There are six ways to create voltage. Some methods are more widely used than others, and some are used just for specific applications.

1. FRICTION — Voltage produced by rubbing certain materials together, like when you rub a balloon on your sweater. This was the first kind of electricity humans recognized. A Van de Graaff generator can produce millions of volts with friction. Friction is not a very practical way to generate useful voltage. Right after the ZAP the voltage disappears.

2. PRESSURE (piezoelectricity) — Voltage produced by squeezing crystals of certain substances like quartz or tourmaline. Pronounced “pee-ay’-zoe”. The microphones in mobile phones are sensitive to sound pressure because they are made with a piezoelectric alloy called $PZT$, or lead zirconate titanate.

3. HEAT (thermoelectricity) — Voltage is produced by heating the junction between two different metals. This device is called a thermistor and it is a common way to measure temperature.

4. LIGHT (photoelectricity) — Voltage is produced when light hits a photosensitive (light sensitive) substance. Semiconductor materials like silicon are light sensitive. A photodiode is in a package with a window on top to allow light to hit the diode. The amount of light changes the conductivity of the diode.

5. CHEMICAL — Voltage can be created by a chemical reaction. We call this a battery. Chemcial action generates the voltages inside your nerve cells.

6. MAGNETISM — Voltage produced when the wire moves through a magnetic field, or a magnetic field moves past a conductor. This is how AC electricity is created in large-scale power plants, hydroelectric dams, and portable/emergency generators.

Reference: Navy Electricity and Electronics Training Series, NEETS, module 1, page 1-28.