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# edexcel PHY6 April 05 (speciman) paper - Help Needed! watch

1. I have a little problem with one of the questions in this paper and the mark scheme isn't helping at all!

the question is:

(f) (i) An electron in a certain betatron gains 180eV while travelling in one orbit of radius 0.85m.
How far does it travel in reaching 20MeV? (2)
(ii) When an electron is accelerated from rest to 20MeV, its kinetic energy is increased to 3.2 x 10 -12 J. What increase in mass does this represent? Hence show that the masses of 20 MeV electrons become 'many times greater than their rest mass'. (5)

This is Q1 in the paper and yeah, i don't understand what the markscheme is saying so if anyone can help...
2. you just have to find the ratio between 180eV and 20MeV, this should turn out as 1.1x10^5

because the radius of the betatron is 0.85, the circumference is pi x (2x0.85)

this means that in 1.7pi metres, it gains 180eV
you multiply the constant of 1.1x10^5 by 1.7pi to find the distance needed for it to gain 20MeV!

hope it helps
3. part 2,
dE=dmc^2
dm=dE/c^2
3.2x10^-12/(3x10^8)^2
= 3.55x10^-29 kg increase

Ok,
restmass of electron = 9.11x10^-31kg
new mass = rest mass + increase in mass = 3.64x10^-29kg

and then find the percentage difference = 2.5%
4. Thanks!

That has helped.. i understand it a little more than before.. but physics just eludes me!

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