Piezoelectricity - why hitting crystals makes electricity

when you press down on the trigger of barbecue lighter you'll notice it's quite hard work like it's not like pressing the button on the remote control you actually have to put some effort into it and there's a reason for that when you press down on the trigger you're working against quite a stiff spring so that all that work you do is stored as potential energy in the compressed spring and in front of that spring is a tiny hammer so that when the mechanism finally gives way and the spring is released or that potential energy stored in the spring is turned into kinetic energy in the hammer the hammer then strikes a special kind 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 and the hammer and the crystal these two wires are coming off the crystal and they're drawing that voltage up to the top of the lighter here there's still a gap between the two wires at the top here but the voltage that you produce when you press down on the piezoelectric crystal is so high that electricity can actually jump across that gap producing a spark if you send a flammable gas up to the top as well then the spark will ignite the gas for a while now I've wanted to understand how the piezoelectric effect works on the molecular level and I finally do and I've also built something to make it easy for anyone to understand how it works so that's what this video is about there's a few different types of piezoelectric crystal that you can buy but I'm gonna look at quartz in particular which is the first piezoelectric crystal to ever be discovered this is what a giant quartz crystal looks like it's also a healing crystal according to the website that I bought it from it will align your chakras I don't know if you take it orally or if you were to take a slice of quartz away from this crystal and compress it you'd be able to measure a voltage across the slice but you'd have to slice it at just the right angle like that I don't know if this is the right angle but it'd be something like that and it's got to be just right and to see why you've got to get the angle just right we need to look at the lattice structure of quartz so quartz is made of silicon dioxide so silicon and oxygen and this is what it looks like at first it looks like quite a complicated structure but as you rotate the crystal you find these symmetries like this one and here's another but there's one particular angle that I'm interested in which is this one notice this hexagonal shape here it's actually a spiral going into the screen but for simplicity we're going to think of it as a ring so these three crunchy peanut butter lids represents silicon atoms and these three smooth peanut butter lids represent oxygen so when you compress a crystal of course you're squishing these hexagons but crucially the bond between oxygen and silicon isn't quite even like the oxygen is a bit more aggressive in the way it holds on to its electrons so the oxygens are a little bit negatively charged and the silicones are a little bit positively charged so think about where the average of all the positive charge is in this diagram it's a bit like the center of mass but it's the centre of charge so it's there in the middle between these three silicon atoms but watch what happens when you compress the crystal the two side silicon's move outwards but they don't move vertically and the bottom silicon moves upwards so the average position of these three positive charges moves upwards slightly and similarly think about the average position of the three negative charges look the two side negative charges move outwards but that top negative charge moves downwards so the average of the three negative charges moves downwards so when you compress a crystal of course in this exact orientation you're slightly shifting all the negative charges in one direction and all the positive charges in the other direction suppose you've got 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 charges and positive charges overlap exactly and so you've got this neutral greenish color but when you compress the crystal the negative charges shift in one direction and the positive charges shift in the other direction in the bulk of the crystal it's still neutral overall but at the faces you've got this buildup of positive and negative charge if you then wire up these faces well the positively charged face will be trying to pull electrons towards it from inside the wire and the negatively charged face will be repelling electrons and if you bring the ends of those wires close enough together the electricity will be able to jump the gap producing a spark so that's how quartz is piezoelectric but in general any crystal can be piezoelectric so long as it fulfils a couple of criteria the first one is that the lattice needs to contain some polar bonds that 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 that's why diamond isn't piezoelectric you can squeeze a diamond as much as you like but all the carbon atoms inside are neutral so there's no charge for you to move around the second criteria is more subtle is 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 a look again at this hexagonal arrangement of atoms from the quartz crystal look at this silicon atom and then draw a line through the central 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 you go through the central point that means it doesn't have point symmetry to see why a lack of point symmetry is important look at this arrangement of atoms that does have point symmetry see how silicon atoms are our opposites silicon atoms oxygen atoms are obviously oxygen atoms when you compress the crystal you move the charges trickly so the average position of those charges doesn't change it stays in the middle the piezoelectric effect has a lot of uses besides just a high-voltage power source like an a barbecue lighter it can also be used as a sensor this is a piezoelectric disc it's probably not quartz it'll be something else and if I press down on the disc I can get a bit of a voltage across it it's sensitive enough to be used as a pickup for sound so I can actually use it as a microphone interestingly the piezoelectric effect is reversible actually in a previous video I mentioned how speakers can be used as microphones and microphones can be used as speakers and it's true of this piezoelectric microphone it can be used as a speaker it's when you apply a voltage across a piezoelectric crystal it will deform it's 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's a tiny crystal of course that vibrates exactly 32768 times per second if you know your powers of two then that might sound familiar but why well this video is already getting too long so my next video is going to be about the amazing mechanism inside a quartz watch if you want to make sure you definitely see that video then don't forget to subscribe if you super want to make sure then consider it clicking the notification bill this video is made possible by my patrons on patreon and curiosity stream a subscription streaming service of our two thousand documents and nonfiction titles including some exclusive originals you can get unlimited access to all of that for as little as $2.99 ring calm foolish nasty mold and use the promo code steve mold when you sign up no spaces although a case that you're the sort of people who like to learn through video and you know sometimes you just want someone to curate the videos for you to just say look these are the good ones right what's these ones and thirsty stream is really good for that it was founded by the person who founded the Discovery Channel and it's just a great collection like if you want to learn about gravity there's a few really good documentaries hosted by Jim al-khalili that I recommend if you don't know about Jim al-khalili he's really good like he's really good at explaining things so I recommend those when you first sign up for your 30-day free trial and you know maybe do it today doing it alright that's it I hope you enjoyed this video if you did don't forget to hit subscribe and I'll see you next time [Music]

Channel: Steve Mould

Views: 1,658,584

Rating: 4.9080734 out of 5

Keywords: explained, understand, piezoelectric effect

Id: wcJXA8IqYl8

Length: 9min 34sec (574 seconds)

Published: Thu May 16 2019

Reddit Comments

[u/SouthernCleopatra]

What if you connect the circuit? Does connecting each face of the crystal have any effect on its compression when you hit it?

[u/Zorcron]

Exploration of an application of this phenomenon to timekeeping in this video.