Most electrical energy is generated by electromagnetic induction. However, electricity can be produced by other means. Batteries use electrochemistry to produce low voltages.

An electrolyte is a solution of chemicals in water such that the chemical separates into positively and negatively charged ions when dissolved. The charged ions react with the conducting electrodes and release energy, as well as give up their charge

A fixed electrode potential is associated with the reaction at each electrode; the difference between the two electrode potentials drives a current around an external circuit. The electrolyte must be sealed into a safe container to make a suitable battery.

‘Dry’ cells use an electrolyte in the form of a gel or thick paste. A primary cell releases electricity as the chemicals react, and the cell is discarded once all the active chemicals have been used up, or the electrodes have become contaminated. A secondary cell uses a reversible chemical reaction, so that it can be recharged to regenerate the active chemicals.

The fuel cell is a primary cell which is constructed so that the active chemicals (fuel) pass through the cell, and the cell can be used for long periods by replenishing the chemicals. Large batteries consist of cells connected in series or parallel to increase the output voltage or current.

Electricity can be generated directly from heat. When two different materials are used in an electrical circuit, a small electrochemical voltage (contact potential) is generated at the junction. In most circuits these contact potentials cancel out around the circuit and no current flows.

However, the junction potential varies with temperature, so that if one junction is at a different temperature from the others, the contact potentials will not cancel out and the net circuit voltage causes current to flow (Seebeck effect). The available voltage is very small, but can be made more useful by connecting many pairs of hot and cold junctions in series.

The thermocouple is used mostly for measurement of temperature by this effect, rather than for the generation of electrical power. The efficiency of energy conversion is greater with semiconductor junctions, but metal junctions have a more consistent coefficient and are preferred for accurate measurements.

The effect can be reversed with suitable materials, so that passing an electric current around the circuit makes one junction hotter and the other colder (Peltier effect). Such miniature heat pumps are used for cooling small components.

Certain crystalline chemicals are made from charged ions of different sizes. When a voltage is applied across the crystal, the charged ions move slightly towards the side of opposite polarity, causing a small distortion of the crystal. Conversely, applying a force so as to distort the crystal moves the charged ions and generates a voltage.

This piezoelectric effect is used to generate high voltages from a small mechanical force, but very little current is available. Ferromagnetic materials also distort slightly in a magnetic field. The magnetostrictive effect produces low frequency vibration (hum) in ac machines and transformers.

Electricity can be produced directly from light. The photovoltaic effect occurs when light falls on suitable materials, releasing electrons from the material and generating electricity. The magnitude of the effect is greater with short wavelength light (blue) than long wavelength light (red), and stops altogether beyond a wavelength threshold.

Light falling on small photovoltaic cells is used for light measurement, communications and for proximity sensors. On a larger scale, semiconductor solar cells are being made with usable efficiency for power generation.

Light is produced from electricity in incandescent filament bulbs, by heating a wire to a sufficiently high temperature that it glows. Fluorescent lights produce an electrical discharge through a low pressure gas. The discharge emits ultraviolet radiation, which causes a fluorescent coating on the inside of the tube to glow.

No comments:

Post a Comment