What is Induced emf: Definition and 251 Discussions

In electromagnetism and electronics, electromotive force (emf, denoted





E




{\displaystyle {\mathcal {E}}}
and measured in volts) is the electrical action produced by a non-electrical source. Devices (known as transducers) provide an emf by converting other forms of energy into electrical energy, such as batteries (which convert chemical energy) or generators (which convert mechanical energy). Sometimes an analogy to water pressure is used to describe electromotive force. (The word "force" in this case is not used to mean forces of interaction between bodies).
In electromagnetic induction, emf can be defined around a closed loop of conductor as the electromagnetic work that would be done on an electric charge (an electron in this instance) if it travels once around the loop. For a time-varying magnetic flux linking a loop, the electric potential's scalar field is not defined due to a circulating electric vector field, but an emf nevertheless does work that can be measured as a virtual electric potential around the loop.In the case of a two-terminal device (such as an electrochemical cell) which is modeled as a Thévenin's equivalent circuit, the equivalent emf can be measured as the open-circuit potential difference, or voltage, between the two terminals. This potential difference can drive an electric current if an external circuit is attached to the terminals, in which case the device becomes the voltage source of that circuit.

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  1. P

    Induced Magnetic Moment (vector) vs. Induced EMF (scalar)

    When I induce magnetic flux through a closed loop, I should expect the lines of flux produced by current in that loop to oppose the change of flux through that loop. But what happens when that loop, say a rectangular loop, is curved into the shape of the letter J (like a candy cane) and my flux...
  2. G

    Calculating Induced EMF in a Bent Loop with Changing Magnetic Field

    Homework Statement The figure below shows a closed loop of wire that consists of a pair of equal semicircles, of radius 7.0 cm, lying in mutually perpendicular planes. The loop was formed by folding a flat circular loop along a diameter until the two halves became perpendicular to each other...
  3. Quadrat

    Induced emf in various rectangular loops

    Homework Statement [/B] Three conducting loops, all with the same resistance ##R## move towards a uniform and constant magnetic field, all with the same velocity ##v##. Their relative sizes can be identified by the grid. As the loops move into the magnetic field an induced current begins to...
  4. I

    What is the maximum induced EMF in a rotating contour near a straight wire?

    Homework Statement In the very long wire conductor as shown on the picture , flows a current magnitude I. Near the wire there is a square shaped contour , with length a ,which rotates around its axis a constant angular velocity. Axis of square contour and a straight line conductor are...
  5. ReidMerrill

    What is the Correct Method for Finding Induced EMF in a Coil?

    Homework Statement A 4.0 cm diameter coil (1000 turns) is at rest in the XY plane. The magnetic field in this region of space is given by B=(0.30i + 0.50t2 k) What is the induced EMF in the coil at t=2.0 seconds Homework Equations ε=dΦ/dt Φ=ABcosθ The Attempt at a Solution ε=[/B]d/dt(...
  6. S

    Induced EMF in a Triangle: Determining EMF and External Force

    Homework Statement Magnetic Field B is going into the plane. Bar is moving to the right with velocity v. Neglect resistance of the conducting bar and all contacts. a.) Determine the induced emf in the circuit. b.) To maintain the uniform motion of the conductive bar, there must be an external...
  7. gsmtiger18

    Line charge creating induced emf and displacement current

    Homework Statement A long, straight wire has a line charge, λ, that varies in time according to: λ = λ0 exp(-βt). A square wire loop of dimension a is located adjacent to the wire at a distance of a from the wire. Calculate expressions for the displacement current at the center of the wire loop...
  8. NihalRi

    Find the induced EMF and Lenz' law

    Homework Statement There is a magnetic field directed inside/ towards the screen. There is also a single rectangular coil parallel to the screen where one of the sides are movable. The magnetc field increases, what happens to this movable side of the coil. This is my description of the...
  9. O

    Induced EMF from moving conducting rod

    Homework Statement Lets say we have two horizontal rails connected by a resistor to the left, and we have a movable conducting rod that slides without friction on the rails.There is a uniform magnetic field going into the page. Homework Equations F = ILB[/B]The Attempt at a Solution Now, I...
  10. B

    What Causes Induced EMF in Moving Conductors and Transformers?

