chemical effects of electric current

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Eddy current

Eddy currents (also called Foucault currents) are currents induced in conductors, opposing the change in flux that generated them. It is caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause a circulating flow of electrons, or a current, within the body of the conductor. These circulating eddies of current create induced magnetic fields that oppose the change of the original magnetic field due to Lenz's law, causing repulsive or drag forces between the conductor and the magnet. The stronger the applied magnetic field, or the greater the electrical conductivity of the conductor, or the faster the field that the conductor is exposed to changes, then the greater the currents that are developed and the greater the opposing field.

The term eddy current comes from analogous currents seen in water when dragging an oar breadthwise: localised areas of turbulence known as eddies give rise to persistent vortices.

Eddy currents, like all electric currents, generate heat as well as electromagnetic forces. The heat can be harnessed for induction heating. The electromagnetic forces can be used for levitation, creating movement, or to give a strong braking effect. Eddy currents can also have undesirable effects, for instance power loss in transformers. In this application, they are minimised with thin plates, by lamination of conductors or other details of conductor shape.

Self-induced eddy currents are responsible for the skin effect in conductors. The latter can be used for non-destructive testing of materials for geometry features, like micro-cracks. A similar effect is the proximity effect, which is caused by externally-induced eddy currents.

History

The first person to observe current eddies was François Arago (1786-1853), the 25th president of France, who was also a mathematician, physicist and astronomer. In 1824 he observed what has been called rotatory magnetism, and the fact that most conductive bodies could be magnetized; these discoveries were completed and explained by Michael Faraday (1791-1867).

In 1834, Heinrich Lenz stated Lenz's law, which says that the direction of induced current flow in an object will be such that its magnetic field will oppose the magnetic field that caused the current flow. Eddy currents develop secondary flux that cancels a part of the external flux.

French physicist Léon Foucault (1819-1868) is credited with having discovered Eddy currents. In September, 1855, he discovered that the force required for the rotation of a copper disc becomes greater when it is made to rotate with its rim between the poles of a magnet, the disc at the same time becoming heated by the eddy current induced in the metal. The first use of eddy current for Non-destructive testing occurred in 1879 when D. E. Hughes used the principles to conduct metallurgical sorting tests.

Explanation

When a conductor moves relative to the field generated by a source, electromotive forces (EMFs) can be generated around loops within the conductor. These EMFs acting on the resistivity of the material generate a current around the loop, in accordance with Faraday's law of induction. These currents dissipate energy, and create a magnetic field that tends to oppose the changes in the field.

Eddy currents are created when a conductor experiences changes in the magnetic field. If either the conductor is moving through a steady magnetic field, or the magnetic field is changing around a stationary conductor, eddy currents will occur in the conductor. Both effects are present when a conductor moves through a varying magnetic field, as is the case at the top and bottom edges of the magnetized region shown in the diagram. Eddy currents will be generated wherever a conducting object experiences a change in the intensity or direction of the magnetic field at any point within it, and not just at the boundaries.

The swirling current set up in the conductor is due to electrons experiencing a Lorentz force that is perpendicular to their motion. Hence, they veer to their right, or left, depending on the direction of the applied field and whether the strength of the field is increasing or declining. The resistivity of the conductor acts to damp the amplitude of the eddy currents, as well as straighten their paths. Lenz's law encapsulates the fact that the current swirls in such a way as to create an induced magnetic field that opposes the phenomenon that created it. In the case of a varying applied field, the induced field will always be in the opposite direction to that applied. The same will be true when a varying external field is increasing in strength. However, when a varying field is falling in strength, the induced field will be in the same direction as that originally applied, in order to oppose the decline.

An object or part of an object experiences steady field intensity and direction where there is still relative motion of the field and the object (for example in the center of the field in the diagram), or unsteady fields where the currents cannot circulate due to the geometry of the conductor. In these situations charges collect on or within the object and these charges then produce static electric potentials that oppose any further current. Currents may be initially associated with the creation of static potentials, but these may be transitory and small.

Eddy currents generate resistive losses that transform some forms of energy, such as kinetic energy, into heat. This Joule heating reduces efficiency of iron-core transformers and electric motors and other devices that use changing magnetic fields. Eddy currents are minimized in these devices by selecting

Current source

A current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source. The term constant-current sink is sometimes used for sources fed from a negative voltage supply. Figure 1 shows a schematic for an ideal current source driving a resistor load.

