Magnetic Field required to seprate to ions

1. Feb 17, 2012

Eng_physicist

Magnetic Field required two seprate to ions

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

Consider a mass spectrometer used to separate the two isotopes of uranium, 238U3+ and 235U3+. Assume the ions enter the magnetic field region with a speed of 5.30X10^5 m/s. What value of B is required to give a separation of 0.6 mm when the ions leave the spectrometer?

2. Relevant equations

L = 6*10^(-4) m and B = ? T v = 5.30*10^5
e = 1.602*10^(-19)
F = qvXB

3. The attempt at a solution

I know that F = qvXB q = 92e since e = 1.602*10^(-19) so F/qv = B ,
q = 92(1.602*10^(-19)), v = 5.30*10^5

I am not sure how to calculate the force

but I get confused because they are both ions they would have a positive charge and want to repel so why should they attract and I do not know where to take it from here.

Last edited: Feb 17, 2012
2. Feb 17, 2012

Mindscrape

Just treat the ions as going through at different times, and get the separation if you could have a switch that shot out one ion or the other.

3. Feb 18, 2012

technician

Do you realise that the ions will be travelling in a circular arc and the force you have calculated is the value of the centripetal force?
In a mass spectrometer the ions travel in a semi circle to reach the detector.
Because of the slight difference in mass the radius of each path will be different ( the centripetal force is the same)
This should enable you to calculate the difference in radius and therefore the separation over the semi circle.
Hope this helps