Mass spectrometer
Why do atoms of the same element land at different spots on the plate?
Ions are first accelerated through a voltage U, then enter a uniform magnetic field at right angles, travel a semicircle and strike a photographic plate. For equal charge, the orbit radius is proportional to the square root of the mass, so different isotopes of the same element land at different spots. This is how a mass spectrometer "weighs" atoms.
Orbit radius vs. accelerating voltage
Radius is proportional to √U; the greater the mass, the larger the radius.
Think about it
After entering the magnetic field, which has the larger orbit radius: neon-20 or neon-22?
Neon-22. r = √(2mU/q)/B, so the greater the mass, the larger the radius; the ratio of the two radii is √(22/20) ≈ 1.05, which puts them a few millimeters apart on the plate.
Model assumptionsIons carry one elementary charge and start from rest · Ion mass = mass number × atomic mass unit
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.