Calculating Momentum of a Particle: Q&A

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Discussion Overview

The discussion revolves around calculating the momentum of a particle, particularly in the context of an electron being accelerated through a potential difference. Participants explore the relationship between rest energy, total energy, and momentum, as well as the implications of potential energy in these calculations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant proposes using the equation (E^2 - (mc^2)^2)^(1/2) = pc to calculate momentum from rest energy and total energy.
  • Another participant agrees that this equation appears correct and questions the effect of potential difference on total energy.
  • A third participant presents an alternative formulation, suggesting that with c=1, the relationship simplifies to E^2 = p^2 + m^2, and asserts that knowing total energy and mass allows for momentum calculation.
  • A later reply discusses a specific example of a 1 MV potential difference, suggesting it would yield an energy of 1.5 MeV for momentum but expresses uncertainty about the correctness of this assumption.
  • Another participant notes that if all energy is considered kinetic, the proposed formula is valid, but cautions that potential energy must also be accounted for if present.

Areas of Agreement / Disagreement

Participants express varying degrees of agreement on the use of specific equations, but there is no consensus on the implications of potential energy in the calculations or the correctness of the example provided.

Contextual Notes

Participants discuss the dependence of momentum calculations on the definitions of total energy and potential energy, as well as the assumptions made regarding energy types involved in the scenario.

Who May Find This Useful

Readers interested in particle physics, energy-momentum relationships, and the effects of electric potential on particle dynamics may find this discussion relevant.

Stickybees
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Say I have the rest energy(mc^2) and the total energy of a particle (E), would getting the momentum energy of the particle be as simple as doing (E^2-(mc^2)^2)^(1/2) = pc?

And when accelerating a electron through a potential difference how would I work out its momentum, given I have its rest energy and the value for potential difference?

Thanks.
 
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That looks like the correct equation to use.

When an electron is accelerated through a potential difference, what happens to its total energy?
 
c=1 you have
E2= p2+m2

so you want to know the momentum you do what you said

p=√ [E2-m2]

by the time you say "total energy E" it means that the kinetical energy is within your already known parameter, so is there a potential, is there not- you know E, you know mass, so you know its mommentum.
 
I am assuming that for a p.d. of for example 1 MV, this will create an energy difference of 1 MeV for an electron and with the rest energy of around 0.5 MeV for an electron, it would simply equal 1.5 MeV/c for the momentum, but I think this is wrong, but I don't understand why.
 
Assuming all the energy was kinetic, your formula works. If there is also potential energy involved, then you have to take that into account as well.
 

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