Posted by **cr** on Friday, April 15, 2011 at 11:06am.

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?

Answer in units of T.

*** 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!

- Physics *work shown ~ pls help! -
**bobpursley**, Friday, April 15, 2011 at 11:56am
I wonder if you are limited to two significant digits because of the 5.1cm, and the 6.4KVolts

- Physics *work shown ~ pls help! -
**cr**, Friday, April 15, 2011 at 12:09pm
What answer do you get when you calculate? Am I doing something wrong in my calculations?

- Physics *work shown ~ pls help! -
**Austin**, Thursday, July 21, 2011 at 1:43pm
You have to change the potential difference to potential energy difference, first. Multiply the potential difference by the charge.

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