The most widely used transduction principles are electromagnetism, electrostatics and piezoelectricity. Electric motors operate on three different physical principles&colon; magnetism, electrostatics and piezoelectricity. There have been hypotheses of glue, friction, suction, electrostatics, micro-interlocking and intermolecular forces. In electrostatics the effects of magnetism are not present. In classical physics, many important forces follow an inverse-square law, such as gravity or electrostatics. One way that enzymatic catalysis proceeds is by stabilizing the transition state through electrostatics. It is used in physics education to demonstrate the principles of electrostatics. Poisson kernels commonly find applications in control theory and two-dimensional problems in electrostatics. Leyden jars are still used in education to demonstrate the principles of electrostatics. LbL deposition has also been accomplished using hydrogen bonding instead of electrostatics. There are applications where a shock wave, and electrostatics, can result in high velocity projectiles. It is the magnetic analogue of electrostatics, where the charges are stationary. The principles of electrostatics are important when designing items of high-voltage equipment. Original electrets were also typically cheaper and lower in technical capability and fidelity than electrostatics. However, there is often a problem with integrating such a woofer with the electrostatics.