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Galvanic cell

A Galvanic cell, or Voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from chemical reactions taking place within the cell. It generally consists of two different metals connected by a salt bridge, or individual half-cells separated by a porous membrane.

Volta was the inventor of the voltaic pile, the first electrical battery. In common usage, the word "battery" has come to include a single Galvanic cell, but a battery properly consists of multiple cells.

History

In 1780, Luigi Galvani discovered that when two different metals (copper and zinc for example) were connected together and then both touched to different parts of a nerve of a frog leg at the same time, they made the leg contract. He called this "animal electricity". The volatic pile invented by Alessandro Volta in the 1800s is similar to the galvanic cell. These discoveries paved the way for electrical batteries.

Description

A Galvanic cell consists of two half-cells. In its simplest form, each half-cell consists of a metal and a solution of a salt of the metal. The salt solution contains a cation of the metal and an anion to balance the charge on the cation. In essence the half-cell contains the metal in two oxidation states and the chemical reaction in the half-cell is an oxidation-reduction (redox) reaction, written symbolically in reduction direction as

Mn+ (oxidized species) + n e|- M (reduced species)

In a galvanic cell one metal is able to reduce the cation of the other and, conversely, the other cation can oxidize the first metal. The two half-cells must be physically separated so that the solutions do not mix together. A salt bridge or porous plate is used to separate the two solutions yet keep the respective charges of the solutions from separating, which would stop the chemical reactions.

The number of electrons transferred in both directions must be the same, so the two half-cells are combined to give the whole-cell electrochemical reaction. For two metals A and B:

An+ + n e|- A
Bm+ + m e|- B
m A + n Bm+n B + m An+

This is not the whole story as anions must also be transferred from one half-cell to the other. When a metal in one half-cell is oxidized, anions must be transferred into that half-cell to balance the electrical charge of the cation produced. The anions are released from the other half-cell where a cation is reduced to the metallic state. Thus, the salt bridge or porous membrane serves both to keep the solutions apart and to allow the flow of anions in the direction opposite to the flow of electrons in the wire connecting the electrodes.

The voltage of the Galvanic cell is the sum of the voltages of the two half-cells. It is measured by connecting a voltmeter to the two electrodes. The voltmeter has very high resistance, so the current flow is effectively negligible. When a device such as an electric motor is attached to the electrodes, a current flows and redox reactions occur in both half-cells. This will continue until the concentration of the cations that are being reduced goes to zero.

For the Daniell cell, depicted in the figure, the two metals are zinc and copper and the two salts are sulfates of the respective metal. Zinc is the oxidized metal so when a device is connected to the electrodes, the electrochemical reaction is

Zn + Cu|2+ → Zn|2+ + Cu

The zinc electrode is dissolved and copper is deposited on the copper electrode (as copper ions become reduced to copper metal). By definition, the cathode is the electrode where reduction (gain of electrons) takes place, so the copper electrode is the cathode. The cathode attracts cations, so has a negative charge when current is discharging. In this case, copper is the cathode and zinc the anode.

Galvanic cells are typically used as a source of electrical power. By their nature they produce direct current. For example, a lead-acid battery contains a number of galvanic cells. The two electrodes are effectively lead and lead oxide.

The Weston cell was adopted as an International Standard for voltage in 1911. The anode is a cadmiummercuryamalgam, the cathode is made of pure mercury, the electrolyte is a (saturated) solution of cadmium sulfate and the depolarizer is a paste of mercurous sulfate. When the electrolyte solution is saturated the voltage of the cell is very reproducible, hence its use as a standard.

Cell voltage

The standard electrical potential of a cell can be determined by use of a standard potential table for the two half cells involved. The first step is to identify the two metals reacting in the cell. Then one looks up the standard electrode potential, E0, in volts, for each of the two half reactions. The standard potential for the cell is equal to the more positive E0 value minus the more negative E0 value.

For example, in the figure above the

Electrochemical cell

An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery". (Actually a single "Galvanic cell"; a battery properly consists of multiple cells.)

