Piezoelectric Effect

 by Bruce Mao

In order to generate a flame, a lighter uses a spark (an electric current) to ignite a flammable gas. (Figure 1) Therefore, by logic, one might think a battery of some sort is needed for the lighter to work since a battery generates an electric current. But in fact, we don’t need a battery. There is a crystal (a solid object) inside the lighter. If you press the crystal, it generates electric current. This phenomenon is called piezoelectric effect (“piezo” comes from the Greek word for “press”) and was discovered by French physicists Jacques and Pierre Curie. [1] In other words, it is the force you put on a crystal that generates the electric current required to ignite the gas! But, how on Earth, does a force cause an electric current?

Figure 1 An animation showing how a lighter works. Image source: https://global.kyocera.com/fcworld/charact/elect/piezo.html

First of all, not every material manifest piezoelectric effect. Only certain types of materials can demonstrate this phenomenon. Quartz is one of them. In order to explain piezoelectric effect, we need to think about the structure of a crystal. Take a quartz crystal as an example. It is composed of an incredibly huge number of small particles called silicon atoms and oxygen atoms. Those atoms have a positively charged nucleus and some negatively charged electrons (Think nucleus as a central ball. Around the nucleus, there are smaller balls called electrons moving around. The nucleus and the electrons together occupy a spherical space called an atom.). Because the nucleus of a silicon atom and the nucleus of an oxygen atom have different abilities to attract the electrons, the electrons are distributed unevenly within the crystal. (Technically, we say that the bonds between silicon atoms and oxygen atoms are polar.) Due to this uneven distribution, inside the quartz crystal, there are areas of negative charge and areas of positive charge. If the crystal just sits somewhere quietly, the average position and amount of the positive charge and the average position and amount of negative charge within the crystal are exactly the same. (Figure 2) Therefore, the positive and negative charges cancel each other out. Overall, the crystal is neutral (i.e., it is uncharged).

Figure 2 The average position of the three positive charges is in the middle. So is the average position of the three negative charges. These two average positions overlap. And since the amounts of positive charge and negative charge are equal, overall, the whole thing is neutral. Image source: https://www.youtube.com/watch?v=wcJXA8IqYl8

However, when one presses the crystal, the force changes the relative position of those atoms a little bit. Because of this, the average positions of both positive charge and negative charge within the crystal changes a little bit and get separated. Hence the charges are no longer cancelled out. One end of the crystal has positive charge, and the other end has negative charges, creating a potential difference so electrical current can be produced. (Figure 3) (Imagine potential difference as a height. If there is a height, water can flow from high to low. This is similar for electric current – If there is a potential difference, it flows from high to low.). So, in short, force deforms the crystal, separating the average positive charge and negative charge within the crystal. Therefore, a potential difference is created, and electricity can be generated. [2]

Figure 3 When the positions of the atoms change, their average position changes too. In this picture, the average position of the three positive charges moves upwards a little bit. Similarly, the average position of the three negative charges moves downwards a little bit. Therefore, the overall effect is that a positively charged end and a negatively charged end are created. Image source: https://www.youtube.com/watch?v=wcJXA8IqYl8

Except a lighter, there are many other applications of piezoelectric effect. For example, in an ultrasound scan, the ultrasound which is essentially many vibrations causes the piezoelectric material to deform and generates a potential difference. This potential difference is used to produce an image of the inside of the body. Also, piezoelectric effect can happen in reverse. When there is a potential difference in some materials, the material deforms. This is how the ultrasound for scanning the body is produced in the first place.

References

[1] Wikipedia (2023), Piezoelectricity [online] Last accessed 20 March 2023: https://en.wikipedia.org/wiki/Piezoelectricity

[2] Steve Mould (2019), Piezoelectricity - why hitting crystals makes electricity [online] Last accessed 21 March 2023: https://www.youtube.com/watch?v=wcJXA8IqYl8