravity is going to make a river flow if there is no water.
This is a very basic and important distinction in electronics too. - between matter (electrons) and force (photons). Voltage does not move. Voltage is like a pressure between two points. You might say current flows through a resistor but it is wrong to think of voltage as doing so. Voltage is across a resistor not through it.
A battery (voltage supply) does not supply current. The electric charge preexists in the wire but lacks organization if you will. A wire is a conductor precisely because it has free electrons. If a voltage is applied across a wire, with a battery, it will push the preexisting electrons in one direction (toward the positive). But the electrons are not the force either. To think this would be the same as confusing sound waves with the air the sound waves travel in. The difference in voltage potential creates an electric field and the electric field is the force. The Electric force moves very quickly. The drift of electrons is not much faster than ketchup. Alternating current vibrates more than flows. At least I think this is the basic model.
I remain,