Half-cells

An electrochemical cell consists of two half-cells. Each half-cell consists of an electrode, and an electrolyte. The two half-cells may use the same electrolyte, or they may use different electrolytes. The chemical reactions in the cell may involve the electrolyte, the electrodes or an external substance (as in fuel cells which may use hydrogen gas as a reactant). In a full electrochemical cell, species from one half-cell lose electrons (oxidation) to their electrode while species from the other half-cell gain electrons (reduction) from their electrode. A salt bridge(i.e. filter paper soaked in KNO3) is often employed to provide ionic contact between two half-cells with different electrolytes—to prevent the solutions from mixing and causing unwanted side reactions. As electrons flow from one half-cell to the other, a difference in charge is established. If no salt bridge were used, this charge difference would prevent further flow of electrons. A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate. Other devices for achieving separation of solutions are porous pots and gelled solutions. A porous pot is used in the Bunsen cell (right).

Equilibrium reaction

Each half-cell has a characteristic voltage. Different choices of substances for each half-cell give different potential differences. Each reaction is undergoing an equilibrium reaction between different oxidation states of the ions—when equilibrium is reached the cell cannot provide further voltage. In the half-cell which is undergoing oxidation, the closer the equilibrium lies to the ion/atom with the more positive oxidation state the more potential this reaction will provide. Similarly, in the reduction reaction, the further the equilibrium lies to the ion/atom with the more negative oxidation state the higher the potential.

Electrode potential

The cell potential can be predicted through the use of electrode potentials (the voltages of each half-cell). (See table of standard electrode potentials). The difference in voltage between electrode potentials gives a prediction for the potential measured.

Cell potentials have a possible range of about zero to 6 volts. Cells using water-based electrolytes are usually limited to cell potentials less than about 2.5 volts, because the very powerful oxidizing and reducing agents which would be required to produce a higher cell potential tend to react with the water.

Electrochemical cell types

Main types

Cells are classified into two broad categories,

  • Primary cells irreversibly (within limits of practicality) transform chemical energy to electrical energy. When the initial supply of reactants is exhausted, energy cannot be readily restored to the electrochemical cell by electrical means.
  • Secondary cells can be recharged; that is, they can have their chemical reactions reversed by supplying electrical energy to the cell, restoring their original composition.

Primary electrochemical cells

Primary electrochemical cells can produce current immediately on assembly. Disposable cells are intended to be used once and discarded. Disposable primary cells cannot be reliably recharged, since the chemical reactions are not easily reversible and active materials may not return to their original forms.

Common types of disposable cells include zinc-carbon cells and alkaline cells. Generally, these have higher energy densities than rechargeable cells, but disposable cells do not fare well under high-drain applications with loads under 75 ohms (75 Ω).

Secondary electrochemical cells

Secondary electrochemical cells must be charged before use; they are usually assembled with active materials in the discharged state. Rechargeable electrochemical cells or secondary electrochemical cells can be recharged by applying electric current, which reverses thechemical reactions that occur during its use. Devices to supply the appropriate current are called chargers or rechargers.

The oldest form of rechargeable cell is the lead-acid cell. This electrochemical cell is notable in that it contains a liquid in an unsealed container, requiring that the cell be kept upright and the area be well ventilated to ensure safe dispersal of the hydrogen gas produced by these cells during overcharging. The lead-acid cell is also very heavy for the amount of electrical energy it can supply. Despite this, its low manufacturing cost and its high surge current levels make its use common where a large capacity (over approximately 10Ah) is required or where the weight and ease of handling are not concerns.

An improved type of liquid electrolyte cell is the sealed valve regulated lead acid (VRLA) cell, popular in the automotive industry as a replacement for the lead-acid wet cell. The VRLA cell uses an immobilized sulphuric acid electrolyte, reducing the chance of leakage and extending shelf life. VRLA cells have the electrolyte immobilized, usually by one of two means:

  • Gel cellscontain a semi-solid electrolyte to prevent spillage.
  • Absorbed Glass Mat(AGM) cells absorb the electrolyte in a special fibreglass matting

