Energy v. Kinetic Energy

Seohyun Park 1L
Posts: 73
Joined: Fri Sep 28, 2018 12:29 am

Energy v. Kinetic Energy

For 1.33 (6th edition), part a asks for the wavelength an electron gives off given its velocity. First, I used wavelength = h/p (de borglie equation) to find the right answer. I tried a different method using the given velocity to find its kinetic energy (KE= 1/2mv^2) , and plugged that into E=h(frequency) to find the wavelength. I got a different, wrong answer using that method. Is the E used in de borglie equation different from kinetic energy? Or can you not use E=hv for electrons?

Brian Chhoy 4I
Posts: 66
Joined: Fri Sep 28, 2018 12:16 am

Re: Energy v. Kinetic Energy

I think the equation for E=hc is only for light, so even though an electron has both waves and photon like properties, you won't get the same answer as if you used the de Broglie equation which is for particles.

Parth Mungra
Posts: 72
Joined: Fri Sep 28, 2018 12:28 am

Re: Energy v. Kinetic Energy

Part A asks for the wavelength and you can substitute the momentum formula (p=mv) into the de broglie formula (wavelength=plancks/p) because you have the mass and the velocity of the electron. So it should look like this:

Wavelength= (Planck's constant)/(mass x velocity)

You don't need the kinetic energy in part a, you need its momentum. Hopefully this helped!

lukezhang2C
Posts: 59
Joined: Fri Sep 28, 2018 12:18 am

Re: Energy v. Kinetic Energy

Electrons have different properties than photons, and while it is near massless, it is often not considered as such when solving for kinetic energy which is why the same kinetic energy equations which work for photons do not work for electrons.