# Wavelenght associated with electron

#### mounica reddy

1.given mass of electron = 9.11*10^-31 ... kinetic energy=1mega electron volt

2. Relevant equations:: k.e=1/2mv^2 and λ=h/mv

3. The attempt at a solution :: am unable to get it....can any one ry...

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#### cupid.callin

1.given mass of electron = 9.11*10^-31 ... kinetic energy=1mega electron volt

2. Relevant equations:: k.e=1/2mv^2 and λ=h/mv

3. The attempt at a solution :: am unable to get it....can any one ry...

You have KE and also expression for it
So you can find v?
and then mv

PS: for energy conversion use: 1eV = e J (e=1.6*10-19)

#### Curious3141

Homework Helper
Big caution here: if you use classical mechanics to compute the velocity of the electron, you get v > c. This is not correct.

I'm afraid that here, you have to use the relativistic formulae ($\Delta{E} = \Delta{m}c^2$ and $m = \frac{m_0}{\sqrt{1 - \frac{v^2}{c^2}}}$) to get the answer. You can't use $E_k = \frac{1}{2}mv^2$.

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#### cupid.callin

Big caution here: if you use classical mechanics to compute the velocity of the electron, you get v > c. This is not correct.

I'm afraid that here, you have to use the relativistic formulae ($\Delta{E} = \Delta{m}c^2$ and $m = \frac{m_0}{\sqrt{1 - \frac{v^2}{c^2}}}$) to get the answer. You can't use $E_k = \frac{1}{2}mv^2$.
Nice catch, i missed that. But its use depends on the level of question ... in basic physics, relativity eqns are usually not used ... Let the OP decide weather he is supposed to use them or not

#### Curious3141

Homework Helper
Nice catch, i missed that. But its use depends on the level of question ... in basic physics, relativity eqns are usually not used ... Let the OP decide weather he is supposed to use them or not
I suppose, but frankly, if the student is expected to understand and apply the de Broglie equation, he/she should be expected to discern when SR should be used rather than Classical Mechanics.

#### Redbelly98

Staff Emeritus
Homework Helper
... in basic physics, relativity eqns are usually not used ...
Usually true, unless the class is learning introductory relativity.

If this is to be solved without relativity, it is the responsibility of the teacher or professor to keep energies and speeds in the nonrelativistic regime. This teacher didn't do that, so the correct way to solve this is to consider relativity.

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