physics question

im tryin to do this question
two protons are accererated towards each other calculate the total kinetic energy that the accelerated protons need so that they will touch

radius of a proton= 1.5*10^-15
charge proton=1.6*10^-19

i used f=kQ^2/r^2

but mark scheme says its kQ^2/r
i dont undestand why its not r^2?????????/
thanks hope my waffle makes sence

Reply 1

VCVT17

2

I think they use the electric field strength to calculate the electric potential of 2 charges ( protons) to find out the Energy = Electric field strength x distance between them.
Is it right?
I hope so....

Reply 2

habosh

13

sarah12345

im tryin to do this question
two protons are accererated towards each other calculate the total kinetic energy that the accelerated protons need so that they will touch

radius of a proton= 1.5*10^-15
charge proton=1.6*10^-19

i used f=kQ^2/r^2

but mark scheme says its kQ^2/r
i dont undestand why its not r^2?????????/
thanks hope my waffle makes sence

w=fx
W=Eqr
W is the work done which is also the kE
so Eq=Force nowso EqQ/r^2=Eq
so q is canceled
now as I said earlier W=Eqr so both sides are like multiplied by r for example so its
kQr/r^2
so one r is canceled and it becomes KQ/r

Reply 3

sarah12345

OP

2

ok thanks i kinda get it but dont understand how u did this bit

W is the work done which is also the kE
so Eq=Force nowso EqQ/r^2=Eq

?????

Reply 4

habosh

13

sarah12345

ok thanks i kinda get it but dont understand how u did this bit

W is the work done which is also the kE
so Eq=Force nowso EqQ/r^2=Eq

?????

now when a body moves ,it actully does work right??
like for example the powersupply in a circuit does a wok on charges so they move with a specific kinetic energy equal to that of the work done by the powersupply
now the work done moving the 2 protons to each other equals the kinetic energy as you have to do work against the potential energy between then as they repel (2 positive charges)
now you know that PE changes to KE and if you noticed to make is easier
Work done=Fx
F=ma
when pe of earth itnequals mgh notice Mg=Ma so it';s actully mah which is fh which is fx which is also KE ... :wink:

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