Other portable rechargeable cells are (in order of increasing power

Reflex arc

A reflex arc is the neural pathway that mediates a reflex action. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs. There are two types of reflex arc - autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles). Monosynaptic vs. polysynaptic When a reflex arc consists of only two neurons in an animal (one sensory neuron, and one motor neuron), it is defined as monosynaptic. Monosynaptic refers to the presence of a single chemical synapse. In the case of peripheral muscle reflexes (patellar reflex, achilles reflex), brief stimulation to the muscle spindle results in contraction of the agonist or effector muscle. By contrast, in polysynaptic reflex pathways, one or more interneurons connect afferent (sensory) and efferent (motor) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the spinal cord. The Patellar Reflex (knee jerk) Patellar reflex: when the patellar tendon is tapped just below the knee, the patellar reflex is initiated and the lower leg kicks forward (via contraction of the quadriceps). The tap initiates an action potential in a specialised structure known as a muscle spindle located within the quadriceps. This action potential travels to the spinal cord, via a sensory axon which chemically communicates by releasing glutamate (see synapse) onto a motor nerve. The result of this motor nerve activity is contraction of the quadriceps muscle, leading to extension of the lower leg at the knee. The sensory input from the quadriceps also activates local interneurons that release the inhibitory neurotransmitter glycine onto motor neurons, blocking the innervation of the antagonistic (hamstring) muscle. The relaxation of the opposing muscle facilitates extension of the lower leg. In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the lateral giant neuron of crayfish.

Varistor

A varistor is an electronic component with a "diode-like" nonlinearcurrent–voltage characteristic. The name is a portmanteau of variable resistor. Varistors are often used to protectcircuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. A varistor is also known as Voltage Dependent Resistor or VDR. A varistor’s function is to conduct significantly increased current when voltage is excessive.

Note: only non-ohmic variable resistors are usually called varistors. Other, ohmic types of variable resistor include thepotentiometer and the rheostat.

Metal oxide varistor

The most common type of varistor is the Metal Oxide Varistor (MOV). This contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behavior is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

Follow-through current as a result of a lightning strike may generate excessive current that permanently damages a varistor. In general, the primary case of varistor breakdown is localized heating caused as an effect of thermal runaway. This is due to a lack of conformality in individual grain-boundary junctions, which leads to the failure of dominant current paths under thermal stress.

Varistors can absorb part of a surge. How much effect this has on risk to connected equipment depends on the equipment and details of the selected varistor. Varistors do not absorb a significant percentage of a lightning strike, as energy that must be conducted elsewhere is many orders of magnitude greater than what is absorbed by the small device.

A varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its "clamping voltage". If a transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This (catastrophic) failure occurs when "Absolute Maximum Ratings" in manufacturer's datasheet are significantly exceeded. Varistor degradation is defined by manufacturer's life expectancy charts using curves that relate current, time, and number of transient pulses. A varistor fully degrades typically when its "clamping voltage" has changed by 10%. A fully degraded varistor remains functional (no catastrophic failure) and is not visibly damaged.

Ballpark number for varistor life expectancy is its energy rating. As MOV joules increase, the number of transient pulses increases and the "clamping voltage" during each transient decreases. The purpose of this shunt mode device is to divert a transient so that pulse energy will be dissipated elsewhere. Some energy is also absorbed by the varistor because a varistor is not a perfect conductor. Less energy is absorbed by a varistor, the varistor is more conductive, and its life expectancy increases exponentially as varistor energy rating is increased. Catastrophic failure can be avoided by significantly increasing varistor energy ratings either by using a varistor of higher joules or by connecting more of these shunt mode devices in parallel.

Important parameters are the varistor's energy rating in joules, operating voltage, response time, maximum current, and breakdown (clamping) voltage. Energy rating is often defined using standardized transients such as 8/20 microseconds or 10/1000 microseconds, where 8 microseconds is the transient's front time and 20 microseconds is the time to half value.

To protect communications lines (such as telephone lines) transient suppression devices such as 3 mil carbon blocks (IEEE C62.32), ultra-low capacitance varistors or avalanche diodes are used. For higher frequencies such as radio communication equipment, a gas discharge tube (GDT) may be utilized.

A typical surge protectorpower strip is built using MOVs. A cheapest kind may use just one varistor, from hot (live, active) to neutral. A better protector would contain at least three varistors; one across each of the three pairs of conductors (hot-neutral, hot-ground, neutral-ground). A power strip protector in the United States should have a UL1449 3rd edition approval so that catastrophic MOV failure would not create a fire hazard.

Hazards

While a MOV is designed to conduct significant power for very short durations (≈ 8/20 microseconds), such as caused by lightning strikes, it typically does not have the capacity to conduct sustained energy. Under normal utility voltage conditions, this is not a problem. However, certain types of faults on the utility power grid can result in sustained over-voltage conditions. Examples include a loss of a neutral conductor or shorted lines on the high voltage system. Application of sustained over-voltage to a MOV can cause high dissipation, potentially resulting in the MOV device catching fire. The National Fire Protection Association (NFPA) has documented many cases of catastrophic fires that have been caused by MOV devices in surge suppressors, and has issued bulletins on the issue.

