Radius of hellical chain (radius of path of electron)?

[delsoo]

Homework Statement

here's the question, is my concept correct? the ans is 9.86 cm, but my ans is 13 cm, can anyone tell me which part is wrong?

Homework Equations

The Attempt at a Solution

 

[mfb]

For your formula, you have to take the perpendicular component of the velocity only.

 

[delsoo]

For your formula, you have to take the perpendicular component of the velocity only.

perpendicular velocity is v sin tetha, so i sub all the values to the working above, i would get r=(mv)/(Bq sin 60) , where m=9.1x10^-31 , v= 4.0x10^7 ms^-1 B=2.0x10^-3 T , q=1.6X10^-9C , i would get 0.13m but not 9.86 cm... what's wrong with my working?

 

[mfb]

At one side you did use the angle, but I think at the other side you did not.

 

[delsoo]

At one side you did use the angle, but I think at the other side you did not.

what do u mean? can you expalin more detail?