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Original title: Physics behind "Eating rock candy will get an electric shock"

Grandma Wu, who doesn't brush the question, showed us the saying that "eating rock candy will get an electric shock" by experiment. The first thing I thought of was piezoelectric effect: when external stress is applied to the material, it will generate voltage. There are many types of piezoelectric materials, but quartz was the first piezoelectric crystal to be discovered, and we will use quartz as an example to describe how these materials exhibit this unique property.

Cut out a quartz crystal and compress it, and we can measure the voltage across the slice, but only if we cut it at the right angle. To understand the importance of cut angles, we need to look at the crystal lattice structure of quartz. At first the structure of quartz looks very complex, but when we rotate it to the right angle, we find the familiar hexagonal shape.

Microstructure First we need to understand a concept called electronegativity. Quartz crystals consist of oxygen and silicon atoms, where the oxygen and silicon atoms share an electron. Because oxygen atoms have a smaller size and share electrons closer to the nucleus of oxygen, oxygen forces electrons more than silicon does. Oxygen is therefore more electronegative than silicon and carries a slight negative charge, which also means that silicon atoms will carry a slight positive charge. Oxygen and silicon atoms together form dipoles in quartz crystals.

These dipoles are connected end-to-end, and they form the hexagon mentioned above. Although this hexagon is not a positive hexagon, it is arranged in such a way that the charge centers of the negative and positive atoms coincide. If we use red to represent negative charge and yellow to represent positive charge, then in the uncompressed crystal red and yellow overlap completely to form neutral green. If we exert a compressive force on the molecule, the charge centers no longer coincide. This is called polarization, which also means that an electric field will be created between the net positive and negative charges, resulting in a small voltage across it, just like a small battery. If we put pressure on the whole quartz, there will be many small cells connected in series and parallel, outputting higher voltages.

In principle, any crystal can be a piezoelectric material, as long as certain conditions are satisfied. The first is that the lattice requires some polar bonds, which means that some atoms carry a slight positive charge and others a slight negative charge. That's why diamonds aren't piezoelectric. All the carbon atoms in them are neutral. The second criterion is more subtle and requires the absence of some type of symmetry, otherwise the average position of the charge will not change when compressed, for example, hexagonal diagonal in quartz are different kinds of atoms.

Piezoelectric effect is widely used in practical life, and the most common one belongs to lighters. When we press the lighter, we press the piezoelectric ceramic inside. It can generate high voltage of kV, after connecting through wires, it can break through air under very small gaps, so that the gas from the lighter can be ignited. In addition, there are also some microphones that use the piezoelectric effect to convert sound vibrations into electrical signals, which are called piezoelectric microphones. At a more advanced level, some devices implanted in the human body use the piezoelectric effect as their energy supply.

Interestingly, the piezoelectric effect is also reversible; its compression produces a voltage, and if a voltage is applied, it compresses or expands. The classic example of this principle is a quartz watch, in which a quartz crystal vibrates 32768 times per second when powered.

In some small devices, because the space is not large enough, can not be installed on the rotating fan for heat dissipation, then a kind of called piezoelectric fan will come in handy. When we apply alternating voltages to it, the piezoelectric material in it vibrates, causing the flakes on it to fan, just like our hand fans.

This article comes from Weixin Official Accounts: Vientiane Experience (ID: UR4351), by Eugene Wang

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