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# Kinetic Energy - Electrons/Electron Gun watch

1. Hi all,

Attempting to revise for physics atm, came across this question and tbh I'm pretty stumped:

1. An electron is accelerated from a speed of 3x10^6 m/s across a potential difference of 20V in an electron gun.

The kinetic energy of the electron having passed through the electron gun is:

A. 0.90 x 10^-18 J
B. 3.20 x 10^-18 J
C. 4.10 x 10^-18 J
D. 7.30 x 10^-18 J
E. 8.20 x 10^-18 J
I tried using Ek=1/2mv^2, using 9.11 x 10^-31kg as mass, getting C as the answer, but apparently it's D. Clearly I'm missing something with the 20V through the electron gun, but unfortunately my notes lack any specific mention of electron guns and I'm clearly too dim to figure it out! Any help is appreciated. Thank you.
2. (Original post by MySweetheartTheDrunk)
Hi all,

Attempting to revise for physics atm, came across this question and tbh I'm pretty stumped:

I tried using Ek=1/2mv^2, using 9.11 x 10^-31kg as mass, getting C as the answer, but apparently it's D. Clearly I'm missing something with the 20V through the electron gun, but unfortunately my notes lack any specific mention of electron guns and I'm clearly too dim to figure it out! Any help is appreciated. Thank you.
You've only worked out the initial kinetic energy. You need to realise that when the electron passes through the potential difference of 20V, work is done, and so its kinetic energy increases. The work done on the electron is QV.
3. (Original post by CallumFR)
You've only worked out the initial kinetic energy. You need to realise that when the electron passes through the potential difference of 20V, work is done, and so its kinetic energy increases. The work done on the electron is QV.
Ah thank you, makes sense Only thing is, I can't see what I use as the value for Q. Is the charge of an electron not just -1?
4. (Original post by MySweetheartTheDrunk)
Ah thank you, makes sense Only thing is, I can't see what I use as the value for Q. Is the charge of an electron not just -1?
For Q you use the charge in coulombs, and for an electron that is -1.6*10^-19 coulombs (you can just ignore the negative sign in this question though)
5. (Original post by CallumFR)
For Q you use the charge in coulombs, and for an electron that is -1.6*10^-19 coulombs (you can just ignore the negative sign in this question though)
Aha thank you, you've been a great help! Now sorted that problem but left with the predicament of the fact I didn't know the charge of an electron, and it isn't in the data sheet at the front of the exam! Perhaps tis one of those things you're expected to know - I hate it being expected that I know something! Merci beaucoup for the help anyhows, much appreciated
6. (Original post by MySweetheartTheDrunk)
Aha thank you, you've been a great help! Now sorted that problem but left with the predicament of the fact I didn't know the charge of an electron, and it isn't in the data sheet at the front of the exam! Perhaps tis one of those things you're expected to know - I hate it being expected that I know something! Merci beaucoup for the help anyhows, much appreciated
It will definitely be on the data sheet at the front for the final exam
7. Since when has physics been multiple choice? o.0
8. (Original post by CallumFR)
It will definitely be on the data sheet at the front for the final exam
Haha right you are, cheers! I was looking at a past prelim paper - they only put in the stuff that was necessary to know in that years paper I think
9. (Original post by You Failed)
Since when has physics been multiple choice? o.0
Um on the Scottish curriculum, not sure exactly how long but quite a while I believe!
10. (Original post by MySweetheartTheDrunk)
Um on the Scottish curriculum, not sure exactly how long but quite a while I believe!

I'm sure the students love it though!
11. (Original post by You Failed)

I'm sure the students love it though!
Got to say actually I'm not a huge fan of multiple choice, in anything! At least the entire paper isn't multiple choice though. For physics it's 20 marks multiple choice, 70 written answer. So I suppose it's not too bad. But yes, the scottish curriculum seem very keen on multiple choice, it was introduced to maths two years ago!

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Updated: February 11, 2010
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