# Thread: Kinetic Energy and the De Broglie Wavelength

1. Just something I noticed a little while ago, and want to know why I keep getting this contradictory result.

From the de Broglie Equation,

lambda (here denoted as l) =h/mv, where m =mass, v =velocity and h = Planck's constant.

But frequency ( here denoted as f) = v/l

Therefore,

f = v/h/mv = mv^2/h

The energy of a wave is given by E= hf.

Therefore, E = hmv^h/h

Therefore, E =mv^2

Yet the energy should be equal to the kinetic energy of the particle, which is mv^2/2.

Why is it that I keep getting this answer?  2.

3. I'm not sure but I think it goes something like this:

-First you omitted the Lorentz factor g=(1 - v^2/c^2)^(-1/2). With it the de Broglie equation is l = h/gmv. It's tempting to just assume that v<<c and therefore g=1, but let's keep it for now. This gives you the result E=gmv^2.

-Second, E=hf gives you the total energy of the particle both kinetic and rest energy. In other words E = KE + RE, where RE=mc^2 with m being the rest mass of the particle. Also we know that E=gmc^2 where g is the Lorentz factor.

Thus we get that for a particle with mass m E=gmv^2=gmc^2. Clearly that cannot be right. The only flaw in your reasoning that I can find is letting f=v/l. It makes sense in classical physics but the de Broglie hypothesis takes relativity into account and things get weird with relativity.  4. Some posts have been split off into a new thread here:

http://thescienceforum.com/De-Brogli...gth-17809t.php

as they pertained to a individual poster's personal theory  5. Well, my discussion with martillo got moved, but I have an answer for you. The velocity of the particle is not the phase velocity of its waveform. Rather the phase velocity of the wave is vp = c^2/v. If you use that to calculate the frequency of the wave and then apply E=hf you will get E=gmc^2 as we should. Again remember that this is the total energy of the particle E = KE + RE.

Thus the kinetic energy is KE = E - RE, where E=gmc^2 and RE=mc^2. Therefore:

KE = gmc^2 - mc^2
= mc^2*(g-1)

If you do a Taylor expansion on g the c^2 will drop out and you will get KE = 1/2*mv^2. The Newtonian kinetic energy is just a first order approximation to the true relativistic kinetic energy.  Bookmarks
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