How Hammering A Crystal Creates Electricity: Piezoelectric effect

Imagine a crystal that can create electricity just by pressing on it. That is what happens with crystals like quartz, but how? This is called the piezoelectric effect and to understand it we must look into crystals at an atomic level.

How was the piezoelectric effect discovered?

In the mid-18th century, scientists had already discovered that heating or cooling certain materials resulted in an electric potential. To explain this more fundamentally, scientists theorized a relationship between mechanical force and electric charge. However, it wasn't until 1880 that Pierre and Jacques Curie demonstrated the piezoelectric effect: they were able to show that hitting certain crystals or applying pressure to them caused them to develop a potential difference. Upon further investigation, it was determined that the piezoelectric effect also works in reverse: the crystal could change shape when a potential difference was applied across it.

Why does this happen?

To understand why this happens, let's focus on crystalline quartz, one of the first piezoelectric crystals to be discovered. Quartz is made up of a lattice of silicon and oxygen, where each silicon atom is bonded to four oxygen atoms and each oxygen atom is shared between two silicon atoms, giving the formula SiO₂. Oxygen is significantly more electronegative than silicon, so it holds a partial negative charge, which induces a partial positive charge in the silicon atoms.When pressure is applied to the crystal, its atoms are compressed closer together. Due to the way in which the atoms in quartz are arranged, this causes the positively charged silicon atoms to move one way and the negatively charged oxygen atoms the other way. The shift in their atomic positions causes opposite charges to build up on opposite faces of the crystal, creating a potential difference across it.

Which crystals are piezoelectric?

A material can be piezoelectric as long as it meets two main requirements:

• The bonds in the material must be polar so that there can be an 'imbalance' of charges when the material is squeezed.

• The crystals must not have a centre of symmetry (point symmetry). Simply put, a positively charged atom must be opposite a negatively charged atom across the centre. If the charges within the line of action are the same, there will be no change in the net charge when force is applied to the material.

  
  

What are the uses of the piezoelectric effect?

One of its primary uses is in creating ultrasound. A piezoelectric crystal can be connected to an alternating voltage supply, causing it to vibrate at the frequency of the alternating voltage. This frequency can be adjusted to produce high-frequency ultrasound. Quartz can also be used in watches, where the crystal can be made to vibrate using small currents from a battery at a precise 32,768 times per second. This makes quartz useful in making accurately timed watches that don't need to be 'wound'.