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Re: [Phys-l] stereo speakers



On Mar 9, 2009, at 8:39 PM, Dan Crowe wrote:

The power transfer from an amplifier to a speaker is maximized when the impedance of the speaker matches the internal impedance of the amplifier, where the internal impedance of the amplifier is the Thevenin equivalent impedance. The Thevenin equivalent of an amplifier is an ideal power source in series with an internal impedance. The speaker completes the circuit, which thus comprises an ideal power supply in series with two impedances: the internal impedance of the amplifier and the load resistance of the speaker.
The power delivered to the load is maximum if the load impedance matches the internal impedance. If the load impedance is less than the internal impedance, then the current through the load increases, but the voltage across the load decreases. If the load impedance is greater than the internal impedance, then the voltage across the load increases, but the current through the load decreases.

Daniel Crowe
Loudoun County Public Schools
Academy of Science
dan.crowe@loudoun.k12.va.us
"Anthony Lapinski" <Anthony_Lapinski@pds.org> 03/09/09 8:17 PM >>>
I was discussing Ohm's law today, and a few students were asking about
stereo speakers and their resistance. Textbooks do few problems involving
speakers. From my understanding, a typical speaker is 8 ohms. This will
get a certain amount of current and voltage, providing a power (P = IV) to
the speakers. If one replaces the speaker with one that has a lower
resistance, does this mean that the current (and power) will increase for
a given voltage? Or is it that for a given power (volume), the current
will increase while the voltage will decrease (since R is now less)? And
the implication is that when the volume is turned up (P increases), an
amplifier must have a better power rating to supply the sound if the
speaker resistance is lower.

Is this correct? Can anyone provide some "basic" physics information about
speakers, amplifiers, and Ohm's law?

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Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


It would probably be useful to ask students about how the heat generation rate, P, depends on the R of the resistor connected to a battery. If R is much smaller than the internal resistance of the source, r, then most of the heat is produced inside the battery. If R>>r then nearly all hear is produced outside the battery (but P becomes negligible when R becomes very large). Let them show that the maximum P is when R=r.

- - - - - - - - - - - - - -
Ludwik Kowalski, a retired physics teacher and an amateur journalist. Updated links to publications and reviews are at:

http://csam.montclair.edu/~kowalski/cf/ http://csam.montclair.edu/~kowalski/my_opeds.html http://csam.montclair.edu/~kowalski/revcom.html