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From Yahoo Answers

Question:friends, electric potential is defined as the work done to bring a unit positive charge from infinity to that point against electric forces. electric potential energy is the work done to assemble the charge. What is the difference between electric potential and electric potential energy?

Answers:Electric potential is the potential energy per unit charge, just as electric field is the force per unit charge. It's a way to describe the system independent of the "test charge". So if it takes 10 eV of energy to move a single elementary charge (like a proton) from A to B, we say the difference in the proton's potential energy is 10 eV between those points. But independent of the proton, we say the difference in potential between those two points is 10 Volts. The charge we're moving has potential energy U. The field we're moving it through has potential. V = U/q, or U = qV.

Question:Why does the biphasic action potential change into the monophasic action potential after nerve injury between the two recording electrodes?

Answers:Let us consider the following situation. An action potential is initiated at the one end of the nerve. The sodium influx associated with the rapid depolarization phase of the action potential in the region where the nerve is depolarized makes the extracellular surface negative with respect to the surrounding regions. However, the regions under the two recording electrodes are still at rest and there is no difference in potential between them. When the action potential conducts into the region under the first (negative) recording electrode, the extracellular surface of the membrane becomes negative with respect to the extracellular surface under the second (positive) recording electrode and the voltmeter records a positive potential difference (V+ - V-). As the action potential conducts along the nerve, the difference in potential will be zero when the action potential is between the two electrodes , negative when the action potential is under the second recording electrode , and zero again when the action potential has conducted past the second recording electrode. Because there are both positive and negative phases to this response, it is referred to as a biphasic action potential. When the nerve is damaged,the action potential remains between the electrodes and doesn't reach the second electrode (i.e potential difference remains positive and doesn't go into negative phase ) , the biphasic action potential changes into monophasic action potential i.e with single phase.

Question:For example, a 3.0v stimulation generates an action potential with a lower amplitude than a 5.0 v stimulation. I thought action potentials were the same no matter how strong the stimulus?

Answers:Are you stimulating a single nerve cell or a bundle of nerves. If it is a single nerve, the AP should be the same amplitude as long as the stimulus exceeds threshold. If it is a bundle of nerves, the resulting compound AP will depend on how many individual nerves were excited by the stimulus. Different nerves in the bundle will have different thresholds. They may also receive a different stimulus strength depending on their distance from stimulating electrode.

Question:(NCV) measures waveform from a stimulating electrode to a recording electrode along the ulnar nerve. How is it that you can see a response if action potentials are "all or none"?

Answers:The all-or-none response pertains to a single neuron. The ulnar nerve, as with most nerves in the human body, contains a large number of neurons. There are two reasons for a graded response from the ulnar nerve. First, there is a spectrum of neurons with different sensitivities to electrical stimulation because of their degree of myelination and diameter. Large myelinated fibers are more sensitive to electrical stimulation than are small, unmyelinated ones. Second, for a given size axon, the ones near the surface of the nerve will respond with a lower stimulus intensity than those at the center of the nerve because they are closer to the stimulating electrode and are less electrically insulated than those in the center of the nerve.

From Youtube

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Peter Ward: The Medea Hypothesis :press.princeton.edu Will we extend life into space before a catastrophe stops us permanently? Those going into space will be impelled by an exploratory urge. But their choices will have epochal consequences. Once the threshold is crossed when there is a self-sustaining level of life in space, then lifes long-range future will be secure irrespective of any of the risks on Earth (with the single exception of the catastrophic destruction of space itself). Will this happen before our technical civilisation disintegrates, leaving this as a might-have-been? Will the self-sustaining space communities be established before a catastrophe sets back the prospect of any such enterprise, perhaps foreclosing it for ever? We live at what could be a defining moment for the cosmos, not just for our Earth. The colonization of space is an evolutionary step similar to the colonization of land This would be as epochal an evolutionary transition as that which led to land-based life on Earth. But it could still be just the beginning of cosmic evolution. The enormous potential of future life is dependent on our actions today The first aquatic creatures crawled onto dry land in the Silurian era, more than three hundred million years ago. They may have been unprepossessing brutes, but had they been clobbered, the evolution of land-based fauna would have been jeopardised. Likewise, the post-human potential is so immense that not even the most misanthropic amongst us would countenance its being ...