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. A

    Induced EMF in Rotating Magnetic Field with B=0.5B0z^

    A magnetic field of B=0.5B0z^ is present when a loop with radius r is rotated around the y-axis with angular velocity of w=9 rad/s. What is the induced emf? Well: flux = SB*dA = SBdAcosq(t) = Bcosq(t)SdA = 0.5B0*pi*r^2cosq(t) d(flux)/dt = 0.5B0*pi*r^2*-sinq(t)*dq(t)/dt =...
  2. K

    Determining the Direction of Induced Emf in an Aeroplane's Wing Tips

    Homework Statement An aeroplane in horizontal flight has a wing span of 50 m and is traveling due north at a speed of 500 kmph. Calculate the emf induced between the wing tips of the aeroplane. The value of the vertical component of the Earth’s magnetic field is 5.5 x 10-5 T. What...
  3. E

    Question in determing the direction of induced emf?

    How to determine the direction of emf induced in an alternator without the need to look at the graph. (i.e) using Lenz's Law or Faraday's Laws of determining the direction? Consider for example the instant when B and dA are perpendicular and no flux is present? Thanks for helping!
  4. B

    What Causes Induced EMF in an Incomplete Ring?

    Dear colleagues, My question here refers to the EMF induced in an incomplete ring that is stationary amidst a time dependent magnetic field. We know from Faraday's law that EMF is created upon changing the magnetic flux, either by changing the intensity of the field, or by changing the...
  5. M

    Magnetic Flux and Induced EMF at South Pole

    Homework Statement The magnetic field strength at the South Pole is around 66μT, and magnetic field lines point out of the Earth perpendicular to the surface. The sled is a metal square of side 0.95 m (with non-conducting wooden slats in between). What is the magnetic flux through the sled...
  6. S

    Find the max induced EMF in the loop by the changing loop

    Homework Statement I have attached the two questions. the first has a diagram. 1. Find the MAX Induced EMF (in volts) in the loop by the changing current? 2. What is the magnitude of the EMF induced in the loop by the changing current at t=.8 (seconds) answer in Volts? Homework...
  7. NaOH

    How Does a Voltmeter Respond to Induced EMF in a Circular Wire Loop?

    Homework Statement It is often discussed that a changing magnetic field will create a non-conservative electric field, which would then induces an EMF and hence an induced current, but what happens when we connect voltmeters? Consider putting a circular loop of wire with some resistance in...
  8. L

    Why is the EMF Zero in a Uniformly Moving Coil Through a Magnetic Field?

    A flat, rectangular coil of dimensions L and w is pulled with uniform speed v through a uniform magnetic field B with the plane of its area perpendicular to the field (the figure (Figure 1) ).How is the emf zero? My reasons as to why it's zero: the trig part forces the problem to be zero...
  9. B

    Potential difference of an inductor equal to minus the induced emf?

    Hi, Two questions I'm finding it hard to get my head around with inductors... Say we have an increasing current, this is going to cause an increase in flux in one direction, which will induce an emf to drive a current which produces a magnetic field to oppose the change in flux. Than doing...
  10. L

    What is the induced emf in one of the windings?

    1. A solenoid of length 3.64 cm and diameter 0.914 cm is wound with 170 turns per cm. If the current is decreasing at a rate of 36.3 A/s, what is the induced emf in one of the windings? this is what I did: Length of the solenoid l = 3.64cm = (3.64...
  11. K

    Conflict: why synchronous machine doesn't produce induced EMF in the DC field wires

    conflict: why synchronous machine doesn't produce induced EMF in the DC field wires Isn't it current path through winding which produce magnetic field (rotating) the must produce EMF in the dc winding of field (same theory of induction) but why isn't true in synchronous
  12. K

    Induced emf and flux in a transformer, conceptual question

    Hey Folks, I am a third year electrical engineering student and just want to clarify a concept involving electromagnetics/transformers. When supplying magnetization current to a transformer (assume sinusoidal), this induces a time changing magnetic flux in the core. The time changing magnetic...
  13. A

