The Student Room Group

Path difference q. Pls help

Could someone pls explain part c?
Edit: Why does the path difference decrease? And why does reflection from other surfaces result in energy loss?



(edited 5 years ago)
Reply 1
Bump
Reply 2
So when the wave reflects off of the metal plate it loses some energy. This means that the amplitude of the wave returning is lower. So the waves dont fully cancel eachother out. Its like if the amplitude of the first wave is 2m and then the reflecting wave is 1.5m there would still cancel out but not fully as there is still an amplitude of 0.5m left. I cant remember why there is energy loss but usually to do with the distance travelled so if i shouted from 100m away and then from 10m away will nothing blocking it the 100m would be a lot quieter. Again it will be due to the extra distance of reflecting. Dunno about part 2
Hope that helped in some way.
Reply 3
Original post by tdixon00
So when the wave reflects off of the metal plate it loses some energy. This means that the amplitude of the wave returning is lower. So the waves dont fully cancel each other out. Its like if the amplitude of the first wave is 2m and then the reflecting wave is 1.5m there would still cancel out but not fully as there is still an amplitude of 0.5m left. I cant remember why there is energy loss but usually to do with the distance travelled so if i shouted from 100m away and then from 10m away will nothing blocking it the 100m would be a lot quieter. Again it will be due to the extra distance of reflecting. Dunno about part 2
Hope that helped in some way.


Oh ok thnx so much! :biggrin: Any idea why path diff decreases?
Reply 4
Original post by Presto
Oh ok thnx so much! :biggrin: Any idea why path diff decreases?


So i think because right now their phase difference would be at 360 degrees because they are completely in phase towards the metal plate. When they reflect it takes time which is likely to not be the exact same time so they have now become i belive 180 degrees out of pahse or another function of that. On why the minimas hit zero for part 2 it could be that the microphone has found the one in which the waves have become perfectly out of phase and so at a node in the wave would be a true minima. Dont quote me on that one though
Reply 5
I don't get how would that lead to a decrease in path difference and the minimas never hit zero, not even in part 2 cos complete cancellation doesn't occur
Reply 6
Bump
Original post by Presto
I don't get how would that lead to a decrease in path difference and the minimas never hit zero, not even in part 2 cos complete cancellation doesn't occur


The path difference of the waves hitting the microphone is the difference in distance between the loudspeaker and the microphone for the emitted waves, and the distance between the loudspeaker, the metal plate, and back to the microphone for the reflected waves. So as the microphone gets closer to the metal plate the distance between the loudspeaker and microphone increases and the distance from the loudspeaker to plate and back to microphone decreases, so the distance the emitted waves and the reflected waves travel get closer together and so the path difference decreases.
If the path difference is an integer number of wavelengths then there's constructive interference, so as the path difference decreases they are falling more out of phase so the waves are having more destructive interference, meaning the maximas get smaller.
Hope that makes sense?
Also make sure you don't get path difference and phase difference mixed up :smile:
Reply 8
Original post by MathsPhysMind
The path difference of the waves hitting the microphone is the difference in distance between the loudspeaker and the microphone for the emitted waves, and the distance between the loudspeaker, the metal plate, and back to the microphone for the reflected waves. So as the microphone gets closer to the metal plate the distance between the loudspeaker and microphone increases and the distance from the loudspeaker to plate and back to microphone decreases, so the distance the emitted waves and the reflected waves travel get closer together and so the path difference decreases.
If the path difference is an integer number of wavelengths then there's constructive interference, so as the path difference decreases they are falling more out of phase so the waves are having more destructive interference, meaning the maximas get smaller.
Hope that makes sense?
Also make sure you don't get path difference and phase difference mixed up :smile:

Thank you so soo much for your comment! :smile:
If the path difference of the waves hitting the microphone is the difference in distance between the loudspeaker and the microphone for the emitted waves, and the distance between the loudspeaker, the metal plate, and back to the microphone for the reflected waves, then why would does the path difference change? One distance is getting bigger as the other is getting smaller so shouldn't the overall difference be the same? Hope you get my q
How do we know that they're getting more out of phase?
Original post by Presto
Thank you so soo much for your comment! :smile:
If the path difference of the waves hitting the microphone is the difference in distance between the loudspeaker and the microphone for the emitted waves, and the distance between the loudspeaker, the metal plate, and back to the microphone for the reflected waves, then why would does the path difference change? One distance is getting bigger as the other is getting smaller so shouldn't the overall difference be the same? Hope you get my q
How do we know that they're getting more out of phase?


Sorry I didn't explain that bit very well, let me try again.
So there are two types of waves hitting the microphone: the emitted waves which travel directly to the microphone, and the reflected waves which travel to the metal plate and are reflect to the microphone. The path difference is the DIFFERENCE in length between the distance these two types of waves travel. So at the beginning they have a large path difference, but as the microphone moves towards the plate the distance these two waves travel gets closer together so the path difference decreases.
As path difference decreases the phase difference will cycle in and out of phase, so when the path difference is say one wavelength they are in phase, but as they path difference decreases they get more and more out of phase so rhere is more destructive interference. Eventually as the path difference decreases more they will come back in phase, but that's not applicable in this questions since the amplitude only trends downwards, and the furthest you can move the microphone is the plate where the phase difference would be zero and there is no standing wave/interference.
Does that make more sense?
Reply 10
Original post by MathsPhysMind
Sorry I didn't explain that bit very well, let me try again.
So there are two types of waves hitting the microphone: the emitted waves which travel directly to the microphone, and the reflected waves which travel to the metal plate and are reflect to the microphone. The path difference is the DIFFERENCE in length between the distance these two types of waves travel. So at the beginning they have a large path difference, but as the microphone moves towards the plate the distance these two waves travel gets closer together so the path difference decreases.
As path difference decreases the phase difference will cycle in and out of phase, so when the path difference is say one wavelength they are in phase, but as they path difference decreases they get more and more out of phase so rhere is more destructive interference. Eventually as the path difference decreases more they will come back in phase, but that's not applicable in this questions since the amplitude only trends downwards, and the furthest you can move the microphone is the plate where the phase difference would be zero and there is no standing wave/interference.
Does that make more sense?

Ohh now I get itt! Thanks sooo much =D
Original post by Presto
Ohh now I get itt! Thanks sooo much =D


Anytime :smile:

Quick Reply

Latest