AQA Physics PHYA5 - Thursday 18th June 2015 [Exam Discussion Thread]

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Does anyone know why with electron diffraction when you calculate the de broglie wavelength of an electron you use λ=hc/E instead of λ=h/mv ? What does c have to do with the electron energy?

Reply 2221

Sam.B57

Original post by gcsestuff

It's to do with the further away the two charged plates the less chance there is of the electron 'jumping' across

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right... I think that makes sense, probably left it a bit late to try and learn that bit

Reply 2222

CD223

Original post by Me123456789

June 2011 Q2b

For the reaction to take place the a particle must come within a distance, d, from
the centre of the aluminium nucleus.

Calculate d if the nuclear reaction occurs when the a particle is given an initial
kinetic energy of at least 2.18 × 10–12 J.

The electrostatic potential energy between two point charges Q1 and Q2 is
equal to where r is the separation of the charges and ε0 is the
permittivity of free space.

In this question the Q1 is 13x1.6x10^-19 , I understand that because that's the number of electrons aluminium has,
but I don't understand why Q2 has a charge of 2x1.6x10^-19. Can anyone explain why?

It's to do with proton number. The aluminium nucleus has a charge of +13e because that's how many protons the nucleus of Aluminium contains.

The alpha particle is equivalent to a Helium nucleus. In a similar way, it has a charge of +2e because it contains two protons.

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Reply 2223

Jimmy20002012

Can anyone help me on second part of this question doing turning points don't seem to know how to do it?

ImageUploadedByStudent Room1434559496.197379.jpg

ImageUploadedByStudent Room1434559496.197379.jpg

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Reply 2224

CD223

Original post by ironic_username

Does anyone know why with electron diffraction when you calculate the de broglie wavelength of an electron you use λ=hc/E instead of λ=h/mv ? What does c have to do with the electron energy?

Both equations give the de Broglie wavelength but the former is easier to calculate from the conditions of the experiment. Electrons have a charge "e" and are accelerated by a potential difference "V". As such, their energy E is equal to eV.

From last year unit 1 you know that:

[br]E = hf[br]

c=f \lambda

[br]f = \dfrac{c}{\lambda}[br]

Subbing this into the first equation gives

[br]E = \dfrac{hc}{\lambda}[br]

[br]\therefore \lambda = \dfrac{hc}{E}[br]

Reply 2225

Me123456789

Original post by CD223

ah yes, the proton number, I got a bit confused :colondollar:

Thank you! completely forgot about helium

(edited 8 years ago)

Reply 2226

CD223

Original post by Me123456789

ah yes, the proton number, I got a bit confused :colondollar:

Thank you! completely forgot about helium

No problem!

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Reply 2227

Sbarron

Original post by MSB47

Firstly because only 1 out of 400 photons are detected multiply the corrected count rate by 400 to give you the count rate of the detector if it detected all photons incident on it. Then divide that by the area of detector because that will give you the intensity of the source at the distance it is from and because it is uniform in all direction the intensity is constant at this point therefore, you can use the inverse square law equation I=I₀/4pi(r)² and re arrange to make I₀the subject and 4pi(r)² is the area of the sphere the gamma photons have spread out at that distance so I₀is activity of source because it gives the number photons emitted per second which is same as activity (number of disintegrations per second)

Hope that helps

So you can use I=Io/area^2 instead of I=Io/distance^2

Reply 2228

Sbarron

Original post by Sbarron

So you can use I=Io/area^2 instead of I=Io/distance^2

Also I don't understand why there is a k constant if we never use it?

Reply 2229

Protoxylic

Original post by AR_95

How much do we need to know about quantum tunnelling?

For Turning points you just need to know as much detail as is necessary to explain STMs and how they work from a basic level

Reply 2230

betbi3etwerrd

Original post by Leonacatherine

Also can anyone explain how electron scattering is used to determine nulcear radius

Here you go $electron deffraction.png$

Reply 2231

tennispatch97

Can someone please explain to me why in the June 2014 paper (question 1)b)ii.) you don't include the mass of the neutrons released in the fission reaction when calculating the energy released, but in the June 2013 and June 2011 papers (both question 1)c)i.) you do have to include the mass of electrons/neutrons released in the reactions when calculating the energy released in order to obtain the correct answer.

Reply 2232

aprocrastinator

Original post by CD223

For those of you asking for it, here's what I'd write for the derivation of kinetic theory:

ImageUploadedByStudent Room1434555044.767375.jpg

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this is great, thank you! :smile:

also I love your handwriting :O

Reply 2233

jf1994

Can anyone doing medical physics explain to me when we should substitute negative values into the lens equation 1/f = 1/u + 1/v?

Reply 2234

betbi3etwerrd

fix: $electron deffraction.png$

Reply 2235

CD223

Original post by aprocrastinator

this is great, thank you! :smile:

also I love your handwriting :O

Really?

thanks haha! No worries.

I've always been told it's messy! (I'm left handed :colondollar:

)

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Reply 2236

a.a.k

Original post by CD223

Really?

thanks haha! No worries.

I've always been told it's messy! (I'm left handed :colondollar:

)

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You havent seen my handwriting.
When people look at it they immidiatly think i want to be a doctor. :awesome:

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Reply 2237

CD223

Original post by jf1994

Can anyone doing medical physics explain to me when we should substitute negative values into the lens equation 1/f = 1/u + 1/v?

I don't do medical, but in Astro it's when the image is virtual, so v becomes negative.

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Reply 2238

knighty437

For the core unit:

June 2010: What goes on in a nuclear reactor for induced fission
June 2011: How pressure is exerted by gasses
June 2012 Comparison of nuclei radius experiments, electron or alpha scattering
June 2013: Determining a radioactive source
June 2014: Half life of radioactive waste and how it should be stored.

Those are the past 6 markers for anyone who's wondering.