Piezoelectricity - why hitting crystals makes electricity

When you press the trigger on a barbecue lighter you will notice that it is quite hard work as it is not like pressing the button on a remote control, you actually have to put a bit of effort into it and there is a reason for that when you press the trigger. The trigger is working against a fairly stiff spring, so all the work it does is stored as potential energy in the compressed spring and in front of that spring is a small hammer so that when the mechanism finally gives way and the spring is released. All that potential energy stored in the spring is converted into kinetic energy in the hammer, then the hammer hits a special type of crystal called a piezoelectric crystal and when you hit one of these things it generates a voltage across the crystal.

This is the inside of a barbecue lighter this is the mechanism that houses the spring, the hammer and the glass these two wires come out of the glass and carry that voltage to the top of the lighter here there is still a space between the two wires on the top here, but the voltage produced when the piezoelectric crystal is pressed is so high that

electricity

can jump across that gap producing a spark. If a flammable gas is also sent to the top, the spark will ignite the gas for a time. Although now I wanted to understand how the piezoelectric effect works at a molecular level and I finally do, and I have also created something so that anyone can understand how it works, which is what this video is about, there are some differences. types of piezoelectric

crystals

you can buy, but I'm going to look at quartz in particular, which is the first piezoelectric crystal ever discovered.

This is what a giant quartz crystal looks like if you removed a slice of quartz from it. glass and compress it, you could measure a voltage across the cut, but you would have to cut it at the right angle like this. I don't know if this is the right angle, but it would be something like this. but it has to be perfect and to see why you have to get the perfect angle, we have to look at the lattice structure of quartz, so quartz is made of silicon dioxide, so it's silicon and oxygen, and that's how it's made. go.

At first it seems like a pretty complicated structure, but as you rotate the crystal you find these symmetries like this and here's another one, but there's one angle in particular that interests me, which is this one. Notice this hexagonal shape here, it's actually a spiral. on the screen, but for simplicity let's think of it as a ring, so these three crunchy peanut butter caps represent silicon atoms and these three soft peanut butter caps represent oxygen, so when you compress a crystal of quartz you are crushing these hexagons. but the most important thing is that the bond between the oxygen and the silicon is not as uniform as if the oxygen were a little more aggressive in the way it holds its electrons, so the oxygens are a little negatively charged and the silicons are a little bit positively charged, so think about where the average of all the positive charge is in this diagram, it's a little bit like the center of mass, but it's the center of charge, so it's in the middle between these three atoms of silicon, but notice what happens when you compress the glass on the two side silicons. they move outwards but they don't move vertically and the bottom silicon moves up so the average position of these three positive charges moves slightly up and similarly think about the average position of the three negative charges.

Look at the two side negative charges move outward but that top. the negative charge moves down, so the average of the three negative charges moves down, so when you compress a crystal, of course, in this exact orientation, you are slightly displacing all the negative charges in one direction and all the positive charges in the other direction, suppose we have a square slice of quartz crystal, let's represent all the positive charge as gold and all the negative charge as red and in an uncompressed crystal those negative and positive charges overlap exactly and then you have this neutral greenish color, but when you compress the glass, the negative charges move in one direction and the positive charge moves in the other direction.

For the most part the crystal is still neutral overall, but on the faces you have this buildup of positive and negative charge if you then connect them. faces well, the positively charged face will try to attract electrons to it from the inside of the wire and the negatively charged face will repel the electrons and if you bring the ends of those wires close enough the

electricity

will be able to jump the gap producing a spark, this is how the Quartz is piezoelectric, but in general any crystal can be piezoelectric as long as it meets a couple of criteria, the first is that the lattice must contain some polar bonds, which just means that some of the atoms end up with a slight positive charge and some of the atoms end up with a slight negative charge like silicon and oxygen in quartz, which is why diamond is not piezoelectric.

You can squeeze a diamond as much as you want, but all the carbon atoms inside are neutral, so there is no charge. To move, the second criterion is more subtle and has to do with symmetry. The crystal needs to have a certain type of symmetry or, more accurately, a lack of a certain type of symmetry. Take another look at this hexagonal arrangement of atoms in the quartz crystal. Look at this silicon atom and then draw a line through the center point to the opposite side and see what's there: it's not another silicon atom, it's an oxygen atom, so there's something different on the opposite side when passes through the center point, that means it does not have point symmetry to see why the lack of point symmetry is important.

Look at this arrangement of atoms that does have point symmetry. See how silicon atoms are opposite to silicon atoms. Oxygen atoms are opposite oxygen atoms. When you compress the crystal, you move the charges symmetrically. so the average position of those charges does not change it remains in the middle the piezoelectric effect has many uses besides just a high voltage power source like a barbecue lighter it can also be used as a sensor this is a piezoelectric disk it is probably not quartz, it's going to be something else and if I press the dial I can get a little bit of voltage through it, it's sensitive enough to use as a sound pickup so I can use it as a microphone, isn't that great. microphone but it is a microphone, curiously, the piezoelectric effect is reversible.

In fact, in a previous video I mentioned how speakers can be used as microphones and microphones can be used as speakers and it is true that this piezoelectric microphone can be used as a speaker, it is not a great speaker but it is a speaker when you apply a voltage through a piezoelectric crystal it will deform is the opposite of the piezoelectric effect. My absolute favorite example of the use of the piezoelectric effect is in the quartz watch inside one of these watches there is a small quartz crystal that vibrates exactly 32,768 times per second, if you know your powers of two then this may sound familiar, but hey, this video is already getting too long, so my next video will be about the amazing mechanism inside a quartz watch.

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