Phase difference

So i've been trying to get my head around phase difference and I still can't understand why it works and how to calculate it for harder problems. I wrote an example question attached and it would be a massive help if someone could state the phase difference and maybe explain why it has that phase difference. Thanks

(edited 7 years ago)

Reply 1

profjb

A and B are separate particles btw.

Reply 2

niamhdoesmaths

Original post by profjb

The phase of a wave is a point on the wave cycle. Two points on a wave are "in phase" if they're a whole number of cycles apart. If two points on a wave are not in phase, there will be a phase difference between them. The phase difference is the fraction of the cycle that separates the points. With that knowledge can you work out the phase difference between A and B yourself now?

Hope that's made sense :h:

Reply 3

profjb

I don't know what you mean by the 'fraction of the cycle' that separates the 2 points. I do understand the other notes but it just dosen't make sense in my head. For e.g. I think the phase difference in the diagram I showed is 450 degrees but why is it 450 degrees and why if the 2 particles are moving in the opposite directions in terms of there displacement is that not took into account.

Reply 4

niamhdoesmaths

Original post by profjb

Displacement doesn't affect it. A wave cycle spans 360 degrees, so it doesn't matter where the two points are displacement-wise, just the spacing between them. 450 degrees is correct here :smile:

Reply 5

profjb

Ok. That explains how to figure out phase difference. Thank you for that :smile:

. I think the main problem is I still don't really understand what phase is. I mean the definition for phase difference is the difference in displacement between 2 points on the same wave or adjacent wave so how can it not involve displacement.

Reply 6

profjb

Ok. The definition in my textbook states the 'difference in displacement' but I don't see how it can be the difference in displacement. It seems almost wrong.
All the other definitions explain it in terms of the difference in angle in radians or degrees between 2 waves.

Reply 7

niamhdoesmaths

Original post by profjb

Ok. That explains how to figure out phase difference. Thank you for that :smile:

It kind of involves displacement - but not really. If two points on a wave are in phase, they are at the same point in the wave cycle - therefore they have the same displacement. But that's all you need to know involving displacement. If two points aren't an exact multiple of 360 degrees apart then they are out of phase. If they're an odd number multiple of 180 degrees apart then they are exactly out of phase - complete opposites in the wave cycle.

The definition for phase difference is "the amount one wave lags behind another" (if it's the phase difference between two waves, that is). Displacement isn't mentioned - it's simply how much of the cycle one wave (or point on the wave) is behind the other. I hope that makes sense! :smile:

Reply 8

profjb

Yes your explanation seems correct and explains why I get that value. Thank you :smile:

. I am wondering if they actually got the definition in the book wrong as the mark schemes have a completely different definition. The textbook has a lot of mistakes in it being the first copy supplied so that might be why I've been so confused.

Reply 9

profjb

I mean for example if it was the difference in the displacement between the 2 points then surely for the diagram I showed the difference in the displacement would be A where A is the maximum amplitude.