Wireless energy transfer or wireless power is the transmission of electrical energy from a power source to an electrical load without artificial interconnecting conductors. Wireless transmission is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible. The most common form of wireless power transmission is carried out using direct induction followed by resonant magnetic induction. Other methods under consideration include electromagnetic radiation in the form of microwaves or lasers.

Energy transfer and efficiency The general principle is that if a given oscillating amount of energy (for example alternating current from a wall outlet) is placed into a primary coil which is capacitively loaded, the coil will ‘ring’, and form an oscillating magnetic field. The energy will transfer back and forth between the magnetic field in the inductor and the electric field across the capacitor at the resonant frequency. This oscillation will die away at a rate determined by the Q factor, mainly due to resistive and radiative losses.

However, provided the secondary coil cuts enough of the field that it absorbs more energy than is lost in each cycle of the primary, then most of the energy can still be transferred. ————————————————- Electric energy transfer An electric current flowing through a conductor carries electrical energy. When an electric current passes through a circuit there is an electric field in the dielectric surrounding the conductor; magnetic field lines around the conductor and lines of electric force radially about the conductor. 3] In a direct current circuit, if the current is continuous, the fields are constant; there is a condition of stress in the space surrounding the conductor, which represents stored electric and magnetic energy, just as a compressed spring or a moving mass represents stored energy. In an alternating current circuit, the fields also alternate; that is, with every half wave of current and of voltage, the magnetic and the electric field start at the conductor and run outwards into space with the speed of light. [4] Where these alternating fields impinge on another conductor a oltage and a current are induced. [3] Any change in the electrical conditions of the circuit, whether internal[5] or external[6] involves a readjustment of the stored magnetic and electric field energy of the circuit, that is, a so-called transient. A transient is of the general character of a condenser discharge through an inductive circuit. The phenomenon of the condenser discharge through an inductive circuit therefore is of the greatest importance to the engineer, as the foremost cause of high-voltage and high-frequency troubles in electric circuits. 7] Electromagnetic induction is proportional to the intensity of the current and voltage in the conductor which produces the fields and to thefrequency. The higher the frequency the more intense the induction effect. Energy is transferred from a conductor that produces the fields (the primary) to any conductor on which the fields impinge (the secondary). Part of the energy of the primary conductor passes inductively across space into secondary conductor and the energy decreases rapidly along the primary conductor.

A high frequency current does not pass for long distances along a conductor but rapidly transfers its energy by induction to adjacent conductors. Higher induction resulting from the higher frequency is the explanation of the apparent difference in the propagation of high frequency disturbances from the propagation of the low frequency power of alternating current systems. The higher the frequency the more preponderant become the inductive effects that transfer energy from circuit to circuit across space.

The more rapidly the energy decreases and the current dies out along the circuit, the more local is the phenomenon. [3] The flow of electric energy thus comprises phenomena inside of the conductor[8] and phenomena in the space outside of the conductor—the electric field—which, in a continuous current circuit, is a condition of steady magnetic and dielectric stress, and in an alternating current circuit is alternating, that is, an electric wave launched by the conductor[3] to become far-field electromagnetic radiation traveling through space with the speed of light.

In electric power transmission and distribution, the phenomena inside of the conductor are of main importance, and the electric field of the conductor is usually observed only incidentally. [9] Inversely, in the use of electric power for radio telecommunications it is only the electric and magnetic fields outside of the conductor, that is electromagnetic radiation, which is of importance in transmitting the message. The phenomenon in the conductor, the current in the launching structure, is not used. 3] The electric charge displacement in the conductor produces a magnetic field and resultant lines of electric force. The magnetic field is a maximum in the direction concentric, or approximately so, to the conductor. That is, a ferromagnetic body[10] tends to set itself in a direction at right angles to the conductor. The electric field has a maximum in a direction radial, or approximately so, to the conductor. The electric field component tends in a direction radial to the conductor and dielectric bodies may be attracted or repelled radially to the conductor. 11] The electric field of a circuit over which energy flows has three main axes at right angles with each other: 1. The magnetic field, concentric with the conductor. 2. The lines of electric force, radial to the conductor. 3. The power gradient, parallel to the conductor. Where the electric circuit consists of several conductors, the electric fields of the conductors superimpose upon each other, and the resultant magnetic field lines and lines of electric force are not concentric and radial respectively, except approximately in the immediate neighborhoodof the conductor.

