
#1
Dec1309, 09:50 PM

P: 10

The noise power in a conductor is given by
P = k.t.B Does this mean the noise power is independant of the conductor length? Also, does this only apply for a conductor with no resistance? Thnks, Rudi 



#2
Dec1309, 10:01 PM

P: 4,664

KTB is just the thermal power in a circuit at temperature T and bandwidth B, not considering any other sources of noise. KTB is about 114 dBm per MHz at 273 kelvin. It has nothing to do with conductor length. See
http://en.wikipedia.org/wiki/Johnson...3Nyquist_noise Bob S 



#3
Dec1309, 10:37 PM

P: 10

Thanks Bob,
Sowhy do we need the relationship P = 4.k.t.R.B If we already know the power in the circuit? 



#4
Dec1309, 10:46 PM

P: 4,664

Noise power in conductorBob S 



#5
Dec1409, 12:29 AM

Emeritus
Sci Advisor
PF Gold
P: 6,238

BTW, if you look at the "current signal", then the "power" is 4 k T B / R The physical power you can *extract* from a voltage source with voltage variance V2, and internal resistance R, is by connecting a loadresistor R to it. The current flowing in the circuit is then sqrt(V2)/(2 R), and the power dissipated in the load resistor is then (I^2 R) or (sqrt(V2) / (2 R))^2 R = 4 k T R B / (4 R) = k T B. (btw, that's with an unphysical ideal resistor with no noise as a load  in reality, the load resistor pumps just as much power back into the "source" resistor as the other way around, so that there is no net power flux if both resistors are at same temperature). 



#6
Feb710, 10:44 PM

P: 10

Thanks vanesch, so
p=k*t*b is the real power. p=4*k*t*r*b is the signal power. But if the power is independent of conductor length, what is to stop me from cutting a conductor into *infininte* (read lots) of peices and getting lots of power? 



#7
Feb810, 02:29 AM

Sci Advisor
PF Gold
P: 1,721

You can cut up your conductors all you want (within a classical limit), each one will be absorbing energy from the environment and emitting it back out. It isn't until there is a resistive load (and a complete circuit) that we interrupt the flow here and redirect some of the absorbed energy as a signal in the circuit. So we need to have a complete circuit involved here. Cutting up conductor lengths does not make sense since now we just have pieces of wire. 


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