[goebki]

Hi,

i have to calculate the molar magnt. suscep. of copper (the result is given, chi= -72,3E-12 m³ mol^-1), that means i have to find the approach to this solution.
This is the exercise: A diamagnetic copper bar with a cross section of 1 cm^-2 is at 298K in air in a gouy balance. After switching on the magnetic field, one measures a force of 3,95E^-4 N, if this magnetic field has a strength of H = 8,00E5 Am^-1 on one end and H = 8,00E3 Am^-1 at the other end. What is the molar magnetic susceptibility of copper, if the molar magnetic susceptibility of Nitrogen -151E-12 m³ mol^-1 and of Oxygen 42,9E-9 m³ mol^-1 is. The composition of air is 79 Vol% Nitrogen and 21 Vol% Oxygen. The density of copper is 8,96 g cm^-3.
In the attachment you can see my solution, the formula of this gouy balance is well known.
My problem is, i don´t know how to calculate the susceptibility of air (Chi (a)). This term must be negative to get the given result (which is a negative term). Can i neglect the paramagnetiv influence of oxygen?

It would be great, if you could give me some hints, to get the result (which is already given).

thanks, goebki