## Sapling Question 22

$\lambda=\frac{h}{p}$

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### Sapling Question 22

I am confused on what equation to use. I know that I need to find the kinetic energy, but usually first we need to find the total energy and then whatever is left is the kinetic energy, but the question does not state how much energy is used. Also, I am not sure about the electron volts, since we always use J to represent energy. Hope someone figured this one out already. Thanks.

Question: You use an electron microscope in which the matter wave associated with the electron beam has a wavelength of 0.0205 nm. What is the kinetic energy of an electron in the beam, expressed in electron volts?

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### Re: Sapling Question 22

For this question, you want to use deBroglie's equation. Convert the given wavelength into meters and plug it in as wavelength in deBroglie's equation. Use the electron's mass (9.11 x 10^-31 kg). Planck's constant is 6.626 x 10^-34 J-s. Then, solve for velocity. Once you solve for velocity, you can plug it into the equation for kinetic energy which is 1/2mv^2. Then, convert Joules into eV by using 1.602 x 10^-19 J for 1 eV.

Hope this helps!

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### Re: Sapling Question 22

In this situation, they are giving you a wavelength and asking you to find the kinetic energy. These are related by the DeBroglie equation, so you first need to use that to find the velocity of the electron. Once you figure out the velocity, you can input that value into the kinetic energy equation to find your answer.

Electron volts are just another measurement of electromagnetic energy, so you can use the conversion factor 1.602x10^-19 J = 1 eV to convert your answer into eV.

Hope this helps!

Stephen Min 1I
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Joined: Wed Sep 30, 2020 10:01 pm

### Re: Sapling Question 22

We need to find the velocity of the electron to solve for KE. In order to do this, you would use the deBroglie's equation, given that the wavelength is equal to 0.0205 nm, to solve for velocity. Also, the conversion from eV to Joules is on the equation sheet that Lavelle gave us.