n*****g
发帖数: 25 | 1 In the text book of undergraduate Physics, one may find that two destructive
wave or 180 out-of-phases (such as water wave) add together to cancel out. I
studied and get good grades.
But the very honesty is that I don't understand it, what happen to the energy
when they add up? Doesn't that break the 1st law of thermdynamics?
Later, I asked my brother still in high school to ask his teacher, she said
the temperature has a slight increase when two destructive waves meet.
I think this is not corre | f*******d
发帖数: 339 | 2 There is no paradox. In the case of reflection, the two waves cancel only
at certain points, such as the surface of reflector, and these points are
called nodes, the energy is reflected backwards, there is no paradox.
【在 n*****g 的大作中提到】
: In the text book of undergraduate Physics, one may find that two destructive
: wave or 180 out-of-phases (such as water wave) add together to cancel out. I
: studied and get good grades.
: But the very honesty is that I don't understand it, what happen to the energy
: when they add up? Doesn't that break the 1st law of thermdynamics?
: Later, I asked my brother still in high school to ask his teacher, she said
: the temperature has a slight increase when two destructive waves meet.
: I think this is not corre
| f*******d
发帖数: 339 | 3 If they travel in the same direction, then yes, they cancel. However, if these
are excited using two or more oscillators, then they can not be all travelling
in exactly the same direction all over the space, there must be spatial
regions
where they do not cancel exactly, and the energy are send into these regions.
This is, for example, how phase modulated array radar works.
destructive
out. I
energy
said
teachers. | f*******d
发帖数: 339 | 4 This device you asked is exactly a perfect reflector, i.e. a mirror. Although
people are usually interested
in the region in FRONT of the mirror where there is reflection, one may also
consider
the region BEHIND the mirror. There is no wave behind a perfect mirror,
because there are actually two waves: one is the original one, the
other is a wave excited by the reflector, which has the opposite phase and
same amplitude.
If the mirror is perfect, the two cancels, as you thought.
Actually, at op |
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