Some Conceptual Questions (a) Physics teachers will often tell students that wav

Question

Some Conceptual Questions

(a) Physics teachers will often tell students that waves can propagate through each other and emerge unaffected. They also tell students that waves interfere with each other and can even completely cancel each other out. How do we reconcile these two seemingly contradictory statements?

(b) Sound travels much faster in water than air. This makes it difficult for divers to use sound as a way to orient themselves. In particular it is harder for divers to discern the direction from which a sound originates. Why would this be harder underwater than in air?

(c) We say that charge is conserved, and this is often stated as “Charge cannot be created or destroyed”. This is in fact a commonly stated, but erroneous version of the conservation of charge idea. A process that violates the statement in quotes is shown on page 622 of your book. Explain why charge can be created and destroyed without violating charge conservation.

(d) Are electrons and protons required by Coulomb’s law? Why or why not?

Explanation / Answer

(a) For two interfering waves, the displacement of the medium at any point is the sum of displacements of the waves at that point. If one wave has displacement and other has no displacement then waves can propagate through each other and emerge unaffected.  If at each point, one wave has displacement exactly opposite to other then waves completely cancel each other out

(b) Our ability to tell which direction a sound is coming from is largely determined by detecting phase differences. A sound source's wavefront will reach one ear at a slightly different time than the other, and by the time that same sound reaches the other ear, the slight delay will be perceived as a phase difference. But sound travels five times faster in water than it does in air. Travelling that fast, the sound is detected by both ears at almost exactly the same moment. That’s one reason that underwater a sound seems to come from all directions at once.

(c) the frictional charge is one of the example in which charge can be created and destroyed without violating charge conservation.

(d) Electrons and protons are not the only things that carry charge. Other particles (positrons, for example) also carry charge. So it is not required.

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