# Question about the wave equation

1. Dec 12, 2012

### Sunfire

Hello,

is it possible for one to assume a straight-line propagation of an e.m. wave and constant velocity c? If so, is it possible to simplify the wave equation

utt=c2uxx

by expressing the spatial variable x through the time variable t?
x must be a function of t, since the motion is rectilinear with constant c.

Then, x = x(t) and x is no longer independent variable. Then the above PDE should be rewritten in terms of t only, since u(x,t)=u(x(t),t)=u(t)

Does this make sense?

2. Dec 13, 2012

### sankalpmittal

Yes , you can convert it into differential forms :

http://en.wikipedia.org/wiki/Wave_equation
http://people.ccmr.cornell.edu/~muchomas/P214/Notes/OtherWaves/node18.html [Broken]
http://hyperphysics.phy-astr.gsu.edu/hbase/waves/waveq.html

Last edited by a moderator: May 6, 2017
3. Dec 13, 2012

### Jano L.

Yes, of course, the light of a pocket laser is a good example of such wave.

The quantity u in the wave equation is some physical quantity ascribed to point in space x at some time t (e.g. electric field), so it is usually thought of as a function of both x and t.

If you have some function x1(t), you can define new function by
$$u1(t) = u(x1(t),t).$$

For example, if x1 is function giving the position of electron, u1 gives the electric field acting on the electron at time t.

However, the function x1 has to be inferred from other sources; there is nothing in the wave equation that would give such a function.

True, there is the motion of the maxima of the wave crests and one could ascribe such function x1(t) to one of them, but there is no good reason for doing so - there is no particle there - so it would seem to be just an empty exercise.

4. Dec 13, 2012

### sophiecentaur

Or the signal at a few wavelengths distant from any radio transmitter. That behaves pretty well like a plane wave.