A parallel-plate capacitor of plate separation 2.0 mm and plate area 15 cm2 is charged to a potential difference of 120 V and then disconnected from the battery.
Inside the metal plates, conduction electrons still move at random thermal speeds on the order of 105ms−1. However, ammeters placed in the leads read zero once the battery is removed.
(a) Describe how the macroscopic electric field and surface charge distribution on the plates differ between the moment just after disconnection and several minutes later, after thermal equilibrium has been re-established.
(b) Explain in detail why the ammeters read zero current even though every conduction electron in the plates continues to move at high thermal speeds.
(c) Identify the physical quantity that must remain constant for net current to be zero and justify your answer by referring to the definition of electric current.
(d) The total charge on one plate is Q=6.4×10−9C. Estimate the numerical value of the average drift velocity that would be required for a steady current of 1.0 mA to flow through a lead of cross-sectional area 1.0mm2, assuming a free-electron density n=8.5×1028m−3. Compare this drift velocity with the thermal speed given above and discuss why thermal motion alone cannot account for macroscopic current.