Resonant energy transfer

Willem Beeker
Two important concepts in physics are energy and forces. Therefore, to understand the world around us we need a thorough understanding of these concepts and how they interact with each other. This article will focus on a system with two balanced forces and a third perturbing force. This is the area in physics that deals with oscillators and resonances that can be commonly found in the world around us. New technologies are being developed in this area are, for instance, wireless power transfer, but more fundamental research also takes place, for instance, the search for metamaterials. The concepts of energy, force, and resonance will be explored in this article. First I will illustrate the concept of a force and energy transfer by discussing a couple of example systems.

A force is needed to change the amount of energy of a certain object. The two forces most encountered in "real life" are gravity and the electromagnetic (EM) force. Once a single force acts upon an object, the energy of the object will either increase or decrease with time. When two forces act upon an object in exactly opposite directions, the object's energy neither increase nor decreases. This equilibrium is either stable or unstable, which becomes apparent after the application of a third force that disturbs the equilibrium (this is often called a perturbing force). If the system was in a stable equilibrium, it will return to its original state once the disrupting force ceases to act. On the other hand, if the system does not return to its equilibrium state, the system was unstable and very small perturbing forces are sufficient to cause it to leave the equilibrium state. A simple example is a sling with solid ropes, where two forces are exactly canceled at both the lowest point (towards the ground) and the highest point. From experience, one already knows that the highest point is unstable. Once disturbed, the sling will move downwards from this unstable position without returning.

If the perturbed system is located near a stable equilibrium point and not heavily damped, the object will oscillate with a certain frequency, called the resonance frequency, around the equilibrium point. If the perturbing force is periodic, then the nature of the system response can be categorized by comparing the frequency of the force with the natural frequency of the system. There are three categories: below resonance, at resonance, and above the resonance frequency. Driving systems on resonance is of interest because energy can be transferred efficiently from the driving force to the system.

The use of a system driven at the resonance frequency often has the purpose of maximizing energy transfer. For example, this is useful in broadcasting technology, where the amount of power received by an antenna can be maximized. This is achieved by shaping antennas so that the current flow in the antenna element is driven resonantly by the incident light field. Recently, researchers presented a new approach to energy transfer that focuses on the magnetic resonance of a system consisting of two helical shaped conducting coils. The researchers made the electrons in a conducting source coil oscillate at the resonance frequency, thereby producing a magnetic field through the coil. At a distance of up to 2 meters they placed a second identical coil to pick up the magnetic field, which then generates a similar current in the second coil. This current was then sent through a conventional light bulb and a power transfer of 60 Watt was measured. This example represents the most important feature of resonant coupling, namely maximum energy transfer. The maximum efficiency of a resonantly coupled system is limited by the coupling constant and the damping coefficient (energy loss through other mechanisms, including the load to be powered by the energy transfer).

The main advantage of using the magnetic resonance is that human tissues do not respond resonantly to this type of radiation, therefore, it can be used safely in everyday life. Small devices like cellular phones could be charged without having to be plugged into an adapter. Small robots with specific tasks, such as cleaning, will probably become commonplace in households in the next 10 years. These robots could get their energy through such radiation, thereby increasing their applicability and effective working time. Actually, any device that uses batteries could be charged by such wireless power transfer, reducing the need to change batteries. A drawback is that to implement this technology, designs will need to be changed; therefore successful implementation will depend on the proper introduction of standards for this technology. So we will need to wait and see whether major companies will start to make use of this.