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In designing a velocity selector that uses uniform perpendicular electric and magnetic fields, you want to select positive ions of charge +5e that are traveling perpendicular to the fields at 8.75 km/s. The magnetic field availabe to you has a magnitude of 0.550 T. (a) What magnitude of electric field do you need? (b) Show how the two fields should be oriented relative to each other and to the velocity of the ions. (c) Will your velocity selector also allow the following ions (having the same velocity as the +5e ions) to pass through undeflected? (i) negative ions of charge -5e? (ii) positive ions of charge different from +5e?

ok totally lost! please help!

  • Physics -

    I assume the velocity selector leaves a certain velocity ion undeflected by having the eE force equal and opposite to the qVB force. Since the E and B fields are perpendicular, the resulting forces can be in the same direction.

    (a) The E field you want is given by
    E = V B

    (b) E and B should both be perpendicular to the V direction as well as each other. In that way, the electric and magnetic forces act to change the beam direction only, and maximum deflection of unwanted velcoities is achieved..

    (c) The size and sign of q does not matter. E and B forces will balance for all q values if the velocity is the value one wishes to select.

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