# Magnetic field

1. Jun 8, 2010

### thereddevils

1. The problem statement, all variables and given/known data

An ion of charge 1.6 x 10^(-19) C and Kinetic energy 2.0k eV enters perpendicularly into a uniform magnetic field .The ion performs a circular path of radius 4.3 cm .Determine the mass of the ion if the magnetic field intensity is 0.5 T.

2. Relevant equations

3. The attempt at a solution

1/2 mv^2 = 2.0 x 10^3

mv^2 = 4.0 x 10^3 ---1

(mv^2)/r=Bev

mv = (0.5)(1.6 x 10^(-19))(0.043)

v = [3.44 x 10^(-21) ]/m ---2

Sub 2 into 1 ,

m{[3.44 x 10^(-21) ]/m }=4.0 x 10^3

Solve for m , i got a very small figure which is wrong .

THe correct answer is 1.84 x 10^(-26) kg

where did i go wrong ?

2. Jun 8, 2010

### tiny-tim

hi thereddevils!

don't you have to convert electron-volts into something else?

3. Jun 9, 2010

### thereddevils

hi tiny tim ,

what do i hv to convert that too ? I thought electron volt is joules which means enerygy ?

4. Jun 9, 2010

### tiny-tim

hi thereddevils!

no, a joule is a coulomb volt, not an electron volt …

it's the energy if you move something with a coulomb of charge through one volt …

an electron volt is the energy if you move something with the charge of an electron through one volt …

joules volts coulombs and so on are SI units, but the electron volt isn't (se the PF Library on electric units ) …

an electron has 1.602 10-19 coulombs of charge,

so 1 eV = 1.602 10-19 joules

5. Jun 9, 2010

thanks !