eli5: how/why does water on a touchscreen have the same effect on it as a finger

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eli5: how/why does water on a touchscreen have the same effect on it as a finger

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Most modern touchscreens work by measuring capacitance. That is, the ability to store an electrical charge. When your finger touches the screen it changes the capacitance of the screen at the point that your skin makes contact. Water will also change the capacitance. So, the screen has a hard time differentiating between the two.

There are many types of touchscreens. The most commonly used today for smartphones are capacitive.

They have a grid of capacitance sensors and they basically project an electrostatic field across the display. When you touch it, it detects the difference in voltage in those capacitor and can locate where you touched.

It works because your finger conducts electricity. Same with water.

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.

Most water is not “pure” water. There are impurities in most water that create a net electrical charge. Even if it’s tiny, a phone screen can register that charge

Water on a touchscreen has the same effect as a finger because it can conduct electricity. When you touch the screen with your finger, the electrical current in your body flows through your finger and into the screen. This changes the electric field on the surface of the screen, which the touchscreen uses to register where you are touching it.

Water is also a conductor of electricity, so when you touch the screen with water on your finger, the same thing happens. The water conducts the electrical current from your body into the screen, changing the electric field on the surface of the screen and causing it to register your touch.