Main article: Electrochemical cell
A voltaic cell for demonstration purposes. In this example the two half-cells are linked by a salt bridge separator that permits the transfer of ions, but not water molecules (Sony Vaio VGN-FZ battery) .
A battery is a device that converts chemical energy directly to electrical energy. It consists of a number of voltaic cells; each voltaic cell consists of twohalf cells connected in series by a conductive electrolyte containing anions and cations (Sony VGP-BPS8 battery) .
One half-cell includes electrolyte and the electrode to whichanions (negatively-charged ions) migrate, i.e. the anode or negative electrode; the other half-cell includes electrolyte and the electrode to which cations(positively-charged ions) migrate, i.e. the cathode or positive electrode. In the redox reaction that powers the battery , reduction (addition of electrons) occurs to cations at the cathode, while oxidation (removal of electrons) occurs to anions at the anode (SONY VGP-BPS8 battery) .
The electrodes do not touch each other but are electrically connected by the electrolyte. Many cells use two half-cells with different electrolytes. In that case each half-cell is enclosed in a container, and a separator that is porous to ions but not the bulk of the electrolytes prevents mixing (Sony Vaio VGN-FZ battery) .
Each half cell has an electromotive force (or emf), determined by its ability to drive electric current from the interior to the exterior of the cell. The net emf of the cell is the difference between the emfs of its half-cells, as first recognized by Volta.[11] Therefore, if the electrodes have emfs and , then the net emf is ; in other words, the net emf is the difference between the reduction potentials of thehalf-reactions (SONY VGP-BPS8 battery) .
The electrical driving force or across the terminals of a cell is known as the terminal voltage (difference) and is measured involts.[24] The terminal voltage of a cell that is neither charging nor discharging is called the open-circuit voltage and equals the emf of the cell (Sony Vaio VGN-FZ battery) .
Because of internal resistance[25], the terminal voltage of a cell that is discharging is smaller in magnitude than the open-circuit voltage and the terminal voltage of a cell that is charging exceeds the open-circuit voltage .[26] An ideal cell has negligible internal resistance, so it would maintain a constant terminal voltage of until exhausted, then dropping to zero (Sony VGP-BPL9 battery) .
If such a cell maintained 1.5 volts and stored a charge of one Coulomb then on complete discharge it would perform 1.5 Joule of work.[24] In actual cells, the internal resistance increases under discharge,[25] and the open circuit voltage also decreases under discharge. If the voltage and resistance are plotted against time, the resulting graphs typically are a curve; the shape of the curve varies according to the chemistry and internal arrangement employed (Sony VGP-BPL11 battery) .[27]
As stated above, the voltage developed across a cell's terminals depends on the energy release of the chemical reactions of its electrodes and electrolyte. Alkaline and carbon-zinc cells have different chemistries but approximately the same emf of 1.5 volts; likewise NiCd and NiMH cells have different chemistries, but approximately the same emf of 1.2 volts (Sony VGP-BPL15 battery) .[28]
On the other hand the high electrochemical potential changes in the reactions of lithium compounds give lithium cells emfs of 3 volts or more (SONY VAIO VGN-FZ4000 Battery) .[29]
For other uses, see Battery.
An electrical battery is a combination of one or more electrochemical cells, used to convert stored chemical energy into electrical energy. Since the invention of the first Voltaic pile in 1800 by Alessandro Volta, the battery has become a common power source for many household and industrial applications (Toshiba PA3399U-2BAS Battery) .
According to a 2005 estimate, the worldwide battery industry generates US$48 billion in sales each year,[1] with 6% annual growth.[2]
Batteries may be used once and discarded, or recharged for years as in standby power applications. Miniature cells are used to power devices such as hearing aids and wristwatches; larger batteries provide standby power for telephone exchanges or computer data centers (Sony VGN-FZ460E battery) .
History
The name "battery" was coined by Benjamin Franklin for an arrangement of multiple Leyden jars (an early type of capacitor) after a battery of cannons.[3] Strictly, a battery is a collection of two or more cells, but in popular usage battery #mce_temp_url#often refers to a single electrical cell (Dell Inspiron E1505 Battery) .[4]
An early form of electrochemical battery called the Baghdad Battery may have been used in antiquity.[5] However, the modern development of batteries started with the Voltaic pile, invented by the Italianphysicist Alessandro Volta in 1800 (HP PAVILION DV9700t Battery) .[6]
In 1780 the Italian anatomist and physiologist Luigi Galvani noticed that dissected frog's legs would twitch when struck by a spark from a Leyden jar, an external source of electricity.[7] In 1786 he noticed that twitching would occur during lightning storms.[8]After many years Galvani learned how to produce twitching without using any external source of electricity (IBM ThinkPad T40 Battery) .
In 1791 he published a report on "animal electricity."[9] He created an electric circuit consisting of the frog's leg (FL) and two different metals A and B, each metal touching the frog's leg and each other, thus producing the circuit A-FL-B-A-FL-B...etc. In modern terms, the frog's leg served as both the electrolyte and the sensor, and the metals served as electrodes. He noticed that even though the frog was dead, its legs would twitch when he touched them with the metals (Apple M9848LL/A battery) .
Within a year, Volta realized the frog's moist tissues could be replaced by cardboard soaked in salt water, and the frog's muscular response could be replaced by another form of electrical detection. He already had studied the electrostatic phenomenon of capacitance, which required measurements of electric charge and of electrical potential ("tension") (Apple A1281 battery) .
Building on this experience, Volta was able to detect electric current through his system, also called a Galvanic cell. The terminal voltage of a cell that is not discharging is called its electromotive force (emf), and has the same unit as electrical potential, named (voltage) and measured in volts, in honor of Volta (Apple A1281 battery) .
In 1800, Volta invented the battery by placing many voltaic cells in series, literally piling them one above the other. This Voltaic pilegave a greatly enhanced net emf for the combination,[10] with a voltage of about 50 volts for a 32-cell pile.[11] In many parts of Europe batteries continue to be called piles (Apple M9848LL/A battery) .[12][13]
Volta did not appreciate that the voltage was due to chemical reactions. He thought that his cells were an inexhaustible source of energy,[14] and that the associated chemical effects (e.g. corrosion) were a mere nuisance, rather than an unavoidable consequence of their operation, as Michael Faraday showed in 1834 (Dell Inspiron 6000 battery ) .[15]
According to Faraday, cations (positively charged ions) are attracted to thecathode,[16] and anions (negatively charged ions) are attracted to the anode (HP Pavilion DV2000 Battery) (Dell Inspiron E1505 battery) .[17]
Although early batteries were of great value for experimental purposes, in practice their voltages fluctuated and they could not provide a large current for a sustained period. Later, starting with the Daniell cell in 1836, batteries provided more reliable currents and were adopted by industry for use in stationary devices, particularly in telegraph networks where they were the only practical source of electricity, since electrical distribution networks did not then exist (Toshiba PA3399U-2BRS battery) .[18]
These wet cells used liquid electrolytes, which were prone to leakage and spillage if not handled correctly. Many used glass jars to hold their components, which made them fragile. These characteristics made wet cells unsuitable for portable appliances. Near the end of the nineteenth century, the invention of dry cell batteries (HP Pavilion DV3 Battery), which replaced the liquid electrolyte with a paste, made portable electrical devices practical (IBM ThinkPad T60 battery ) .[19]
Since then, batteries have gained popularity as they became portable and useful for a variety of purposes.[20]
No comments:
Post a Comment