Currents Definition and 497 Threads

Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material. When graphed, these circular currents within a piece of metal look vaguely like eddies or whirlpools in a liquid.
By Lenz's law, an eddy current creates a magnetic field that opposes the change in the magnetic field that created it, and thus eddy currents react back on the source of the magnetic field. For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to eddy currents induced in the surface by the moving magnetic field. This effect is employed in eddy current brakes which are used to stop rotating power tools quickly when they are turned off. The current flowing through the resistance of the conductor also dissipates energy as heat in the material. Thus eddy currents are a cause of energy loss in alternating current (AC) inductors, transformers, electric motors and generators, and other AC machinery, requiring special construction such as laminated magnetic cores or ferrite cores to minimize them. Eddy currents are also used to heat objects in induction heating furnaces and equipment, and to detect cracks and flaws in metal parts using eddy-current testing instruments.

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  1. K

    Finding unknown currents in a circuit, mass of wire

    Homework Statement The following two problems concern the diagram that is attached. In the circuit shown, the batteries have negligible internal resistance. The currents through the 6.0 and 2.0 Ohm resistors are 5.00 and 3.00 A respectively and their directions are shown. The Emf of one...
  2. L

    Question about oscillating currents. (the sign of the phase constant)

    Homework Statement This is more of a conceptual problem I'm having with in a problem. I'm trying to find the phase constant of the equation for a capacitor that is charging at time zero. I'm having trouble with figuring out what sign the phase constant should have. Homework Equations...
  3. O

    How to calculate power loss due to eddy currents

    Homework Statement What equation do i use to calculate the power loss due to eddy currents? I can't find an equation anywhere online.
  4. F

    Retarding force of eddy currents in a disc

    Homework Statement How to calculate braking force generated by eddy currents. If there is a disc of radius r with conductivity K, with a magnet located at a distance r-d from the center of the disc with a magnetic field B, what is the retarding force of the magnetic field created by the eddy...
  5. J

    Net Force Produced By Currents In Wire And Square Loop

    Homework Statement A long, straight wire carries a current of 14A. Next to the wire is a square loop with sides 1.0m in length, as shown in the figure. The loop carries a current of 2.5A in the direction indicated. http://img69.imageshack.us/i/walker2260.jpg/ Calculate the magnitude of...
  6. A

    Understanding Phase & Line Currents in Star Connections

    in star connecting load why the phase current is equal to line current? advanced thanks.
  7. B

    Magnetic Fields & Metal: Exploring Currents & Brakes

    let's say we have a square piece of metal and we put this piece in a variable magnetic field, would it get heated? is this the idea of a current? If we have a rotating metal disk (as the case of a saw for instance), and then we put this rotating disk in a constant uniform magnetic field such...
  8. T

    Why do eddy currents flow the opposite direction to the right-hand-screw-rule?

    I've noticed in demonstrations that when you've got a magnetic field going through an iron core, say coming out of the page at you, that the eddy currents circulating perpendicular to them will flow clock-wise. But if you had a wire loop and put a current through it clockwise that it would...
  9. B

    Can Magnetic Fields Heat Metal & Create Eddy Currents?

    let's say we have a square piece of metal and we put this piece in a variable magnetic field, would it get heated? is this the idea of a current? If we have a rotating metal disk (as the case of a saw for instance), and the we put this rotating disk in a constant uniform magnetic field such...
  10. W

    Magnetic field and currents problem

    Homework Statement 8) A long straight horizontal wire is oriented along the north-south direction. Conventional current flows through the wire from south to north. a) What is the direction of the magnetic field at a point 5.0 cm above the wire? b) A compass placed 5.0 cm above the wire...
  11. PainterGuy

    Understanding Drift and Diffusion Currents in PN Junctions

    hello everyone,:wink: is this definition correct? drift current is flow due to applied voltage across P-N junction & Diffusion is flow due to different concentration of hole & electron across P-N junction. warning: [i read this definition somewhere, hence not my own work] 1:-- the...
  12. W

    Eddy currents and current induction questions

    Homework Statement A solenoid is connected to a power supply and a ring of metal is sitting on top of the solenoid. When the power supply is turned on the ring jumps up off the solenoid. a) Suppose that when you look down at the solenoid from above. The current due to the power supply is...
  13. C

    Why is the RC circuit homework statement giving different results than expected?