    Difference between induced emf and counter emf

    What is the difference between induced electromotive force and counter electromotive force? The way they are generated seems the same to me. Can someone tell me the difference and how they are different?
  14. 1

    Find induced emf (among other things) in this current + motion

    Homework Statement Attached Homework Equations The Attempt at a Solution I'm pretty confused about this one. First, I thought that maybe the magnetic field's non uniformity didn't matter since the rate of change of flux was going to be the rate of change of area, but that's...
  15. S

    Induced emf when dropping a magnet into a solenoid

    I am having some trouble understanding why and how the emf changes when a bar magnet is dropped into a solenoid. As the magnet moves down (have not entered the solenoid yet) it experences an increase in magnetic field. As it enters the magnet it experiences an increase in magnetic still until...
  16. N

    Calculating Induced EMF for a Changing Magnetic Field Loop

    1. A rectangular wire loop (length 60 cm, width 40 cm) lies completely within a perpendicular and uniform magnetic field of magnitude of 0.5 T. If the length of the loop starts increasing at a rate of 20 mm/s at time t = 0, while the width is decreasing at the same rate, what is the magnitude...
  17. O

    Nonsinusoidal Induced EMF: Can a Rotating Loop Create a Non-Sinusoidal EMF?

    Homework Statement The book describes a rectangular loop of wire in a uniform magnetic field created by a pair of coils. It then discusses the emf that is produced when the rectangular loop rotates with constant angular velocity, noting that the induced emf varies sinusoidally with time. At...
  18. T

    Faradays law/Lens law: how does the induced emf 'act'

    Hello everyone. I'm trying to understand how an 'induced' emf behaves on a current carrying conductor. Basically the example that's giving me problems is in the case of a motor, if a current starts to pass through a coil it will get an induced 'emf' to oppose the motion. And hence it...
  19. E

    Exploring Induced EMF: Polarity and Direction of a Bicycle Wheel Experiment

    Homework Statement A student is experimenting with a bicycle wheel. He turns the bicycle upside down and spins the wheel in a vertical plane at a constant rate. The diagram shows the wheel. At the place where the experiment is performed, the Earth‟s magnetic field is in a horizontal direction...
  20. P

    Induced emf is my answer correct ( not sure about b value)

    Homework Statement The magnetic field perpendicular to a circular wire loop 6.8cm in diameter is changed from +0.39 T to -0.62 T in 160 m/s , where + means the field points away from an observer and - toward the observer. Homework Equations e=Blv The Attempt at a Solution e=...
  21. G

    Induced EMF on a moving rod next to a line of current.

    Homework Statement I'm currently reviewing some of the old E&M material that I haven't seen in a while and have gotten stuck on this problem. I'm sure I'm just making a simple mistake but I can't seem to make any headway. Here's the problem: Figure 35-32 shows a copper rod moving with...
  22. A

    Calculating Induced Current in an Expanding Loop

    Homework Statement Imagine a pliable round metal loop that can expand or contract. In a region with a constant magnetic field B0 that is oriented perpendicular to the plane of the loop, suppose that the loop expands, with its radius growing with time as r = r0(1+at2). As the loop expands and...
  23. K

    How can I find the number of turns in a solenoid winding?

    A 30mH solenoid inductor is wound on a form 0.4 m in length and 0.1 m in diameter. A coil is tightly wound around the solenoid at its center. the coil resistance is 5.0 ohms. The mutual inductance of the coil and solenoid is 60μH. At a given instant, the current in the solenoid is 0.3A and is...
  24. Y

    Any difference between induced EMF of an air coil vs with magnet core?

    Faraday's Law of Induction: EMF=-\frac{\partial \Phi}{\partial t} If two identical coil with same turns of wire and same dimension ( same cross section area in the middle), one has only air and the other has a magnet core. If the two placed in an uniform varying magnetic field, is there...
  25. P

    Nit-Picky Questions on Induced EMF, and Electromagnetism in General

    I just need some clarification on the induced EMF Formula.. I know that the negative sign in front of N * dFlux/dt is just an indicator of Lenz's law, but if we were given the magnetic flux as a function of time, would there be situations where -d/dt(F(t)) would be different than just...
  26. E

    Faraday's Law of Induction: Why is EMF Inversely Proportional to Time?

