### Photoelectric Post Assessment Module 30C

Posted:

**Fri Oct 13, 2017 3:11 pm**For 30C, to find the frequency, do we have to use the equation where Energy of the photon = (Threshold energy)(kinetic energy)?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=16&t=22183

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Posted: **Fri Oct 13, 2017 3:11 pm**

For 30C, to find the frequency, do we have to use the equation where Energy of the photon = (Threshold energy)(kinetic energy)?

Posted: **Fri Oct 13, 2017 6:59 pm**

you can use the equation

frequency= (.5 x mass of electron x velocity of electron squared +work function) /plancks constant

i got that from

Energy of photon - work function = kinetic energy of electron

energy of photon = h x frequency

and

KE = .5mv^2

frequency= (.5 x mass of electron x velocity of electron squared +work function) /plancks constant

i got that from

Energy of photon - work function = kinetic energy of electron

energy of photon = h x frequency

and

KE = .5mv^2

Posted: **Sun Oct 15, 2017 1:39 pm**

For Part C, you need to combine several equations to find the frequency. By combining KE= E-threshold, E=hv, and KE=1/2mv^2, you get

frequency= (1/2mv^2 + threshold)/ h (Planck's constant)

Remember to use the mass of an electron: 9.11 x 10^-31 kg for the mass and to square velocity when plugging into the calculator.

You should get 6.78 x 10^14 Hz.

Hope this helps!

PS work out the "combined equation" since this it might sound confusing

frequency= (1/2mv^2 + threshold)/ h (Planck's constant)

Remember to use the mass of an electron: 9.11 x 10^-31 kg for the mass and to square velocity when plugging into the calculator.

You should get 6.78 x 10^14 Hz.

Hope this helps!

PS work out the "combined equation" since this it might sound confusing

Posted: **Sun Oct 15, 2017 5:03 pm**

Cassandra Mullen 3E wrote:For Part C, you need to combine several equations to find the frequency. By combining KE= E-threshold, E=hv, and KE=1/2mv^2, you get

frequency= (1/2mv^2 + threshold)/ h (Planck's constant)

Remember to use the mass of an electron: 9.11 x 10^-31 kg for the mass and to square velocity when plugging into the calculator.

You should get 6.78 x 10^14 Hz.

Hope this helps!

PS work out the "combined equation" since this it might sound confusing

I did the same thing, but ended up with B, 2.27 * 10^38... What did you use as the "threshold"? I added the kinetic energy to the work function, which is what I assume the threshold is, but could the threshold possibly be something else, or did I make a mistake elsewhere?

Posted: **Tue Oct 17, 2017 1:29 pm**

JonathanLam1G wrote:Cassandra Mullen 3E wrote:For Part C, you need to combine several equations to find the frequency. By combining KE= E-threshold, E=hv, and KE=1/2mv^2, you get

frequency= (1/2mv^2 + threshold)/ h (Planck's constant)

Remember to use the mass of an electron: 9.11 x 10^-31 kg for the mass and to square velocity when plugging into the calculator.

You should get 6.78 x 10^14 Hz.

Hope this helps!

PS work out the "combined equation" since this it might sound confusing

I did the same thing, but ended up with B, 2.27 * 10^38... What did you use as the "threshold"? I added the kinetic energy to the work function, which is what I assume the threshold is, but could the threshold possibly be something else, or did I make a mistake elsewhere?

For the threshold/work function value, I used 2.50 x 10^-19 J.