A mass spectrometer (see the figure) is designed to measure the mass m of the 238U+ ion. A source of 238U+ ions (not shown) sends ions into the device with negligibly small initial kinetic energies. The ions pass between parallel accelerating plates and then through a velocity selector designed to allow only ions moving at speed v to pass straight through. The ions that emerge from the velocity selector move in a semicircle of diameter D in a uniform magnetic field of magnitude B, which is the same as the magnetic field in the velocity selector. (Express your answers in terms of quantities given in the problems and universal constants as necessary.)
The accelerating plates have area A and are a distance d apart.
(a) What should the charges on the plates be so the ions emerge at speed v, ignoring their initial kinetic energies? Indicate which plate is positive and which negative.
(b) Sketch the electric field lines between the plates.

Accelerating plates Plate areaA -d Velocity selector Ions Detector