## sapling hw problem

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

Brianna Martilla 1C
Posts: 103
Joined: Wed Sep 30, 2020 9:58 pm

### sapling hw problem

The E. coli bacterium is about 1.9 μm long. Suppose you want to study it using photons of that wavelength or electrons having that de Broglie wavelength.

What is the energy of the electron?

I figured out the answer to the first part of the problem, but I'm having trouble with this part. The feedback it telling me to use $E=1/2mv^2$ and $lambda=h/mv$ to solve. I rearranged them and changed them to $E=\frac{1}{2}m(\frac{h}{m\cdot lambda})^{2}$. Then I plugged in the values for h, m, and lambda (converted the given wavelength to meters). I ended up getting $6.7\times 10^{-44}$, which seems pretty small.

Any help is appreciated.

Posts: 159
Joined: Wed Sep 30, 2020 9:32 pm
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### Re: sapling hw problem

Your new equation looks correct. I used the equation you listed and got 6.7x10^-26 J, so maybe check your calculations because all your work before that looks correct. Make sure you converted the wavelength into meters, that could just be a small error.

Hope this helps!

Allan Nguyen 2G
Posts: 100
Joined: Wed Sep 30, 2020 9:49 pm

### Re: sapling hw problem

For the second part of this problem, you want to use the de Broglie equation and rearrange it to find the velocity of the electron. So after rearranging the equation it should be v = h/m λ. Convert the wavelength to meters, and plug in the values and you should get 382.84 m.s^-1. I would then use the equation to find the kinetic energy of an electron which is E = 1/2mv^2. I plugged in the velocity I got from using the de Broglie equation and got 6.68 x 10^-26 J. That's weird, I got the same value as you but it's different in scientific notation. Maybe you converted the micrometers to meters wrong, there are 1 x 10^6 micrometers in a meter.

nayha a 1E
Posts: 96
Joined: Wed Sep 30, 2020 9:54 pm

### Re: sapling hw problem

Can someone explain to me how we find the value for m? What would we plug in there?

Lesly Lopez 3A
Posts: 95
Joined: Wed Sep 30, 2020 9:54 pm
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### Re: sapling hw problem

nayha a 1L wrote:Can someone explain to me how we find the value for m? What would we plug in there?

Hi there, I am not sure where this number came about but someone in another question on this website said that the mass od an electron was: 9.109 x 10-31 kg. Hope that helps!

Lesly Lopez 3A
Posts: 95
Joined: Wed Sep 30, 2020 9:54 pm
Been upvoted: 1 time

### Re: sapling hw problem

Hi! always try to look back at the reference sheet. I was having trouble too until Someone told me it was on the reference sheet. All the masses are there in units of kg.