# Homework Help: Mass spectrometer conceptual question

1. Oct 10, 2009

### clairez93

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

It's not really a book problem, just a question I encountered in my reading.
For a mass spectrometer, if r = mv / qb, then r is directly proportional to mass and inversely proportional to charge. However, my reading says that the higher the mass, the less deflection. I think I may be confusing deflection with r, then. What is deflection? Why is it different from r? I had formerly thought the amount of deflection referred to r.

2. Relevant equations

3. The attempt at a solution

2. Oct 10, 2009

### Delphi51

Yes but r proportional to m means less deflection with higher m.
An infinite mass would have infinite radius, meaning it goes in a straight line and has no deflection. A small mass will be deflected a lot into circular motion with small r.

3. Oct 10, 2009

### clairez93

Yes but if r is directly proportional to m, if m goes up, then r must go up, according to the equation, no? And if q goes down, then r goes up? Am I making a conceptual mistake here?

4. Oct 10, 2009

### clairez93

Unless r is not deflection, in that case, what is the difference?

5. Oct 10, 2009

### Delphi51

You have MORE deflection when r gets SMALLER.
Zero deflection is when it keeps going in a straight line with infinite r.
I guess r could be called the inverse of deflection.