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higher cfe physics help

When light of frequency f is shone on to a certain metal, photoelectrons are ejected with a maximum velocity v and kinetic energy Ek.

Light of the same frequency but twice the irradiance is shone on to the same surface.

Which of the following statements is/are correct?
I Twice as many electrons are ejected per second.
II The speed of the fastest electron is 2 v
III The kinetic energy of the fastest electron is now Ek.

A I only
B II only
C III only
D I and II only
E I, II and III
The answer is A, I only know this from getting it wrong (I put C) and looking at the marking scheme. Can someone please explain to me why the answer is A.Thank you.

Reply 1

JP298

The energy of an electron is dependant on the frequency of light shone upon the metal. From the equation E=hf . Energy = Plancks constant * the frequency of light. If the frequency stays the same it will have no change on the kinetic energy of the electron and thereby no effect on the velocity. So if the irradiance is doubled it has no effect on the energy of electron but only the number of electrons that will be emitted in 1s. So in this case twice as many electrons will be emitted. Hope this helps, if you need anything else just quote me.

(From an A band 1 higher physics and Current advanced higher student)

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Reply 2

JP298

Original post by molly221

Hi. Sorry to bother you again but would it be alright if you could explain how to do the following question to me?

Screen Shot 2016-01-11 at 17.53.47.png

Is it just the momentum equation?

Yes, you are completely correct, this question is based on Conservation of Momentum. So the law states that Total Momentum Before = Total Momentum After in the absence of all External Forces. So, have a go at the question and if you get the correct answer then that's great! If not post your working up and I'll help you to see where you might of gone wrong and how to fix it! Just quote us if you need anymore help, happy to do so!

Reply 3

molly221

Original post by JP298

Thank you. This has really helped.

I am also unsure on another question, and i was wondering if you could explain? Its alright if you can't as i understand i have already asked you quite a bit.

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Reply 4

JP298

Original post by molly221

Hi again,

No, I'm more than happy to help. So what I would recommend doing is that you look up online the difference between proportional, inversely proportional and inversely squared proportional. As that is what you're going to need to know for higher physics and learn what the graphs look like of each. The answer is C. Quote me if you need any more help!

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Reply 5

molly221

Original post by JP298

Thank you! I have a physics test tomorrow, that is testing us on the complete topic of particles and waves, so your help has been really valuable.

Reply 6

molly221

Original post by JP298

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Reply 7

JP298

Original post by molly221

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Have a look at your notes and have a go at the question and post your working and explanation. I'll then help you with anything you might not be grasping

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Reply 8

molly221

Original post by JP298

Have a look at your notes and have a go at the question and post your working and explanation. I'll then help you with anything you might not be grasping

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I got it correct! thanks anyway :smile:

Reply 9

JP298

Original post by molly221

I got it correct! thanks anyway :smile:

Well done! That's great! If you need any more help just quote us!

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Reply 10

molly221

Are you by any chance good at chemistry?

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Reply 11

molly221

Original post by JP298

Well done! That's great! If you need any more help just quote us!

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Are you by any chance good at chemistry?

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Reply 12

JP298

Original post by molly221

Are you by any chance good at chemistry?

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Unfortunately not, hahaha!

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(edited 8 years ago)

Original post by molly221

At least you're amazing at physics! I wish I was. Haha. Thanks anyway. 😄

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Haha, thanks a lot. I never took it past N5 level, even then I wouldn't be confident giving you any advice. Just keep at the past papers and you'll be at grade A standard by the exam!

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Reply 15

molly221

Hi, can someone please explain to me how to do solve the question.
I have tried to do it and got the answer of B but marking scheme says D.
I will post my working.
Thanks.

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Original post by JP298

(edited 8 years ago)

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Reply 16

Labrador99

Volunteer Team

Original post by molly221

Hi, can someone please explain to me how to do solve the question.
I have tried to do it and got the answer of B but marking scheme says D.
I will post my working.
Thanks.

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Here is the answer with full working... Hope this helps
Vrms= 12V
P= 24W
***When calculating the power of a circuit with an alternating current, the rms values are used for both voltage and current (as the rms values are the AC equivalents of the DC supply) ***
P = I x V
24 = 12 x I
I = 24/12
I = 2A **This is the Irms**
Ipeak = √2 x Irms
Ipeak = √2 x 2
Ipeak = 2.8A
Therefore, answer D

Reply 17

molly221