4 ohms and 8 ohms

I am always wondering how this works:
Speakers came with different ohms and so does amps, how does amp knew which ohms to use when there's no switch on the amp?
Example: speakers rated at 4 ohms and the amp rated at 4 ohms and 8 ohms without switch of ohms, how does amp knows which WPC (8 ohms 100wpc and 4 ohms 200wpc) to use?
What happend when speakers rated at 2, 5, 6 ohms?
Thanks a lot for your explanation. ^_^ ^_^ ^_^
I don't know in detail the technical side of your question, but the amps receive a load from the speakers and they adjust the power they put out accordingly
Transistor amps adjust automatically to the impedance of various speakers.

Here is an analogy that might help. Your typical 120 volt wall outlet in a home is capable of delivering up to 15 amps of power to any device you plug in. That's about 1800 watts of power.

However, when you plug in a clock radio that only needs 15 watts to run, that is all the wall socket delivers - 15 watts. If you unplug the clock and then plug in a toaster that needs 1000 watts, the wall socket sends 1000 watts to the toaster. There is nothing for you to "adjust" on the wall socket in either case.

This analogy isn't perfect (wall sockets don't have volume controls) but it gives you an idea. A lower impedance speaker (4 ohms) will attempt to draw twice as much current from a transistor amp as an 8 ohm speaker. This translates into the 4 ohm speaker receiving more watts.

Some power amps are not comfortable with low impedance loads so you shouldn't hook up a speaker to an amp if the speaker has a lower impedance than what the power amp's owner's manual indicates is appropriate.

Tube amps are generally a different story. The output tubes hook up to the speakers through an output transformer. These transformers often have one set of connections for 4 ohm speakers and a different set for 8 ohm speakers. Use the connections that provide the closest match for your particular speakers.
Solid state amps output a voltage which, within the power capability of the amp, is unaffected by the load. Maintaing the voltage with a low speaker impedance requires output of more current (amps), so the low impedance speaker will draw more power from the amp. It just happens... no switch required.

Tube amps make their output via a transformer which matches the tubes (which generate lots of voltage but little current) to the load impedance. A low impedance speaker will not draw more power from a tube amp. To get full rated power out of a tube amp you need to connect the speaker to the appropriate output terminals...4 ohms, 8 ohms, etc.
Power delivered to speakers is equal to output voltage squared divided by speakers impedance. It means that power at 4 Ohm will be 2x power at 8 Ohms (if amplifier has the same output voltage). In reality amplifier's voltage drops a bit with lower ohms and power is not exactly proportional. At 2 Ohms it will be close to quadruple power if amplifiers current limit protection won't kick in. Loudspeakers have impedance changing with the frequency and often impedance of 8 Ohm loudspeaker (shown at 1kHz) varies from 4-16 Ohms with frequency. Don't pay too much attention to power - remember scale is not linear (to make it 2x louder you need 10x power). Higher impedance speakers are easier to drive and damp.
Forget the ratings for a second. An amplifier presents a voltage to the speaker. A speaker presents a resistance (ohms) to the amplifier. This pairing causes a current to flow from the amplifier to the speaker. The current is directly dependent on both the voltage from the amp and the speaker load. This is what defines the power - how much current flows into the speaker load. Or, correctly, how much current the speaker load draws from the available voltage. But defining the power presents a small problem.

One problem is the speaker does not present a steady load resistance because the speaker is not like a light bulb or a space heater, which is pure resistance. A speaker has, in addition to resistors, capacitors and inductors (coils of wire). The speaker driver itself is an inductor. So the speaker, as a whole, is an "inductive load".

Another problem is that the music signal is not a steady voltage. It's ac, which means the voltage's frequency varies. As the frequency varies, the load varies (because of the inductive nature of the load) and the power varies as well. So it is very difficult to define the amplifier power. However, if the speaker were a pure resistance of 8 ohms, the power would be constant over the entire 20hz to 20 khz range.

That's why amplifiers are specified for WPC not only for a given load, but a given frequency as well. For example "200 WPC at 1khz into an 8-ohm load, 400 WPC at 4-ohms".

The way to look at it is that the signal from the source is the "switch" that determines power.
Dear all:
Thanks a lot for all your information.

Reference "M1sst1 & Eldartford", for tube amp, what happen if wrongly plug 4 ohms speaker into 8 ohms output (not enough current)? or 8 ohms speakers into 4 ohms output (too much of current)?
Which ohms to use if the speakers rated at 6 ohms?

Reference "Kijanki", what would you suggest WPC for 4 ohms speakers with 84 or 85 dB?

Reference "Gs5556", looks like to add an a/c conditioner will bring up more steady current and produce more steady "WPC", isn't it?
It brings up another question. Does "hi-end" power cord really help to improve the "sound of music"? or just kept the current more steady into amp?

///// Guess I ask too many questions /////

Maybe someone can suggest a website or books that is easy to read and is easy to understand about my "questions / problems"?

Thanks a lot
Have a wonderful day

If an 8 ohm speaker is connected to the 4 ohm tap of the amp, the voltage delivered will be less than the amp could deliver at the 8 ohm tap, so the available power is reduced.

If a 4 ohm speaker is connected to the 8 ohm tap the current delivered will be insufficient for full rated power. (This is a bit like driving your car up a steep hill in high gear).

Either way, there will be no damage. Some folk claim that their 8 ohm speakers sound better on the 4 ohm tap. If you don't need full rated power this might be OK. However, the load presented to the output tubes via the transformer will not be what the designer intended. It will be too low (High impedance...confusing isn't it). I know that tube amps "don't like" operating into an open circuit (nothing connected) so perhaps this light loading may have adverse effects.