eli5: how/why does water on a touchscreen have the same effect on it as a finger
In: 8
Modern touch screens are capacitive. The screen js actually many little capacitors, and when your finger touches it, the capacitors where you touch it increase capacitance, and therefore are able to hold more charge. This causes the capacitors to draw current, and the device uses that current to tell where on your screen you are touching.
Capacitance can be measured by C=εA/d, C is capacitance, ε is the electrical permittivity (we will explain this) A is the area of one side of the capacitor, d is the distance between the two sides.
Capacitance is also C=q/V, q is charge, V is voltage applied to the capacitors, so if we change C, V is constant, the charge changes, a change in charge means current.
ε is what we change. ε0 is a constant used in a lot of electrostatic equations, which is the electrical permittivity of free space, but when something that isn’t a vacuum is between the two halves of the capacitor, it’s not free space. This is called a dielectric, and the properties of the material determine the value of ε. Your finger, or the water, acts as the dielectric, changing ε, changing C, changing q, changing I (current), which we measure, and use that to know you’re touching the screen.
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