# Physics ~work shown pls help!

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An electron is accelerated by a 6.4 kV potential difference. The charge on an electron is 1.60218 × 10^−19 C and its mass is 9.10939 × 10^−31 kg. How strong a magnetic field must be experienced by the electron if its path is a circle of radius 5.1 cm?

*** I tried using B=(1/r)Sqrt(2mV/q^2) and I'm not sure what I am doing wrong.

centripetal force= magnetic force
m v^2/r=Bqv

B= m/q * v/r

you have to change the potential difference to velocity first
6.4E3*q=1/2 m v^2

or v=sqrt 2*6.4E3*2/m

1/r * sqrt (m*2*6.4E3/q^2)

What am I doing wrong? Sqrt (9.10939x10^-31)x(2)x(6400)/(1.60218x10^-19)sqrd =673966.5256

ans * (1/.051) = 13215029.91 Wrong answer. I have entered four wrong answers. I can not enter another wrong answer and pass this homework. pls help!

What answer do you get when you calculate? Am I doing something wrong in my calculations?

• Physics ~work shown pls help! -

I get 1.3E7 m/s

• Physics ~work shown pls help! -

According to my homework this is incorrect too. What could be wrong.