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Q.

An electron and a proton are detected in a cosmic ray experiment, the first with kinetic energy 10 keV and the second with 100 keV. Which is faster, the electron or the proton? Obtain the ratio of their speeds.
(electron mass =9.11×10–31kg, proton mass = 1.67 10 kg-27 , 1eV=1.60×10 J -19)

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a

13.53 
 

b

3.53 
 

c

3.5 
 

d

14

answer is A.

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Detailed Solution

Given,

Mass of electrons, me=9.11×10-31kg

Mass of proton, mp=1.67×10-27kg

The kinetic energy of the electron is,

Ee=10keV=104eV

Ee=104×1.6×10-19=1.60×10-15J

The kinetic energy of the proton is,

Ep=100keV=105eV=1.60×10-14J

The relationship between an electron's kinetic energy and velocity, ve, is as follows:

Ee=12mve2

ve=2×1.6×10-159.11×10-31=5.93×107m/s

The relationship between a proton's kinetic energy and velocity, vp, is as follows:

Ep=12mvp2

vp=4.38×106m/s

Electrons move more quickly than protons because their velocities are greater.

Thus the ratio,

vevp=5.93×1074.38×106=13.531

Hence the correct answer is 13.53.

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