TBR CBT 7 Question

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Joker88

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28.Assuming Theory 1 to be correct, the energy of a single photon could be determined from:

A. ∆msystemc2
B. ∆msystemc2 +½(∆mve+2 + ∆mve-2 - ∆mvη2)
C. ∆msystemc2 + ½(mve+2 + mve-2 - mvη2)

C is the best answer. There are three types of energy to consider in this nuclear reaction: the kinetic energies of the particles, the energy of any photons, and the energy associated with mass change. If Theory 1 were correct, then the energy of a single photon could be found by taking the initial energy of the system and subtracting the final energy of the system (energy of the neutrino and any energy gained by mass loss), not including the photon. The electron and positron both became energy, so all of their kinetic energy must be a part of the final system. In other words, we do not have to consider ∆mass for the electron, proton, or neutron, given that they are present on only one side of the reaction. The relationship is as follows:

KEe++ KEe- = Ehƒ + KEη - ∆msystemc2

½mve+2 + ½mve-2 = hνγ + ½mvη2 - ∆msystemc2

½mve+2 + ½mve-2 - ½mvη2 + ∆msystemc2 = hνγ

½(mve+2 + mve-2 - mvη2) + ∆msystemc2 = hνγ

which becomes:

hνγ = ∆msystemc2 + ½(mve+2 + mve-2 - mvη2)


The answer is C. But I'm confused to why the equation E=MC^2 is added to that side of the equation or included in the formula at all. Its question 28 on TBR 7 btw.
 
Hope this helps. I am honestly not all that fantastic with this portion of the MCAT, but here is what I have based on what TPR gave as an explanation:

TPR say that the mass defect must also be accounted for. Remember, matter can never be created or destroyed, but it can be transformed. In other words, the mass that is lost in the reaction doesn't disappear, it is instead transformed into energy. That is where the E=mc^2 comes in, because the mass defect ("m" in the equation, aka the change in mass) and the speed of light can be used to calculate the energy that comes from the change in mass.
 
I had a question on this too... I don't understand why mc^2 is subtracted on the final energy side. Does anybody know?
 
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