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# Physics! Help :) watch

1. Hey guys, Im stuck on this question:

Calculate the maximum speed of a photoelectron emitted from the surface of potassium by photons of this energy.

Work function of potassium: 2.90*10^-19 j

Mass of an electron: 9.11*10^-31 kg

Planck Constant: 6.6*10^-34 js

Frequency of the radiation: 5.01*10^14 hz

I'm pretty sure I will have to use the equation:

hf= work function + 0.5 mv^2

2. Sorry to intrude on your thread, but how the hell do i work out the speed of an electron?
3. (Original post by scholarshipkid)
Hey guys, Im stuck on this question:

Calculate the maximum speed of a photoelectron emitted from the surface of potassium by photons of this energy.

Work function of potassium: 2.90*10^-19 j

Mass of an electron: 9.11*10^-31 kg

Planck Constant: 6.6*10^-34 js

Frequency of the radiation: 5.01*10^14 hz

I'm pretty sure I will have to use the equation:

hf= work function + 0.5 mv^2

Umm...u just plug the values into the formula. You are using 6.6 for plancks constant. Use 6.63 and you will get the right answer.
4. (Original post by 2710)
Umm...u just plug the values into the formula. You are using 6.6 for plancks constant. Use 6.63 and you will get the right answer.

ahh just done it, first bit of the answer is right i.e. 3.04 but i'm 10^5 out!! I dont understand why this is happening
5. haha is this Alevel..seriously?
And i'm having trouble with AS :P
6. (Original post by Demonic Entity)
Sorry to intrude on your thread, but how the hell do i work out the speed of an electron?
eV = 1/2 mv^2

m being the mass of the electron. should be in the data booklet.
7. (Original post by scholarshipkid)
ahh just done it, first bit of the answer is right i.e. 3.04 but i'm 10^5 out!! I dont understand why this is happening
Make sure you are typing the powers in correctly on your calculator, making sure everything is bracketed alrite. If it still doesn't work, type each step on you calculator separately.
8. (Original post by 2710)
Make sure you are typing the powers in correctly on your calculator, making sure everything is bracketed alrite. If it still doesn't work, type each step on you calculator separately.
I think my method is wrong rather than the actual calculations? I've done this:

2(hf-work function)/(9.11x10^-31)

Then square root the whole thing and I get: 3.04x10^10!!! Should be ^5
9. (Original post by scholarshipkid)
I think my method is wrong rather than the actual calculations? I've done this:

2(hf-work function)/(9.11x10^-31)

Then square root the whole thing and I get: 3.04x10^10!!! Should be ^5
I just did it that way and got ^5 >___>
10. (Original post by 2710)
I just did it that way and got ^5 >___>

hmmm

2(4.2x10^-10) / (9.11x10^-31), then square root it all, are you sure?
11. (Original post by scholarshipkid)
hmmm

2(4.2x10^-10) / (9.11x10^-31), then square root it all, are you sure?
Howd you get this? I get 4.2x10^-20
12. (Original post by 2710)
Howd you get this? I get 4.2x10^-20
You're probably right but please point out my error:

I did hf which was (6.63x10^-34) x (5.01x10^24) and then subtracted the work function of (2.90x10^-9) which gave the answer: 4.2x10^-10

Which I then proceeded to use.
13. (Original post by scholarshipkid)
You're probably right but please point out my error:

I did hf which was (6.63x10^-34) x (5.01x10^24) and then subtracted the work function of (2.90x10^-9) which gave the answer: 4.2x10^-10

Which I then proceeded to use.
Lol, can't you spot it? Its not 5.01x10^24, its ^14. You even wrote it in the first post lol

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