Which formulae should I use when calculating the frequency of an electron given its speed?

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If I know the speed of an electron, how should I go about calculating its frequency?

For example, if the electron was travelling at $v$ $ms^{-1}$ there are two options I could take.

Calculate the momentum of the electron using $p=m_ev$. Then use the formula for de Broglie's wavelength to find the wavelength of the electron using $\lambda=\frac{h}{p}$. Then use the formula for wave speed, $v=f\lambda$ to get the frequency of the electron.
Alternatively, I could find the kinetic energy of the electron using the formula $KE=(1/2)m_ev^2$. Then substitute the kinetic energy calculated into $E=hf$ to get the frequency of the electron.

The first method doesn't give me the right answer, but the second method does. Which step of the first method is incorrect and why?

edited Mar 28 '20 at 06:52

Qmechanic

asked Mar 27 '20 at 22:39

I've denoted $v$ to refer to the speed of the electron, and $f$ to refer to the frequency of the electron. – Hypatia of Alexandria Mar 27 '20 at 22:56
The $v$ in the formula $p=m_ev$ is different from the $v$ in $v=f \lambda$. The former denotes the speed of particle in a wave and the latter denotes the speed of the wave – Tammikēlu Mar 28 '20 at 04:51
Possible duplicate: https://physics.stackexchange.com/q/34214/2451 – Qmechanic Mar 28 '20 at 06:50
Is your problem, you speak of correct answer, with very high velocities? – anna v Mar 28 '20 at 07:07

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