    Homework Statement The Attempt at a Solution As at t=0 the capacitor behaves as a wire ... so the circuit at t=0 will be like: q at any time for this circuit will be, q \ = \ CE(1-e^{-t/\tau}) + Q_oe^{-t/\tau} differentiating it will give current, i \ = \ \frac{Ee^{-t/\tau}}{R} \ - \...
  14. 1

    Question on directions of currents and magnetic fields

    How do I find the direction of these things? The right hand rule only works when I have the current and it's asking for clockwise/counter-clockwise, and the left hand rule only works when I have two of the three: Current, Magnetic Field, and Force. Example...
  15. B

    Kirchoff's Laws to find all currents flowing in the cct

    1) Using Kirchoff's Laws, find all the currents flowing in the cct. 2) Hence, determine the voltages across all resistors and check that each loop complies with KVL
  16. PainterGuy

    Understanding Electric Currents in Fig 5.13 & 5.14(b)

    hello, this is a pdf from a book:-- https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B_XrsbDdR9NEOGQzZTEzMmMtMDdhNS00OTJkLTljZmItY2UyYzFjZTllMzM0&hl=en&authkey=CNeg85sN in fig 5.13 12v supply and 8v supply are connected to each other. perhaps the ground symbol tells us...
  17. B

    Determine currents through all resistors

    1) Using Ohm's Law determine the eqv resistance across the battery. 2) Determine the currents through all resistors
  18. T

    How Do Split Brake Discs Affect Eddy Currents and Braking Force?

    Hey guys, the other day i was reading about eddy currents and it was getting intersting, except i had a few questions going through my mind that i couldn't find answers to and wanted to ask: say we have a circular bit of aluminium which is our brake disc and an electromagnet which induces the...
  19. R

    Currents and voltages with resistors

    Homework Statement Two identical long bulbs are wired in series and connected to a 4.5-volt battery pack. The resistance of each bulb is 50 ohms. a. Determine the voltage across each bulb. b. Determine current through each bulb. c. How does this compare to the current flowing out of...
  20. B

    Finding Currents in a Circuit: Kirchhoff's Law

    Homework Statement Find the currents [PLAIN]http://img695.imageshack.us/img695/7042/78217840.png Homework Equations Using Kirchhoff's Law, Loop, Junction ruleThe Attempt at a Solution The problem here is the inside circuit .I try to solve it as one emf, one resistance, but honestly I do not...
  21. N

    Calculating the currents in each resistor

    Homework Statement Calculate the currents in each resistor in this figure: http://session.masteringphysics.com/problemAsset/1084690/2/GIANCOLI.ch26.p32.jpg Homework Equations Kirchhoff's rules In a closed loop: the sum of voltage is 0 A a junction: the sum of current is 0 The...
  22. B

    Find terminal voltage, currents of multiple batteries.

    Homework Statement Find terminal voltage, currents of each battery [PLAIN]http://img26.imageshack.us/img26/8667/questionl.jpg Homework Equations \epsilon=I(R + ri) Vterminal= \epsilon \pm I*ri The Attempt at a Solution Find current: I=\frac{\epsilon1+\epsilon2+\epsilon3}{R+r1+r2+r3} Fine...
  23. M

    Loop Currents for Circuits (using matrix)

    Homework Statement The question is just asking to find the current through each of the resistors. Our teacher taught us this loop current method, and then we have to use a matrix to solve for the currents. i1 denotes loop 1 and current, etc. Homework Equations Loop 1: 4i1 - 2i2...
  24. N

    How Do You Calculate Charge in Parallel Capacitors?

