EXAMPLE 23.6 | The electric field inside a capacitor Two 1.0 cm X 2.0 cm rectangular electrodes are 1.0 mm apart. The positive plate must be charged to +3.5 nC and the negative What charge must be placed on each electrode to create a uniform plate to -3.5 nC. In practice, the plates are charged by using a electric field of strength 2.0 X 10 N/C? How many electrons must battery to move electrons from one plate to the other. The number be moved from one electrode to the other to accomplish this? of electrons in 3.5 nC is MODEL The electrodes can be modeled as an ideal parallel-plate 3.5 X 10 9 C capacitor because the spacing between them is much smaller than N = = 2.2 X 10 electrons e 1.60 X 10 19 C/electron their lateral dimensions. Thus 2.2 X 10" electrons are moved from one electrode to the other. SOLVE The electric field strength inside the capacitor is E = O/EA. Note that the capacitor as a whole has no net charge. Thus the charge to produce a field of strength E is ASSESS The plate spacing does not enter the result. As long as the Q = (8.85 X 10 12 C2/Nm2) (2.0 X 10-4 m?) (2.0 X 106 N/C) spacing is much smaller than the plate dimensions, as is true in this = 3.5 X 10 9 C = 3.5 nC example, the field is independent of the spacing