    I read many different explanation but none of them is satisfying. There is two cases: 1. I move an iron rod in a magnetic field, between two bar magnet for example. To this case I read that when I move the rod the electrons on it also move and moving charges produce magnetic field which...
  11. C

    Zero Induced EMF: Find Rate of B Change for Problem 9

    Homework Statement Question I need help with: If wire AB of problem 9 moves with a velocity of 50.0 m/s to the right, at what rate should B change when the wire is at a distance of 5.00 cm from the left end of the loop, such that there will be no induced emf in the circuit? Answer: 300 T/s...
  12. PhiowPhi

    Would the induced EMF & current change?

    From this diagram: If a conductor of length(##L##) is moving with a velocity(##v##) inside constant magnetic field(##B##), there is an induced EMF as indicated at the top of copper slab, and connected to a load and current will flow. I've been curious with the way the wires are connected to...
  13. A

    Parasitics in Solenoids: The Relationship Between AC Frequency and Induced EMF

    If you have a one solenoid inside another (with the same number of coils of wire) the current is going through the inner one. How does the induced EMF in the outer one depend on the frequency of AC in the inner one
  14. Kerrigoth

    Calculating multiple magnitudes for induced electric field

    Let's say I place two different size conducting loops inside of a uniform magnetic field. For the sake of the diagram I have the loops share a point, but just treat them as though they are not touching. Then I decrease the value of the magnetic field very quickly. \frac{dB}{dt} =...
  15. PhiowPhi

    Induced EMF in a small volume of a conductor?

    Assume we have the following copper slab, with volume ##V##, and introduced a magnetic field covering a small area of it like so: There is change in magnetic flux(##\phi##) due to the magnetic field increasing/decreasing, and via Faraday & Len'z law of electromagnetism, there is an induced...
  16. Cookiey

    Induced Emf due to changing magnetic flux

    If we have a conducting loop, and a magnet (say North pole first) is pushed into it; that induces and emf and hence a current in the coil due to the changing magnetic flux, right? According to Lenz' law, the direction of the current is such that it opposes the change. So from the side the magnet...
  17. TheJfactors

    Induced EMF due to changing current in straight wire

    I see a lot of problems on constant current inducing a current in another wire due to its associated magnetic field, but not so much a single wire's induced electric field. If the current along an infinitely long wire is flowing out of the page and inducing a counter-clockwise spinning magnetic...
  18. T

    Finding Induced EMF of secondary coil

    Homework Statement There's a primary and secondary coil. The primary coil has 15 turns, and the secondary coil has 16000 loops. The 25000 V is applied to the primary coil, what will be the induced emf in the secondary coil. Homework EquationsThe Attempt at a Solution Very confused as to how...
  19. Salman72

    Why is there an induced EMF in this case?

    Hey people I was wondering why is an emf induced in a loop which moves perpendicular to a uniform magnetic field. I mean, say the loop is a square of area r^2, and moves in the xy plane, where there is a uniform magnetic field B in the z direction. Why will an emf be induced? The flux remains...
  20. N

    Calculating Induced EMF in a Long Wire Using Flux and Lenz Law

    Hi everyone, I am currently stuck on this problem, and I was wondering if I could get assistance, at least in getting started. It'd be greatly appreciated!
  21. R

    MRI Electromagnet Interference in a Figure-8 cable

    The prevailing wisdom is that coiling excess signal cable under an MRI magnet for storage in a figure-8 configuration will reduce or eliminate induced noise onto the cable from the pulsing fields from gradient coils. I am starting to doubt this, however. A quick diagram of a figure-8 looped...
  22. I

    Direction of induced EMF in loop

    Homework Statement Homework Equations Faraday's and Lenz' law. emf = -d(ф)/dt The Attempt at a Solution The current is increasing upwards during that time interval. Thus, by Right-Hand-Rule, the magnetic field goes into the page at the loop. Lenz' law says that the loop's induced emf...
  23. Shinwasha

    Calculate the magnitude of the induced EMF in the loop

    Homework Statement A square conducting loop lies in the xy plane of an xyz coordinate system. The loop is in a uniform magnetic field that points in the positive z direction and is decreasing at a rate of 0.070 T/s. What are (a) the magnitude of the induced EMF in the loop and (b) the direction...
  24. I

    Induced EMF Problem 6.6 Solution Help

    Homework Statement It's Problem 6.6 on this page: http://web.ece.ucdavis.edu/~lxgliu/eec130a/2012winter/hw9s.pdf It has the solution and everything. I don't understand part (b) though. Homework EquationsThe Attempt at a Solution [/B] For part (b), I thought the direction of the induced...
  25. PhiowPhi

    Induced EMF within a part of a parallel circuit?

