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I have a bit of trouble understanding the electrical laws which apply when I touch a capacitive touchscreen with my finger.

Apparently, there is an electric current running through electrodes below the glass screen. When my finger is close to the glass, it modifies the electrical field below the glass and the phone can detect the exact position of the modification.

First, why do my fingers modify the electrical field? Is it because my electrical potential will always be different than the one on the phone? If so, why?

Does my finger really pick up charges from the phone? If so, there should be a closed-circuit for the current to flow. But as I understand it, the capacitor is made of : my body (1st terminal), the insulator (the glass), and the electrodes below the screen (2nd terminal). As the first terminal is connected the earth, a current can flow. But what if I do not touch earth?

Finally, can a capacitive stylus work without being held by a human hand?

Thanks

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Why do my fingers modify the electrical field?

Because they consist of:

  1. A conductive part,
  2. An insulating part that has the dielectric coefficient different than air.

Either of those two conditions would be sufficient by themselves to change the capacitance measured between the electrodes of the capacitive touch screen.

The electrical field is modified because the charges in the finger change position -- they get displaced via induction. The electrostatic field is a consequence of positions and quantities of all charges, including those in the finger.

Does my finger really pick up charges from the phone

It doesn't need to, and it doesn't. Your finger is at all times insulated from the capacitive plates of the touchscreen by a thin layer of glass. So no charge can really flow through such an insulator. But the charge certainly can be redistributed within the glass, and within your finger.

Your finger acts as both:

  1. An additional solid dielectric for the capacitor - that's the dead, dry top layer of skin.

  2. An additional conductive plate that for the capacitor.

The dielectric as well as the plate change their embedded charge distribution due to electric induction from the screen. No charge flow needs to be involved for that. The electrostatic induction works just fine through vacuum (as does electromagnetic induction, of course).

Finally, can a capacitive stylus work without being held by a human hand?

Of course. All you need is a conductive plate. You can cut a little square of aluminum foil and put it on the screen. It'll be detected as a touch-and-hold event.

How do capacitive touchscreens really work?

According to what it says on the box, er, according to their name. They really are sensitive, 2D capacitance meters. It's not hyperbole nor misdirection (although, tangentially, it's very good that you question everything).

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