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Unformatted text preview: Mass spectrometer Damien Martin January 7, 2008 1 Questions: The configuration shown below is called a velocity selector (or Wein filter) be- cause it allows particles of a particular velocity to pass through it. We are going to find the magnetic field that we need to allow a particular mass particle to get through this apparatus. The plates on the left are going to accelerate the particles, and then the wanted particles are going to travel through the horizon- tal plates. The horizontal plates labelled “upper” and “lower” have a voltage difference of 15 Volts and are separated by 15 cm. Part of your job will be to figure out which plate has the higher potential. 2 cm 70 V-30 V L upper lower v i = 0 1. If an α-particle (two neutrons, two protons) is released at rest in the middle of the plates (as shown) what is the final speed of the particle as it leaves the accelerating plates? The mass of a proton or neutron can be taken as approximately 1 . 7 × 10- 27 kg. 2. Draw a force diagram of the wanted particle as it passes through the horizontal plates (hint: if you want it to get through, which direction can you not have a net force?) 1 3. Determine which of the horizontal plates has a higher voltage, and which has a lower voltage. 4. Determine the magnitude of the magnetic field required to allow the α- particle through the detector. 5. Where would the α-particle end up (i.e. where should we put the detec- tor)? 6. How would the path differ for a more massive particle?...
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