    A Power supply is connected to a parallel circuit with resistance R, there are three wires connected in parallel where the voltages would be the same,however, the current would be dived based on the number of wires and the resistance is the same. What would happen if wire (c) was placed in a...
  26. C

    The physics behind an electric motor and induced emf?

    I am having trouble understanding a process in the electric motor. using this website as a frame of reference (http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/mothow.html), as an electric current is passed into that magnetic field, shouldn't the current also be creating a magnetic field, and...
  27. W

    Induced EMF in Coaxial Loops: Calculating Separation Distance

    1. A very large loop of metal wire with radius 1meter is driven with a linearly increasing current at a rate of 200amps/second . A very small metal wire loop with radius 5centimeter is positioned a small distance away with its center on the same axis (the loops are coaxial). The small loop...
  28. Blitzmeister

    Induced emf of Sliding Wire Down Rails

    Homework Statement [This is the best image I could find] The link on the bottom part of the rails is now removed creating two parallel wires that are not connected (so no longer U shaped but just two rails). Now a wire of length L, mass m and resistance R slides without friction down parallel...
  29. T

    Induced EMF, bar moving through magnetic field

    Homework Statement A long, straight wire as shown below carries a constant current I. A metal bar with length L is moving at a constant velocity V as shown. Point ‘a’ is a distance ‘d’ from the wire. a) Derive an expression for the EMF induced in the bar. b) Which point ‘a’ or ‘b’ is at a...
  30. PhiowPhi

    Would a wire's magnetic field increase the net-B field?

    From the following diagram: When a wire is placed in a magnetic field ##B##, and current starts to flow within that wire. It creates it's own magnetic field(##B_w##). Will ##B_w## interact with the external field ##B##? If it does, is the result of the interaction the Lorentz force? Also...
  31. arpon

    Does Increasing the Area of a Loop in a Magnetic Field Induce Current?

    Homework Statement [/B] Suppose, there is a 'constant' magnetic field ##B## in the upward direction. A loop of conductive wire(XYZ in the picture, which is connected to a closed circuit with resistance R) is placed horizontally on it. The area of the loop is being increased with time. Will...
  32. J

    Is an induced emf always produced by a change in flux?

    Faradays law tells us that a change in flux induces an emf. Now consider the phenomenon of motional emf. It is observed across the ends of an open conductor (ie one which is not in a circuit). It is always discussed in connection to faradays law. Where is the change in flux in the case of a...
  33. Ahmad Kishki

    Which Method to Use for Calculating Induced EMF: Flux Rule or Lorentz Force?

    to calculate emf i can use the flux rule -d(magnetic flux)/dt or the lorentz force line integral across the circuit. Is there a gudeline to follow for knowing when to use either (or both) of these methods? Thanks :D
  34. Feodalherren

    Faraday's Law - induced emf of a rotating loop

    Homework Statement Consider the loop in the figure below. What is the maximum induced emf in each of the following cases if A = 600 cm2, ω = 31.0 rad/s, and B = 0.490 T? Rotating about x,y,z? Homework Equations Faraday's law The Attempt at a Solution This should be relatively easy since the...
  35. P

    How Does Induced EMF Vary Inside a Solenoid?

    Homework Statement The current in a solenoid (you may treat it as a long solenoid of length 2.0 m, turns, and radius 0.5 m) is decreasing at a rate of 1.7 A/s. What is the EMF at a point .35m inside the solenoid? Homework Equations B=N(mu_0*I)/(2a) where B=magnetic field, N=turns per...
  36. L

    Why Does Induced EMF Change in Generators but Not Moving Conductors?

    why does induced emf change in case of generators but remains constant in case of moving conductor?
  37. H

    What's the direction of induced emf here?