Ideal current sources

In circuit theory, an ideal current source is a circuit element where the current through it is independent of the voltage across it. It is a mathematical model, which real devices can only approach in performance. If the current through an ideal current source can be specified independently of any other variable in a circuit, it is called an independent current source. Conversely, if the current through an ideal current source is determined by some other voltage or current in a circuit, it is called a dependent or controlled current source. Symbols for these sources are shown in Figure 2.

|- align="center" |style="padding: 1em 2em 0;"| |style="padding: 1em 2em 0;"| |- align="center" | Voltage source | Current Source |- align="center" |style="padding: 1em 2em 0;"| |style="padding: 1em 2em 0;"| |- align="center" | Controlled Voltage Source | Controlled Current Source |- align="center" |style="padding: 1em 2em 0;"| |style="padding: 1em 2em 0;"| |- align="center" | Battery of cells | Single cell

An independent current source with zero current is identical to an ideal open circuit. For this reason, the internal resistance of an ideal current source is infinite. The voltage across an ideal current source is completely determined by the circuit it is connected to. When connected to a short circuit, there is zero voltage and thus zero power delivered. When connected to a load resistance, the voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit). Thus, an ideal current source, if such a thing existed in reality, could supply unlimited power and so would represent an unlimited source of energy.

No real current source is ideal (no unlimited energy sources exist) and all have a finite internal resistance (none can supply unlimited voltage). However, the internal resistance of a physical current source is effectively modeled in circuit analysis by combining a non-zero resistance in parallel with an ideal current source (the Norton equivalent circuit). The connection of an ideal open circuit to an ideal non-zero current source does not represent any physically realizable system.

Physical current sources

Resistor current source

The simplest current source consists of a voltage source in series with a resistor. The current available from such a source is given by the ratio of the voltage across the voltage source to the resistance of the resistor. For a nearly ideal current source, the value of this resistor should be very large but this implies that, for a specified current, the voltage source must be very large. Thus, efficiency is low (due to power loss in the resistor) and it is usually impractical to construct a 'good' current source this way. Nonetheless, it is often the case that such a circuit will provide adequate performance when the specified current and load resistance are small. For example, a 5 V voltage source in series with a 4.7 kilohm resistor will provide an approximately constant current of 1 mA (±5%) to a load resistance in the range of 50 to 450 ohm.

Active current sources

Active current sources have many important applications in electronic circuits. Current sources (current-stable resistors) are often used in place of ohmic resistors in analog integrated circuits to generate a current without causing attenuation at a point in the signal path to which the current source is attached. The collector of a bipolar transistor, the drain of a field effect transistor, or the plate of a vacuum tube naturally behave as current sources (or sinks) when properly connected to an external source of energy (such as a power supply) because the output impedance of these devices is naturally high when used in the current source configuration.

JFET and N-FET current source

A JFET can be made to act as a current source by tying its gate to its source. The current then flowing is the IDSS of the FET. These can be purchased with this connection already made and in this case the devices are called current regulator diodes or constant current diodes or current limiting diodes (CLD). An enhancement mode N channel MOSFET can be used in the circuits listed below.

Simple transistor current source

Figure 3 shows a typical constant current source (CCS). DZ1 is a zener diode which, when reverse biased (as shown in the circuit) has a constant voltage drop across it irrespective of the current flowing through it. Thus, as long as the zener current (IZ) is above a certain level (called holding current), the voltage across the zener diode (VZ) will be constant. Resistor R1 supplies the zener current and the base current (IB) of NPN transistor (Q1). The constant zener voltage is applied across the base of Q1 and emitter resistor R2. The operation of the circuit is as follows:

Voltage across R2 (VR2) is given by VZ - VBE, where VBE is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by

I_{R2} (= I_{E}) = \frac{V_{R2}}{R2} = \frac{V_{Z} - V_{BE}}{R2}

Since VZ is constant and VBE is also (approximately) constant for a given temperature, it follows that VR2 is constant and hence IE is also constant. Due to transistor action, emitter current IE is very nearly equal to the collector curr

Residual-current device

A residual-current device (RCD), similar to a residual current circuit breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethalshock can result from these conditions. RCDs are designed to disconnect quickly enough to mitigate the harm caused by such shocks although they are not intended to provide protection against overload or short-circuit conditions.

