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OCR Physics A AS Forces+Motion/Electrons & Photons revision thread watch

1. Kirchoffs laws which are

1st: The sum of the currents flowing into any point in a circuit is equal to the sum of the currents flowing out of that point.

2nd: The sum of the e.m.f’s around any loop in a circuit is equal to the sum of the P.d’s around that loop.
1st. -> conservation of charge.

2nd. -> conservation of energy.

I've seen questions come up regarding that
2. I have forces and motion past papers jan03, jun03, jan04, jun04 so i might type up some hard questions nearer the exam date.
3. Oh and IIRC for Electrons&P, you need to know the wavelengths and corresponding frequencies of the electromagnetic spectrum. But only bother learning either frequencies OR wavelengths and use to calculate the other.

Tip:
If you have to convert between eV and joules, remember:
V=W/Q
=> W=VQ
or W = eV
ie W=(1.6*10^-19)V

Great thread btw Chats
4. (Original post by Widowmaker)
I have forces and motion past papers jan03, jun03, jan04, jun04 so i might type up some hard questions nearer the exam date.
Mate, send them to oxymoron and he'll upload 'em.
5. (Original post by endeavour)
1st. -> conservation of charge.

2nd. -> conservation of energy.
I've seen questions come up regarding that
I've added the other bit you said, but cant add anymore, the post has reached its 10,000 word limit I wonder if a mod can make the first post longer, or put in a second post so I can add future stuff there.

(Original post by Widowmaker)
I have forces and motion past papers jan03, jun03, jan04, jun04 so i might type up some hard questions nearer the exam date.
yeh that would be good, thanks
6. (Original post by endeavour)
Great thread btw Chats
Thanks just hope it helps everyone!
7. (Original post by endeavour)
Mate, send them to oxymoron and he'll upload 'em.
yeah I would but I don't have a scanner.
8. (Original post by chats)
I've added the other bit you said, but cant add anymore, the post has reached its 10,000 word limit I wonder if a mod can make the first post longer, or put in a second post so I can add future stuff there.

yeh that would be good, thanks
just copy and paste the stuff and just make a new thread, with a module per post.
9. (Original post by chats)
*bump*

if anyone needs help
if uve done the jan 05 paper of electrons and photons....please can u tell me as many questions that u remember are in that paper. Cheers mate,
Vinnie
10. (Original post by vinnie11)
if uve done the jan 05 paper of electrons and photons....please can u tell me as many questions that u remember are in that paper. Cheers mate,
Vinnie
No sorry I didnt do that paper
11. Thanks lol
12. You also need to know the equation;
hf = work function + Ekmax
work function - the minimum amount of energy required to remove an electron from the surface of a material.
hf - the energy of the photon.
Ekmax - the maximum kinetic energy of the electron that has left the material (0.5m(vmax)^2)
13. (Original post by Widowmaker)
You also need to know the equation;
hf = work function + Ekmax
work function - the minimum amount of energy required to remove an electron from the surface of a material.
hf - the energy of the photon.
Ekmax - the maximum kinetic energy of the electron that has left the material (0.5m(vmax)^2)
Thanks
14. I also have all mark schemes, 2001 - 2005 if anyone needs them.
15. Great thread, i'm doing Forces, Electrons and Waves on the same day
16. Does anyone know the boundaries to get an A/B/C and stuff? Or is it always 80%
17. (Original post by marthafocker)
Does anyone know the boundaries to get an A/B/C and stuff? Or is it always 80%
Can you help with answers this from my mock exam

Monochromatic light of wavelenth 450nm falls on cathode of photocell at a rate of 25microwatts. Only 10% of photons produice an electron and all these electrons produce a current I in an external circuit.
a) calculate
i) energy of one photon in J
ii) number of photons falling on cathode per second
iii) the current, I
b) same amount of energy per second falls on cathode in form of light of wavelength 600nm. the Work functon of the material of cathode is 3x10^-19J. SHOW that a photoelectric current will also flow.
18. Monochromatic light of wavelenth 450nm falls on cathode of photocell at a rate of 25microwatts. Only 10% of photons produice an electron and all these electrons produce a current I in an external circuit.

a) calculate
i) energy of one photon in J

E = hf = h[c/λ]
E = 6.63 x 10-34 x (3 x 108/450 x 10-9)
E = 4.42 x 10-19J
19. ii) number of photons falling on cathode per second
Power = 25 x 10-6W
Energy = Pt = 25 x 10-6 x 1 = 25 x 10-6J

Number of photons
= 25 x 10-6J/4.42 x 10-19J
= 5.66 x 1013 photons per second

iii) the current, I
Number of electrons = 0.1 x 5.66 x 1013 = 5.66 x 1012

Charge = 5.66 x 1012 x 1.6 x 10-19 = 9.05 x 10-7C

I = Q/t = [9.05 x 107C]/[1s] = 9.05 x 10-7A = 905nA

b) same amount of energy per second falls on cathode in form of light of wavelength 600nm. the work functon of the material of cathode is 3x10-19J. SHOW that a photoelectric current will also flow.

E = hc/λ
E = 6.63 x 10-34 x (3 x 108/600 x 10-9)
E = 3.32 x 10-19J

Energy of photon > work function therefore electrons will be removed and a current will flow.
20. This is a great revision thread ...
Though im dreading all my exams

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