    Homework Statement A 4.0-µF and an 7.0-µF capacitor are connected in parallel across a 25-V battery. Find the total charge stored on the two capacitors. Homework Equations q=Cv The Attempt at a Solution i did the eqivalent capactiy*25 volts to get 275 C as the charge but that...
  25. B

    How Do You Calculate Eddy Currents in a Circular Petri Dish?

    Homework Statement I am doing undergraduate research in a Biomedical Engineering Lab. We are working on nervous tissue engineering and looking at how different stimuli effect nerve growth. I am struggling with some of the physics. I am wondering if anyone could teach me how to calculate or...
  26. Femme_physics

    The point of alternating currents?

    I'm not sure I understand the point of alternating currents. From what I understand, it's electrons spinning in circles. So, is alternating currents essentially the way we store electrons? That way we keep them spinning around themselves?
  27. P

    Why Do Eddy Currents Form Circular Loops?

    i know i haven't understood the concept of eddy current and that is why i am posting this question: why are EDDY currents CIRCULAR (why form a loop in circular form)? please also explain this answer that statement that i found on : http://wiki.answers.com/Q/What_is_an_eddy_current "When...
  28. P

    Unraveling the Mystery of Eddy Currents

    i know i haven't understood the concept of eddy current and that is why i am posting this question: why are EDDY CURRENTS circular?
  29. B

    Why do two wires with currents in the same direction attract?

    I understand that using F = qv x B and the fact that positive currents generate counter clockwise magnetic fields, it can be concluded that two parallel wires carrying currents in the same direction attract. But what about when you think about it in the frame of reference of the charges in...
  30. L

    Show Noether Currents Given Dirac Eq & Conjugate

    Given the Dirac equation and its conjugate, how do I show that j^\mu = \bar{\psi} \gamma^\mu \psi is a Noether current? Is there even a standard formula for Noether currents or does it vary depending on each individual case? And if it does vary, how do I go about figuring how to apply it to...
  31. R

    What is the correct equation for calculating the currents in this circuit?

    Homework Statement Use Kirchhoff's rules to find the currents I1, I2 and I3 shown in the circuit below: [PLAIN]http://img811.imageshack.us/img811/3640/13608736.gif The Attempt at a Solution Kirchhoff's junction law gives: I1+I2-I3=0 I have calculated the voltage rise/drop in the...
  32. M

    AC vs DC: Exploring the Effects of Earth's Magnetic Field on Currents

    Homework Statement Hello i just have an interesting question, and was wondering if my answer to it had any validity. Ok so i did an experiment The Current balance experiment(for everyone not familiar with it, you can google it and get all the info you want) to try and estimate mu naught...
  33. J

    Engineering Determine currents of a circuit

    Homework Statement For the circuit in the figure, determine i1to i5 http://i55.tinypic.com/dokoc5.png Homework Equations current division: in = (Requivalence / Rn) * itotal Requivalence in parallel = R1*R2 / R1+R2 The Attempt at a Solution || (parallel), and +s (series) 1ohms ||...
  34. M

    Currents in the Universe: Understanding Their Effects

    It is usually said that gravitation is the dominant "force" on large scale, because Universe is electrically neutral on large scale. But what about currents? Maybe effects of currents are disappearing statistically, but I think that this is not trivial. What is the truth?
  35. J

    Calculating currents homework question

    Homework Statement I have a closed circuit, with 7 bulbs, all of unequal resistances. I know three currents, 300, 200 and 500. I have 4 unknown currents, those fllowing through A, b, C and D which I need to find. There is a graphic for this which I don't know how to insert, but the...
  36. A

    What Is the Direction of the Magnetic Field at Points a, b, and c?

    Homework Statement Four electric currents, equal in magnitude are arranged at the corners of a square as shown in the figure...
  37. A

    What Determines the Magnetic Field Direction in a Square Current Loop?