Between parallel conductors they are conjugate of circles. Neither the power consumption in the conductor, nor the magnetic field, nor the electric field, are proportional to the flow of energy through the circuit. However, the product of the intensity of the magnetic field and the intensity of the electric field is proportional to the flow of energy or the power, and the power is therefore resolved into a product of the two components i and e, which are chosen proportional respectively to the intensity of the magnetic field and of the electric field.

The component called the current is defined as that factor of the electric power which is proportional to the magnetic field, and the other component, called the voltage, is defined as that factor of the electric power which is proportional to the electric field. [11] In radio telecommunications the electric field of the transmit antenna propagates through space as a radio wave and impinges upon the receive antenna where it is observed by its magnetic and electric effect. [11] Radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X rays and gamma rays are hown to be the same electromagnetic radiation phenomenon, differing one from the other only in frequency of vibration. [3][12] Electromagnetic induction Energy transfer by electromagnetic induction is typically magnetic but capacitive coupling can also be achieved. Electrodynamic induction method The electrodynamic induction wireless transmission technique is near field over distances up to about one-sixth of the wavelength used. Near field energy itself is non-radiative but some radiative losses do occur. In addition there are usually resistive losses.

With electrodynamic induction, electric current flowing through a primary coil creates a magnetic field that acts on a secondary coil producing a current within it. Coupling must be tight in order to achieve high efficiency. As the distance from the primary is increased, more and more of the magnetic field misses the secondary. Even over a relatively short range the inductive coupling is grossly inefficient, wasting much of the transmitted energy. [13] This action of an electrical transformer is the simplest form of wireless power transmission.

The primary and secondary circuits of a transformer are not directly connected. Energy transfer takes place through a process known as mutual induction. Principal functions are stepping the primary voltage either up or down and electrical isolation. Mobile phone and electric toothbrush battery chargers, and electrical power distribution transformers are examples of how this principle is used. Induction cookers use this method. The main drawback to this basic form of wireless transmission is short range. The receiver must be directly adjacent to the transmitter or induction unit in order to efficiently couple with it.

The application of resonance increases the transmission range somewhat. When resonant coupling is used, the transmitter and receiver inductors are tuned to the same natural frequency. Performance can be further improved by modifying the drive current from a sinusoidal to a nonsinusoidal transient waveform. [14] Pulse power transfer occurs over multiple cycles. In this way significant power may be transmitted between two mutually-attuned LC circuits having a relatively low coefficient of coupling. Transmitting and receiving coils are usually single layer solenoids or flat spirals with series capacitors, which, in ombination, allow the receiving element to be tuned to the transmitter frequency. Common uses of resonance-enhanced electrodynamic induction are charging the batteries of portable devices such as laptop computers and cell phones, medical implants and electric vehicles. [15][16][17] A localized charging technique[18] selects the appropriate transmitting coil in a multilayer winding array structure. [19] Resonance is used in both the wireless charging pad (the transmitter circuit) and the receiver module (embedded in the load) to maximize energy transfer efficiency.

This approach is suitable for universal wireless charging pads for portable electronics such as mobile phones. It has been adopted as part of the Qi wireless charging standard. It is also used for powering devices having no batteries, such as RFID patches and contactless smartcards, and to couple electrical energy from the primary inductor to the helical resonator of Tesla coil wireless power transmitters. Electrostatic induction method Electrostatic or capacitive coupling is the passage of electrical energy through a dielectric.

In practice it is an electric field gradient or differential capacitance between two or more insulated terminals, plates, electrodes, or nodes that are elevated over a conducting ground plane. The electric field is created by charging the plates with a high potential, high frequency alternating current power supply. The capacitance between two elevated terminals and a powered device form a voltage divider. The electric energy transmitted by means of electrostatic induction can be utilized by a receiving device, such as a wireless lamp. 23][24][25] Tesla demonstrated the illumination of wireless lamps by energy that was coupled to them through an alternating electric field. [26][27][20] “Instead of depending on electrodynamic induction at a distance to light the tube . . . [the] ideal way of lighting a hall or room would . . . be to produce such a condition in it that an illuminating device could be moved and put anywhere, and that it is lighted, no matter where it is put and without being electrically connected to anything. I have been able to produce such a condition by creating in the room a powerful, rapidly alternating electrostatic field.

