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Quick physics question

A photon can convert into a positron and an electron, each of mass 9.11 x10-31 kg.

Calculate the minimum energy for the photon to be able to do this.

Ok... i used E = mc2 with a mass of 18.22 x10-31 (mass of positron and electron)

This isn't right though... the question only used a mass of 9.11 x10-31 kg.

Can someone explain to me why??
Reply 1
Avatar for mackin boi
mackin boi
1
u gots me?!?! :frown:

so u knw tha mass of a photon?
Reply 2
Avatar for drw25
drw25
Maybe the question means that a photon is capable of turning into a positron or an electron, both of which have a mass of 9.11 x10^-31 kg, and wants you to calculate the energy required for one-such conversion to occur.

That said, charge wouldn't be conserved if that was the case, which can't be a good thing, so I'm probably wrong...
Reply 3
Avatar for Revenged
Revenged
OP
15
Nope... i thought the mass of photon would be equal to the mass of BOTH the products but it isn't... and i really dont get why
Reply 4
Avatar for mackin boi
mackin boi
1
this is an impossible Q...ive mulled over it like a fine wine...and nowere...the m in E=mc2 is the mass defect so u have to know the mass of a photon (google comes up with all this jargon about whether light has a mass so that was a pointless line of questionin)...ne1 hu can do this is Crazy :biggrin:
Reply 5
Avatar for El Stevo
El Stevo
11
Revenged
A photon can convert into a positron and an electron, each of mass 9.11 x10-31 kg.

Calculate the minimum energy for the photon to be able to do this.

Ok... i used E = mc2 with a mass of 18.22 x10-31 (mass of positron and electron)

This isn't right though... the question only used a mass of 9.11 x10-31 kg.

Can someone explain to me why??


when a positron and an electron annihilate two photons are normally produced with equal and opposite momentum. i'm guessing two photons are needed.
Reply 6
Avatar for Revenged
Revenged
OP
15
Not necessarily, you can create an pair by firing one gamma ray near a nucleus - which allows momentum to be conserved
Reply 7
Avatar for Revenged
Revenged
OP
15
mackin boi
ne1 hu can do this is Crazy :biggrin:


Totally agree... i was told a photon doesnt have mass but for these calculation it seems that they do...

Is physics weird or is it just OCR?
I would have gone for E=(2me)c² too.

Must be a mistake in the book?
Reply 9
Avatar for Revenged
Revenged
OP
15
OCR Physics B (Advancing Physics) June 2001, Field and Particle Physics, p. Q2.

I don't have the mark scheme (i just have my teacher marking it wrong!)

Could someone check if i did get it right if you have the mark scheme
Reply 10
Avatar for nas7232
nas7232
19
They use 9.11x10-31kg for this reason:

When you annihlate two photons the energy required in the equation = 2E

so from e=mc^2

2e=2mc^2 [now divide by 2]

you get

E=mc^2
Reply 11
Avatar for Revenged
Revenged
OP
15
???
Reply 12
Avatar for nas7232
nas7232
19
what don't you understand, by doing ??? i can't really help you. The book is correct, and i explainned it above.

If you are still unsure, i suggest you look up particle physics phy3 - edexcel, there will be some similiar questions in them papers...

2m is from 9.11x10^-31kg x 2.
Reply 13
Avatar for nas7232
nas7232
19
They use 9.11x10-31kg for this reason:

When you annihlate two photons the energy required in the equation = 2E

so from e=mc^2

2e=2mc^2 [now divide by 2]

you get

E=mc^2

m = 9.11x10^-31kg
c = 3x10^8ms^-1

therefore

E= 9.11x10^-31kg.[3x10^8ms^-1]^2 = The answer required.
No I disagree.
The question says, "A photon can convert..."

So you consider one photon...

(if the book is correct, then the question is worded very badly)
If it means all three have a mass of 9.11....etc then you have a mass defect of 9.11 left,
9.11 ->9.11+9.11, difference of 9.11. this is the value you input into e=mc^2, sorry if this is what someone else said but my simple mind couldn't understnad your complex explanations.
Reply 16
Avatar for Willa
Willa
11
I can tell you why you're wrong, but I cant confirm the correct method just yet, because I am trying to get rid of a root 3 from my working.

But you're wrong because the energy of the electron and positrons are not mc^2....that is their rest mass energy, the particles however MUST be moving to conserve the momentum of the photon.

Photons have momentum = E/c
so you need to satify E/c = g1mv1 + g2mv2 (where g1 and g2 are the gamma correction factors), AS WELL AS: E = (g1+g2)mc^2

I'm currently trying to handle the jiggery pokery of it all, it involves assumptions about equal speeds etc (NOT equal velocities) I'll hopefully have an answer for you soonish.

but just to confirm, the answer they have given is the numerical equivilent of mc^2 (where m= mass of electron)??
Reply 17
Avatar for El Stevo
El Stevo
11
Willa
I can tell you why you're wrong, but I cant confirm the correct method just yet, because I am trying to get rid of a root 3 from my working.

But you're wrong because the energy of the electron and positrons are not mc^2....that is their rest mass energy, the particles however MUST be moving to conserve the momentum of the photon.

Photons have momentum = E/c
so you need to satify E/c = g1mv1 + g2mv2 (where g1 and g2 are the gamma correction factors), AS WELL AS: E = (g1+g2)mc^2

I'm currently trying to handle the jiggery pokery of it all, it involves assumptions about equal speeds etc (NOT equal velocities) I'll hopefully have an answer for you soonish.

but just to confirm, the answer they have given is the numerical equivilent of mc^2 (where m= mass of electron)??


:smile: if the question is an A level question (which i'm lead to believe) then i think your answer may go a little beyond the syllabus Willa. OCR tells us to use rest masses even when the particles are obviously moving... and gamma corrections are none existent as far as we "know"
Willa
I can tell you why you're wrong, but I cant confirm the correct method just yet, because I am trying to get rid of a root 3 from my working.

But you're wrong because the energy of the electron and positrons are not mc^2....that is their rest mass energy, the particles however MUST be moving to conserve the momentum of the photon.

Photons have momentum = E/c
so you need to satify E/c = g1mv1 + g2mv2 (where g1 and g2 are the gamma correction factors), AS WELL AS: E = (g1+g2)mc^2

I'm currently trying to handle the jiggery pokery of it all, it involves assumptions about equal speeds etc (NOT equal velocities) I'll hopefully have an answer for you soonish.

but just to confirm, the answer they have given is the numerical equivilent of mc^2 (where m= mass of electron)??

But at this level we're only after the theoretical minimum rest mass energy. In practice as you say the products will have to have some kinetic energy. And yes, the answer seems to be (m_e)c^2.

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