A few questions

[Homoochan]

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I'm having trouble with these questions... I have the answers but can't figure out the logic..
Can anyone help please?

1. A limiting factor to speed of audible sound in air is:
A. the root mean square velocity of the molecules.
B. the wavelength of the sound
C. the frequency of the sound
D. the pitch of the sound

Answer: A

2. The source of a sound wave is stationary. The observer is moving toward the source. There is a steady wind blowing from the observer to the source. How does the wind change the observed frequency?

A. The wind magnifies the Doppler Effect and increases the frequency.
B. The wind minimizes the Doppler Effect and increases the frequency.
C. The wind magnifies the Doppler Effect and decreases the frequency.
D. The wind minimizes the Doppler Effect and decreases the frequency.

Answer: A

3. Which of the following will result in the greatest increase in the observed frequency of sound?

A. The source moves toward the observer at 30m/s
B. The observer moves toward the source at 30m/s
C. The source moves away from the observer at 30m/s
D. A and B will result in exactly the same frequency change.

Answer: A

4. In change in entropy, why is it important to have the "reversible" heat?
What's would be different if I used "irreversible" heat in the equation: q/t?

5. Gibb's free energy is for non-PV work. What does this mean?

Thanks guys in advance!

 

[cwfergus]

Alright lets give this a try:

1. Alright so the speed of sound is constant, The wavelength and frequency do not affect it. The main thing that affects the speed of sound in air is temperature. At 0 degrees celcius, we know that it is 331. While as you go up/down, the speed will vary, and the root mean square is: v= root(3RT/M) which directly relates the temperature in it

2. For two, well haha i cant really explain this, but of course the answer is A. You can cancel B and C directly out just because they dont even follow the doppler effect. Then you can think, will it magnify or minimize? i'd guess magnify. LIke IF the wind was blowing from the source to the observer whom is approaching the source, it would definitely make sense because the wind would cause the sound to arrive faster at its destination.

3. Well you should just know that when a source is approaching a stationary object, there will be a greater frequency shift as opposed to if the observer was moving to the source. you could do trial and error even. But this is something you should just know, so that cancels out B and D. C is just plain absurd and totally destroys the doppler effect.

I'm having trouble with these questions... I
have the answers but can't figure out the logic..
Can anyone help please?

1. A limiting factor to speed of audible sound in air is:
A. the root mean square velocity of the molecules.
B. the wavelength of the sound
C. the frequency of the sound
D. the pitch of the sound

Answer: A

2. The source of a sound wave is stationary. The observer is moving toward the source. There is a steady wind blowing from the observer to the source. How does the wind change the observed frequency?

A. The wind magnifies the Doppler Effect and increases the frequency.
B. The wind minimizes the Doppler Effect and increases the frequency.
C. The wind magnifies the Doppler Effect and decreases the frequency.
D. The wind minimizes the Doppler Effect and decreases the frequency.

Answer: A

3. Which of the following will result in the greatest increase in the observed frequency of sound?

A. The source moves toward the observer at 30m/s
B. The observer moves toward the source at 30m/s
C. The source moves away from the observer at 30m/s
D. A and B will result in exactly the same frequency change.

Answer: A

4. In change in entropy, why is it important to have the "reversible" heat?
What's would be different if I used "irreversible" heat in the equation: q/t?

5. Gibb's free energy is for non-PV work. What does this mean?

Thanks guys in advance!

 

[cwfergus]

4. Alright, well for entropy, it is a measure of randomness and disorder. Well you just have to think of it, entropy will always be reversible until it hits equilibrium which by then it can do no work.
I dont really know what you mean about if it were irreversible? any question about it?
DeltaS = q/T

5. It just means it is a different type of work than the work done on Pressure-Volume type of work (isobaric/isochoric (though no work is done anyways) /isothermal/Adiabatic)

These last two maybe if you had a specific question or something i could try harder

sorry

 

[spyderracing32]

Sound is dependent on the transmission of energy through air molecules. Thus, sound is dependent on the collisions between air molecules (as close to perfectly elastic as possible).

For one, the RMS velocity of the air molecules matters because the higher the RMS velocity the faster collisions are being made and the the faster sound travels through air.

Two is kind of abstract, but if you know sound is the result of collisions of air molecules it would make sense that if a wind were blowing in the direction of a propagating sound wave it would compress the air through which the sound wave is traveling and increase the frequency of the wave.

Three is just the basic doppler effect.

Four, just know that reversible heat and work are always greater than irreversible heat and work.

Five, non-PV work is any work that is not the direct result of a pressure or volume change (chemical energy, heat energy, etc.)