For this purpose I suspend a sheet of metal a distance from the ceiling on insulating cords and connect it to one terminal of the induction coil, the other terminal being preferably connected to the ground. Or else I suspend two sheets . . . each sheet being connected with one of the terminals of the coil, and their size being carefully determined. An exhausted tube may then be carried in the hand anywhere between the sheets or placed anywhere, even a certain distance beyond them; it remains always luminous. [28] The principle of electrostatic induction is applicable to the electrical conduction wireless transmission method. “In some cases when small amounts of energy are required the high elevation of the terminals, and more particularly of the receiving-terminal D’, may not be necessary, since, especially when the frequency of the currents is very high, a sufficient amount of energy may be collected at that terminal by electrostatic induction from the upper air strata, which are rendered conducting by the active terminal of the transmitter or through which the currents from the same are conveyed. [29] Electromagnetic radiation Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). The main reason for longer ranges with radio wave and optical devices is the fact that electromagnetic radiation in the far-field can be made to match the shape of the receiving area (using high directivity antennas or well-collimated Laser Beam) thereby delivering almost all emitted power at long ranges.

The maximum directivity for antennas is physically limited by diffraction. Beamed power, size, distance, and efficiency The size of the components may be dictated by the distance from transmitter to receiver, the wavelength and the Rayleigh criterion ordiffraction limit, used in standard radio frequency antenna design, which also applies to lasers. In addition to the Rayleigh criterion Airy’s diffraction limit is also frequently used to determine an approximate spot size at an arbitrary distance from the aperture.

The Rayleigh criterion dictates that any radio wave, microwave or laser beam will spread and become weaker and diffuse over distance; the larger the transmitter antenna or laser aperture compared to the wavelength of radiation, the tighter the beam and the less it will spread as a function of distance (and vice versa). Smaller antennae also suffer from excessive losses due to side lobes. However, the concept of laser aperture considerably differs from an antenna. Typically, a laser aperture much larger than the wavelength induces multi-moded radiation and mostly collimators are used before emitted radiation couples into a fiber or into space.

Ultimately, beamwidth is physically determined by diffraction due to the dish size in relation to the wavelength of the electromagnetic radiation used to make the beam. Microwave power beaming can be more efficient than lasers, and is less prone to atmospheric attenuationcaused by dust or water vapor losing atmosphere to vaporize the water in contact. Then the power levels are calculated by combining the above parameters together, and adding in the gains and losses due to the antenna characteristics and the transparency and dispersion of the medium through which the radiation passes.

That process is known as calculating a link budget. Advantages of Bluetooth * Firstly, it is wireless, thus eliminating all the cords used in connections. Line of sight is not required, as opposed to that in Infrared communication. This unwired form of communication reduced the clutter of wires to a great extent. * Secondly, with Bluetooth headsets, you can communicate hands-free. You can use your cell phone without the use of your hands. That makes it safe to talk on phone while your hands are engaged in other activities.

Thanks to Bluetooth technology, you are not required to be physically close to the device you are using. * Bluetooth devices are fairly inexpensive. There is no special cost incurred in using this technology. * The next concern is interoperability. Bluetooth is a standardized specification. Bluetooth enabled devices are highly compatible with each other. They ‘understand’ each other so well, that no human intervention is required. When within range, they sense each other and start communicating on their own. The process of connection setup is automatic. Then comes efficiency! Bluetooth uses low power signals, thus requiring less energy. Due to spread-spectrum frequency hopping, interference with other wireless devices is not a question at all. * Bluetooth Special Interest Group has been working on upgraded versions, which are backward compatible. So higher versions are no cause of worry. * Moreover, Bluetooth communication is secure. Security rules will not allow the devices to communicate unless pre-approved by the user. Bluetooth Limitations * Communication speed is not that great with Bluetooth technology.

Wi-Fi and Infrared communication can happen at much higher speeds compared to that of Bluetooth. * Though Bluetooth communication is considered secure, if a user happens to leave his/her device in the ‘discoverable’ mode, a hacker can eavesdrop or establish a connection with the ‘discovered’ device without the user knowing about it. * If Bluetooth is in the ON mode, it does drain the battery. So one needs to be careful about switching Bluetooth ON only for the time span of communication and turn it OFF after the communication ends.

This practice also reduces the security risk involved with Bluetooth technology. Signal Transfer Bluetooth uses frequency hopping in timeslots, which means that the Bluetooth signals avoid interference with other signals by hopping to a new frequency after transmission or reception of every packet. One packet can cover up to five time slots. Bluetooth can support an asynchronous data channel, or up to 3 simultaneous synchronous voice channels, or a channel, which concurrently supports asynchronous data and synchronous voice.