velocity selector/mass spectrometer

[tym]

Hi guys. I have a question about the velocity selector/mass spectrometer. I did a little research and couldn't find the answer. Hope someone can help. Thank you!!

Here is a pic of the mass spectrometer: (http://www.schoolphysics.co.uk/age16-19/glance/Atomic physics/Mass_spectrometer/index.html) I understand that in region II, v = E/B. In region III, r = mv/qB. I'm not sure how they relate to each other. Is the v fixed after it left region II? Or it's affected by m too in region III?

If in region III, m affects v, and both m and v affect r (based on r = mv/qB), then the relationship between r and m won't be linear any more. Isn't that the point of the mass spectrometer?

Thanks!! Any suggestion will be greatly appreciated.

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[SpaceBuff12]

II, v = E/B. In region III, r = mv/qB. I'm not sure how they relate to each other. Is the v fixed after it left region II? Or it's affected by m too in region III?

If in region III, m affects v, and both m and v affect r (based on r = mv/qB), then the relationship between r and m won't be linear any more. Isn't that the point of the mass spectrometer?

Why do you think that m affects v? The velocity selector (region II) only allows particles of a certain velocity to pass through, i.e. at a given velocity, v, the forces due to the electric field and magnetic field are equal and opposite and thus the ion passes through the slit.

An ion in a magnetic field will experience a force of F = q(v x B). Also remember that the force of an object in a circular motion is equal to Fcircle = mv^2/r. Since the magnetic field is perpendicular to the path of the particle at all times this simplifies to mv/r = qB; or the radius of the circle that the ion will trace is r = mv/qB.

 

[tym]

Why do you think that m affects v? The velocity selector (region II) only allows particles of a certain velocity to pass through, i.e. at a given velocity, v, the forces due to the electric field and magnetic field are equal and opposite and thus the ion passes through the slit.

An ion in a magnetic field will experience a force of F = q(v x B). Also remember that the force of an object in a circular motion is equal to Fcircle = mv^2/r. Since the magnetic field is perpendicular to the path of the particle at all times this simplifies to mv/r = qB; or the radius of the circle that the ion will trace is r = mv/qB.

Thanks SpaceBuff12!! One more question: is the speed in region III equal to the exiting speed from region II? Does the speed stay constant after it left region II?

 

[SpaceBuff12]

Thanks SpaceBuff12!! One more question: is the speed in region III equal to the exiting speed from region II? Does the speed stay constant after it left region II?

Yes, the speed of the particle is constant. A magnetic field cannot increase the speed of a charged particle.

 

[tym]

Yes, the speed of the particle is constant. A magnetic field cannot increase the speed of a charged particle.

Thanks! Thats very clear!