In the United States and Canada, a residual current device is also known as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI). In Australia they are sometimes known as "safety switches" or simply "RCD" and in the United Kingdom they can be referred to as "trips" or "trip switches". In the previous edition of the IEE Electrical Wiring Regulations (16th Edition) they were used to add extra fault protection to socket outlets. The current edition (17th Edition) of the regulations state that all new installations, as well as a change of distribution board or the installation of new circuits in a property wired to any previous installation, must have a split load distribution board with two RCDs covering the installation, with upstairs and downstairs lighting and power circuits spread across both RCDs in case of a fault on one RCD, therefore leaving power to at least one lighting and power circuit.

Purpose and operation

RCDs are designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5–30 milliamperes) than the currents needed to operate conventional circuit breakers or fuses (several amperes). RCDs are intended to operate within 25-40 milliseconds, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.

In the United States, the National Electrical Code requires GFCI devices intended to protect people to interrupt the circuit if the leakage current exceeds a range of 4–6 mA of current (the trip setting is typically 5 mA) within 25 ms. A GFCI device which protects equipment (not people) is allowed to trip as high as 30 mA of current; this is known as an Equipment Protective Device (EPD). In Europe, the commonly used RCDs have trip currents of 10–300 mA.

RCDs operate by measuring the current balance between two conductors using a differential current transformer. This measures the difference between the current flowing out the live conductor and that returning through the neutral conductor. If these do not sum to zero, there is a leakage of current to somewhere else (to earth/ground, or to another circuit), and the device will open its contacts.

Residual current detection is complementary to over-current detection. Residual current detection cannot provide protection for overload or short-circuit currents.

RCDs with trip currents as high as 500 mA are sometimes deployed in environments (such as computing centers) where a lower threshold would carry an unacceptable risk of accidental trips. These high-current RCDs serve more as an additional fire-safety protection than as an effective protection against the risks of electrical shocks.

In some countries, two-wire (ungrounded) outlets may be replaced with three-wire GFCIs to protect against electrocution, and a grounding wire does not need to be supplied to that GFCI, but the outlet must be labeled as such. The GFCI manufacturers provide tags for the appropriate installation description.

Example

The photograph depicts the internal mechanism of a residual current device (RCD). The device pictured is designed to be wired in-line in an appliance power cord. It is rated to carry a maximum current of 13 amperes and is designed to trip on a leakage current of 30 mA. This is an active RCD; that is, it doesn't latch mechanically and therefore trips on power failure, a useful feature for equipment that could be dangerous on unexpected re-energisation.

The incoming supply and the neutral conductors are connected to the terminals at (1) and the outgoing load conductors are connected to the terminals at (2). The earth conductor (not shown) is connected through from supply to load uninterrupted.

When the reset button (3) is pressed the contacts ((4) and hidden behind (5)) close, allowing current to pass. The solenoid (5) keeps the contacts closed when the reset button is released.

The sense coil (6) is a differential current transformer which surrounds (but is not electrically connected to) the live and neutral conductors. In normal operation, all the current down the live conductor returns up the neutral conductor. The currents in the two conductors are therefore equal and opposite and cancel each other out.

Any fault to earth (for example caused by a person touching a live component in the attached appliance) causes some of the current to take a different return path which means there is an imbalance (difference) in the current in the two conductors (single phase case), or, more generally, a nonzero sum of currents from among various conductors (for example, three phase conductors and one neutral conductor).

This difference causes a current in the sense coil (6) which is picked up by the sense circuitry (7). The sense circuitry then removes power from the solenoid (5) and the contacts (4) are forced apart by a spring, cutting off the electricity supply to the appliance.

The device is designed so that the current is interrupted in a fraction of a second, greatly reducing the chances of a dangerous electric shock being received.

The test button (8) allows the correct operation of the device to be verified by passing a small current through the orange test wire (9). This simulates a fault by creating an imbalance in the sense c

Electric power

Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt.

When electric current flows in a circuit, it can transfer energy to do mechanical or thermodynamic work. Devices convert electrical energy into many useful forms, such as heat (electric heaters), light (light bulbs), motion (electric motors), sound (loudspeaker), information technological processes (computers), or even chemical changes. Electricity can be produced mechanically by generation, or chemically, or by direct conversion from light in photovoltaic cells, also it can be stored chemically in batteries.

Mathematics of electric power

Circuits

Electric power, like mechanical power, is represented by the letter P in electrical equations. The term wattage is used colloquially to mean "electric power in watts."

Direct current

In direct current resistive circuits, electrical power is calculated using Joule's law:

P = VI \,

where P is the electric power, V the potential difference, and I the electric current.