    Homework Statement Four electric currents, equal in magnitude are arranged at the corners of a square as shown in the figure...
  38. L

    Eddy currents due t into the core of the Earth

    Hello, Those days I wonder if the magnetic field from the Sun, could make the Eddy's currents inside the core (ferromagnetic) of our planet, because of the rotation of the Earth. (like a induction cooker) It would result a magnetic field on the Heart, and if the magnetic field from the Sun...
  39. P

    Surface charges and surface current on conductors carrying steady currents

    It is known that on a wire carrying steady current there are surface charges (and hence electric field outside the wire, but let's forget about it). This surface charges play an important role to maintain a uniform electric filed along the whole wire. There are only few geometries for which one...
  40. P

    Calculating Force Between 2 Conductors with Constant Currents

    Homework Statement Is given two parallel conductors of equal and finite length L. The distance between them is a. On these conductors flow constant currents I_1, I_2 in same dirrections. If the line which connecting their centars built angle \alpha whith dirrection of conductor find the force...
  41. J

    Force between two perpendicular currents

    Homework Statement Why is the force of a wire to another wire that is perpendicular to it zero? Homework Equations biot-savart law The Attempt at a Solution
  42. G

    Difficult problem about eddy currents and terminal velocity

    Hey guy, Ive been recently investigating the phenomena of eddy currents, particularly that of a magnet falling down a copper tube. To the people who haven't seen it before, basically the magnet reaches terminal velocity in the copper tube extremely quickly, and takes a long time to leave the...
  43. A

    Needing help with calculating currents

    Homework Statement The currents in a DC circuit is to be calculated, and I end up with a system of 5 equations and 5 unknowns. This is where my headache starts. Homework Equations (1) I_{1}=I_{2} + I_{3} (2) I_{2}=I_{4} + I_{5} (3) 6 - 2I_{1} - I_{3} = 0 (4) 4.5 - 5I_{2} - 3I_{5} +...
  44. G

    Noether currents associated with diffeomorphism invariance

    Having some generic curved spacetime, what are the Noether currents that are guaranteed to exist by diffeomorphism invariance? Is the energy-momentum tensor such a current?
  45. J

    Understanding Alternating Currents Motors - Issues & Solutions

    Hi,I am new here^^17 years old... I got a little problem with understanding how simple alternating currents motors/AC Motors work... Anyone has a good Table or Figure?? Magnetic problems: 1)Whats the difference between inducing current and normal electric current?? 2)Whats the meaning...
  46. M

    Designing an AC Ammeter with 0.15V Voltage and Currents of 200mA, 500mA, 1A

    Hello all! I must design an ac ammeter which uses successive approximation ADC,has a specified voltage of 0,15V and individual entrance resistors for measuring currents of 200mA,500mA,1A. Any suggestions on how to build such a device would be highly appreciated!
  47. G

    Understanding Household Currents

    Currents! Homework Statement Why can't the potential difference in household currents be changed? Homework Equations Conceptual. The Attempt at a Solution Is it just because the fuse will ignite if overheated?
  48. S

    Coaxial cable with unequal currents

    1. Homework Statement I just have a question concerning an unbalanced coaxial cable. By unbalanced, I mean that the inner and outer currents are not equal magnitudes of the opposite direction of each other. My homework problem has an inner current of +100[mA] and an outer current of 90[mA] in...
  49. L

    Calculating Currents in a 3-Phase 3-Wire System

    The figure below shows three 1-phase appliances connected in star to a 3-phase 3-wire system. The supply voltage is star-connected with a line-to-line voltage of 415V, 50Hz. I want to calculate: \overline{I_a}, \overline{I_b}, \overline{I_c} The problem is I'm getting wrong phases that...
  50. J

    Alternating currents, what's the use?

    today i was introduced to AC currents in my electricty/magnetism class. Perhaps we're not to this point yet, but I'm not sure what's the use of an alternating current, compared to DC currents. dc currents can only go in one direction, but what's the practical use for alternating one? sorry...
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