What other ways besides using magnetic fields can we use to convert mechanical energy to electrical energy and vice versa?

I have just started to read Electric machinery fundamentals by Stephen Chapman and there is one question in my mind after reading the first paragraph.
What are the other mechanisms, besides using magnetic fields, used in motors and generators for converting energy? I'm asking this because the book uses "Almost all" at the beginning of the sentence, whereas in other places it has used more general terms like "For any given".
P.S. I would appreciate if you explain your answers in detail or provide material for further reading.

• Although low power, piezoelectric comes to mine. That converts mechanical motion directly to electric energy. May 19 at 21:43
• There are also electrostatic generators, and the triboelectric effect. May 19 at 21:52
• Wait, why are you limiting yourself to mechanical energy for generators? I admit it's pretty difficult to think about a motor that isn't mechanical since the purpose of a motor is to move stuff, but a generator's purpose is to generate electricity and not anything inherently mechanical. RTGs, for example use the thermoelectric effect and heat from nuclear decay. No mechanics. There are also muscle wires which use heat (which can come from electric currents or other sources) to make special metal alloys change shape. May 19 at 21:55
• @DKNguyen Well the previous lines are as such: "An electrical machine is a device that can convert either mechanical energy to electrical energy or electrical energy to mechanical energy. When such a device is used to convert mechanical energy to electrical energy, it is called a generator. When it converts electrical energy to mechanical energy, it is called a motor". The sentences are not inclusive definitions, but it got me wondering about the conversion between these two types of energy using something other than magnetic fields. May 19 at 22:14
• Web search for "Kelvin water dropper" - a machine that uses energy from falling water to generate electrical energy by electrostatic means. May 20 at 1:47

The Van der Graaff generator is one example.

Image source: Wikimedia.

1. hollow metallic sphere (with positive charges)
2. electrode connected to the sphere, a brush ensures contact between the electrode and the belt
3. upper roller (Plexiglass)
4. side of the belt with positive charges
5. opposite side of the belt with negative charges
6. lower roller (metal)
7. lower electrode (ground)
8. spherical device with negative charges, used to discharge the main sphere
9. spark produced by the difference of potentials

Unlike most generators, applying voltage to it does not create a motor.

• It's one way in much the same way that most worm-gears are one way, the inefficiency is so high that back-driving doesn't happen in practice. Given low enough belt flexing and pulley friction losses, a VDG will run as a motor, or at least continue to run as a motor once given a direction. May 20 at 4:57
• Magnetic - Generator

• Chemical - battery.

• Pressure - Piezoelectric cell.

• Light - Photoelectric cell.

• Heat - Thermocouple.

• Friction - Static Electricity.

Only the first two are sources of electricity. Next 3 produce a voltage, but not enough to power anything real, mainly sensors. And the last is nature.

Electrostatic machines that are the duals of electromagnetic machines have been constructed. They use electric fields rather than magnetic fields.

Look at my answer to this question

• Seems the electrostatic forces attract COVID spike protein to attach cell walls. Where does chemistry end and electrostatics begin? May 19 at 22:23
• @glen-geek; The electric machine context of the posted question is about continuous conversion between electrical and mechanical energy. In a way that the converted energy is transmitted for use in a general-purpose way. May 19 at 23:07