The diagram shows a mass spectrometer used for measuring the masses of isotopes. It consists of an ion generator and accelerator, a velocity selector and an ion separator, all in a vacuum.
In one experiment, tin ions, each of which carries a charge +1.6 × 10-19C, are produced in the ion generator and are then accelerated by a p.d. of 20kV. Tin has a number of isotopes, two of which are tin-118 (118Sn) and tin-120 (120Sn).
(a.) In practice, all ions produced by the ion generator have a range of speeds. A velocity selector is used to isolate ions with a single speed. In the velocity selector, the isolated ions go through undeviated, though being acted upon by forces from both the electric and magnetic fields.
(i.) Write down the condition and equation which must be satisfied if the ions are to emerge from the exit hole of the velocity selector. Define the terms in the equation. Redraw the section of the
velocity selector and indicate the different forces acting on the ions there and use it to explain your equation.
(ii.) What magnetic flux density is required if ions travelling with a speed of 17 7kms-1 are to be selected?
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