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Speed/Energy of Electrons

Posted: Sat Oct 14, 2017 10:43 pm
by Leah Thomas 2E
Need a quick answer. So the speed of light is constant and is what we use in our equation. However, when dealing with the photoelectric effect, energy is emitted unto a metal at a different a certain speed and is it emitted off at a slower speed/energy since there is some sort of threshold energy (work) that slows the speeds at which electrons are emitted back out? Totally confused on how this work and why we use the speed of light in some cases and not others...

Re: Speed/Energy of Electrons

Posted: Sat Oct 14, 2017 11:32 pm
by Rachel Formaker 1E
I think your confusion is mainly mixing up speed, energy, and frequency.

The speed of light is constant (3.00 x 108 m/s), and is equal to the frequency times wavelength of light.
The frequency(v) of light is different for different types of light, and the energy(E) of light is related to the frequency of light by the equation E=hv.

In the photoelectric effect, light of a certain frequency or energy (either can be given in a problem) will be emitted unto a metal, and if the energy of the light is greater than the work function (aka threshold energy), an electron will be emitted with the kinetic energy 1/2mv2.
The kinetic energy of the electron will always be less than the energy of the emitted light, so in that way it could be thought of as "slower" but it really means that it has less energy. The kinetic energy can be given by the equation Ek= hv -work function

Re: Speed/Energy of Electrons

Posted: Sun Oct 15, 2017 10:35 am
by Nicole Nikolov 1K
I think I'm also kind of confused between speed and frequency. If light has a higher frequency doesn't that mean that it's going to be faster?

Re: Speed/Energy of Electrons

Posted: Sun Oct 15, 2017 12:19 pm
by SitharaMenon2B
The frequency refers to the number of waves per second that pass a point in space , but the speed of light (in a vacuum) stays constant for every wave of light (it doesn't change for different frequencies).

Re: Speed/Energy of Electrons

Posted: Sun Oct 15, 2017 8:59 pm
by Ruohan Sun 3I
The speed of electron ejected is related to its kinetic energy, which is the energy of photon subtracted by the threshold energy. It's not related to the speed of light. You can use the speed of light to calculate the frequency and energy of photons.