    This thing has confused me for long .I have not come across any satisfactory answers on my own or while browsing through the internet. Everybody has seen the example of a rod moving while on parallel rails, with magnetic field perpendicular to the plane of loop formed by rod and rails...
  38. A

    Induced Emf in an Inductor in an LRC circuit

    Homework Statement In the circuit shown in the figure, the switch S is closed at time t=0 with no initial charge on the capacitor. What is the induced emf if the inductor at time t=0? Homework Equations ε=-LdI/dt The Attempt at a Solution Initially the inductor acts like a broken...
  39. N

    Induced EMF in metal loop near RC circuit

    Homework Statement (Challenge Problem 80, Ch33) A rectangular metal loop with 0.050 ohms resistance is placed next to one wire of the RC circuit shown in the below figure. The capacitor is charged to 20 V with the polarity shown, then the switch is closed at t = 0s...
  40. L

    Induced EMF Question: Calculate the Answer

    Homework Statement A rectangular wire loop, 24mm by 36mm, is placed in a magnetic field. The flux density perpendicular to the loop is decreasing in magnitude at a rate of 0.2Wb/m^{2}/s. Calculate the induced emf around the loop. Homework Equations The problem is I don't know which...
  41. S

    Metal bar on rails, induced emf

    A metal bar with length L, mass m, and resistance R is placed on frictionless metal rails that are inclined at an angle α above the horizontal. The top end of the rails are connected with a conducting wire. The resistance of the rails and wire are negligible. The rails are embedded in a uniform...
  42. K

    Induced EMF in house-hold WIFI

    Hi everybody, I was wondering how much induced EMF would be produced in a 20cm diameter coil with 100 loops produce. Has anyone conducted such an experiment? I am interested in quantitative values of the experiment. Thanks, KB
  43. C

    Finding the induced EMF on a bar - Faraday¡s law

    Homework Statement A conducting bar of length L moves with velocity v, in a rectangular region with a uniform and stationary magnetic field B_1. Near the bar, there is a cylindrical region of radius R with another magnetic field B_2 (it may vary on time). Both magentic field are normal to the...
  44. J

    Time dependence of induced emf in a loop

    Homework Statement A uniform magnetic field B is perpendicular to the plane of a circular wire loop of radius R. The magnitude of the field varies with time according to B=B0exp(-t/τ) where B0 and τ are constants. The time dependence of the induced emf in the loop is a) exp(-t2/τ2) b)...
  45. B

    Induced EMF on a rotating coil

    Homework Statement a coil starts perpendicular to a magnetic field of 0.365 tesla , it has a length of 0.15 cm and a height of 0.25 cm , it rotates with a frequency of 1800 rotation / min . it also has 30 turns calculate the mean induced Emf during quarter a rotation Homework Equations...
  46. M

    Electrodynamics question - Induced EMF

    > A very long straight wire carries a current I. A plane rectangular coil of high resistance, with sides of length $a$ and $b$, is coplanar with the wire. One of the sides of length $a$ is parallel to the wire and a distance $D$ from it; the opposite side is further from the wire. The coil is...
  47. V

    Average induced EMF and average induced current of a flipped loop?

    Homework Statement A wire loop of resistance R and area A has its normal along the direction of a uniform magnetic field, B. The loop is then flipped over in a time Δt so that its normal is opposite to the field. a) Calculate the average induced EMF. b) Find the average induced current. c) If...
  48. K

    Induced emf in coil with decreasing current

    A single-turn circular loop of radius R = 0.197 m is coaxial with a long 1740 turn solenoid of radius 0.0410 m and length 0.890 m, as seen in the figure below. (picture in URL)...
  49. PsychonautQQ

    Why is the answer to this GRE physics problem choice A?

    Homework Statement I don't understand why the answer in this problem is choice A. http://grephysics.net/ans/9277/57 I'm confused. Lets say the semi circle is FULLY outside of the circle. For awhile after that, the semi circle would be accelerating the rate at which a certain % of it is...
  50. J

    Calculating Induced EMF in Moving Wire

    In the classic problem of the induced EMF in a moving conductor (as in the picture above), the calculation for the induced EMF is as follows E=\frac{d\Phi_B}{dt}=\frac{d(BA)}{dt}=\frac{Bd(A)}{dt}=\frac{Bd(Lx)}{dt}=\frac{BLdx}{dt}=BLv. The derivation assumes that the magnetic field B is...
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