Determine the radius of the path, physics homework help

Question Description

I’m stuck on a Physics question and need an explanation.

An electron has a kinetic energy of 5.40  10-17 J. It moves on a circular path that is perpendicular to a uniform magnetic field of magnitude 7.80  10-5 T. Determine the radius of the path? 

answer in m

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Final Answer

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This is the equation to find the radius

r = mv/ QB 

but you are missing v.

E= 5.4 *10^-17 J

B = 7.80 *10^-5

mass of an electron = 9.109 * 10^-31 kg

Q=1.6 *10^-19 

(1 joule = 1 kg*m^2/s^2

So, you should note that E = (m*v^2) / 2  OR v = sqrt(2E/m) 

v = sqrt(2* (5.4 *10^-17 J) / (9.109 * 10^-31 kg) 

once you have v, you plug it back into r = mv/ QB 

r is given in meters

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