PHYS-P 150 Lecture Notes - Lecture 34: Touch Id, Capacitor, Touchscreen
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Capacitor: two pieces of conducting material which voltage occurs between: positive charges: accumulate on conductor with higher voltage, negative charges: accumulate on conductor with lower voltage. Capacitance: charge/voltage: depends on area of conductors, also inversely on distance between conductors, closer conductors = more capacitance. Constantly scanning these conductors to see if there is a change in capacitance. Phones have to track many fingers at the same time now-see capacitors below. If your finger is wet the capacitance is diminished and the pattern can"t be recognized: touch id uses capacitance sensor but with a higher resolution. Measures acceleration as suggested by the name: operates based on newton"s second law: force = mass x acceleration. Basically a plate supported by springs, which connect it to a frame. Capacitors measure the position of the plate which is pulled in different directions because of its motion. 3-axis accelerometer operates on the x-y-z axes to determine which way the phone is oriented.