In the case of resistive (Ohmic, or linear) loads, Joule's law can be combined with Ohm's law (I = V/R) to produce alternative expressions for the dissipated power:

P = I^2 R = \frac{V^2}{R},

where R is the electrical resistance.

Alternating current

In alternating current circuits, energy storage elements such as inductance and capacitance may result in periodic reversals of the direction of energy flow. The portion of power flow that, averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known as real power (also referred to as active power). That portion of power flow due to stored energy, that returns to the source in each cycle, is known as reactive power.

The relationship between real power, reactive power and apparent power can be expressed by representing the quantities as vectors. Real power is represented as a horizontal vector and reactive power is represented as a vertical vector. The apparent power vector is the hypotenuse of a right triangle formed by connecting the real and reactive power vectors. This representation is often called the power triangle. Using the Pythagorean Theorem, the relationship among real, reactive and apparent power is:

\mbox{(apparent power)}^2 = \mbox{(real power)}^2 + \mbox{(reactive power)}^2

Real and reactive powers can also be calculated directly from the apparent power, when the current and voltage are both sinusoids with a known phase angle between them:

\mbox{(real power)} = \mbox {(apparent power)}\cos(\theta)
\mbox{(reactive power)} = \mbox {(apparent power)}\sin(\theta)

The ratio of real power to apparent power is called power factor and is a number always between 0 and 1. Where the currents and voltages have non-sinusoidal forms, power factor is generalized to include the effects of distortion.

In space

Electrical power flows wherever electric and magnetic fields exist together and fluctuate in the same place. The simplest example of this is in electrical circuits, as the preceding section showed. In the general case, however, the simple equation P = IV must be replaced by a more complex calculation, the integral of the cross-product of the electrical and magnetic field vectors over a specified area, thus:

P = \int_S (\mathbf{E} \times \mathbf{H}) \cdot \mathbf{dA}. \,

The result is a scalar since it is the surface integralof thePoynting vector.



From Yahoo Answers

Question:Can you guys help me describe the effect of too much and too little electric current running through our bodies?

Answers:Too much current can cause tissue burns, stop the heart, and stop breathing. Your nervous system works by electrical impulses. If you had none, you'd die.

Question:Describe an experiment to determine the relationship between the magnetic effect of an electric current and the angle of deflection of a magnetic compass.

Answers:when current passes through a wire a megnetic feild is created around it in a circle like u point ur thumb of right hand in direction of current and curl ur fingures the tips of fingures will show direction of megnetic feild so when u bring a comppass near wire it will arrange itself in the direction of megnetic feild i-e, making tangent to wire

Question:Does being tased hurts very much? How about Electroshock Therapy? Which hurts more, the therapy or taser?

Answers:Being "tased" is meant to incapacitate a suspect; there are many different kinds -- some of the older ones, which used actual steel barbs with (serial numbers on them) had to be removed by paramedics, EMTs, RNs, or physicians, and caused trauma to the skin. Despite the kind of taser used, the subject does feel pain (unless s/he is chemically impaired with drugs like methamphetamine, ketamine, etc). ECT (electroconvulsive shock therapy) is done under general anesthesia so patients do not feel or remember any pain that would have accompanied seizure induction. Because a general anesthetic agent (e.g. pentothal, pentobarbital) is used, (and sometimes a short-acting analgesic, depending on the anesthetist), patients are unaware of the pain of the shocks themselves (and have no memory of having been shocked with the scalp electrodes). Neuromuscular blocking agents (e.g. rocuronium) prevent the violent muscular contractions that would otherwise be induced (and which would have caused muscular pain). Post ECT, patients may have a headache that requires acetaminophen (a.k.a. Tylenol, Paracetamol) or acetaminophen + codeine (e.g. Tylenol No. 3). So, in answer your question, "being tased" hurts more.

Question:IF i had a length of high resistance wire (let's say Nichrome), would would cause it to conduct and there for heat up, a higher voltage or current?

Answers:It's the current in a conductor which causes it to heat up. The higher the voltage, the lower the current assuming the power load stays the same. This is why electric distribution cables are at a very high voltage, so the current isn't as high, otherwise the cables would have to be impractically thick and expensive. If the voltage through a circuit is increased, then so is the current (and also the power, measured in Watts), which would cause an increase in heat. So increasing the voltage across your high resistance wire would cause the heat ( power load ) to increase because the current would also be increased.