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The Mass Spectrophotometer And you!! RECALL: • When a charged particle is moving within a magnetic field there is an interaction of the moving charge with the interacting magnetic field: • FB = qo v B • Units: N c m/s T Application • • • • The mass spectrophotometer Used to determine the masses of isotopes Or the mass of a molecule Or the different molecular masses in a sample so you can have insight into the composition of the sample • Or to purify samples (like enrichment of uranium...) How does it work???? • Charged particles (with known charge) are accelerated using a particle accelerator (do you recall those from the last unit of study?) • The charged particles pass into parallel plates with E field of given magnitude Diagram the following: + Detection plate + + + - What direction is the E field? What direction is the force on the charge? Where will the charged particle go? + Detection plate + + + E Fele - So... a B field is used to balance the Fele What direction should the B field be so that it can balance the Fele??????????(think 2nd RHR) + Detection plate Fb + + + F ele - IF YOU ARE GIVEN • The V of the charged particle – then you can calculate the magnitude of the B by setting • Fele = Fb • q E = q v B • But for most of the problems you face – you will be given B and use it to calculate the v So... What path will the charged particle take once it leaves the (balanced) E field region (where only the B field acts on the charge)? + Detection plate + + + - F ele r Fb So now we can calculate the V of the charged particle... • Set Fele qE SO: V = E B = Fb q V B Knowing: E, B, q, r, and calculated v • We can calculate the mass of the charged particle... • Recall your studies of circular motion... • Fc = Fb m v2 r = qvb Hey guys...I have a great idea... Let’s try a sample calculation • An ion with a +2 charge is passed through an E field with a magnitude of 5.1 x 104 N/C, and a B field with a magnitude of .75 T. The measured r of the ion’s trajectory is 15.2 cm. What is the mass of the ion??? Just remember that: • You can solve by setting Fc = FB • But you will have 2 unknown variables (m and v) • So solve for v first by setting Fele = FB DID you find: • V = 6.8 x 104 m/s ????????????? • And m = 5.37 x 10 -25 kg ??????? • GOOD JOB!!!!!!!!!!!!!!!! • This type of problem might just be on the test – • Just saying.......................................................................................... ..................................................................................................... .................................................................................................... So... What path will the charged particle take if it is more massive than the first particle? • Larger r !!! + Detection plate + + + + - F ele Fb