    Hi, I am doing coursework on Faraday's law of induction. My assignment was to carry out experiments which confirm Faraday's law and also to explain the physics of how faraday's law works... My experiments all worked perfectly, producing straight line graphs showing that: \epsilon\propto N...
  27. M

    Induced Emf in a Rectangular Coil

    Homework Statement A rectangular coil of 90 turns has dimensions of 22 x 65 cm and is located in a uniform 1 T magnetic field. In 1.7 s, the plane of the coil is rotated from a position where it makes an angle of 19° with the magnetic field to a position where it makes an angle of 64°...
  28. S

    Polarity of Induced EMF in a Conducting Ring

    I have a question regarding a conducting loop of radius r in a changing magnetic field B. I understand and can determine the direction of the induced current which implies the existing of an electric field that is tangential to the loop. Since this is a closed loop, I am having trouble...
  29. S

    Induced emf in a solenoid from a current carrying wire nearby

    Say I have a solenoid that has a magnetic field Bo Sin(ωt) passing through its axis. What will be the EMF induced in the solenoid as a result ? I know it'll be a function of Cos (ωt) but I am more interested in how my choice of the solenoid will affect the EMF that I'll ultimately measure...
  30. S

    Average induced emf in coil with turns

    Homework Statement A closely wound rectangular coil of 95.0 turns has dimensions of 29.0 cm by 40.0 cm. The plane of the coil is rotated from a position where it makes an angle of 39.0 degrees with a magnetic field of 1.60 T to a position perpendicular to the field. The rotation takes 0.120 s...
  31. DocZaius

    Induced EMF through variable-resistance-wire due to Faraday's Law

    Hello, While considering Faraday's Law of Induction, I tried to think of a situation which would expose some difficulties I have with the notion that there is an induced EMF without clear regions of relatively high and low voltage (as in a battery). Here is what I thought would get me started...
  32. M

    Induced EMF through a coil How/Why?

    Homework Statement In a lab I passed a magnet through a coil and measured the voltage. I need/want to know why when moving a magnet through a coil you induce a current. I don't necessarily need a direct answer - I more just want to know if my argument holds and where to look if I'm...
  33. jegues

    Do Induced EMF Direction and Voltage Signs Contradict?

    Homework Statement Are these two things not contradicting? Homework Equations The Attempt at a Solution See figure attached. Aren't these contradicting one another? They need a negative sign in there, correct?
  34. jegues

    Direction of Induced EMF (Faradays Law) Confusion

    Homework Statement See figure attached. Homework Equations The Attempt at a Solution If we attempt to solve part a) in this question we encounter several confusions. If we assume i(t) to be a positive current, the magnetic flux will flow through the core in a counter...
  35. jegues

    Induced EMF Moving Current Loop

    Homework Statement See figure attached. Homework Equations The Attempt at a Solution I'm having some confusion about the negative sign within this problem. If you compute the curl of the expression shown in the figure you'll find that it's equal to, -10v_{0} \hat{k} But...
  36. A

    Repeat Q: Ignore. Show what the magnitude of induced emf

    Show what the magnitude of induced emf Homework Statement Consider a magnetic field B = K(x3z2,0, -x2z3)sinωt in the region of interest, where K and ω are positive constants and t is variable time. Show that the magnitude of the induced emf around a circle R in the plane z = a with its center...
  37. A

    Show what the magnitude of induced emf

    Someone who knows what they are talking about: Show what the magnitude of induced emf Homework Statement Consider a magnetic field B = K(x3z2,0, -x2z3)sinωt in the region of interest, where K and ω are positive constants and t is variable time. Show that the magnitude of the induced emf...
  38. G

    Induced emf with no change in flux

    If a loop is moved in a uniform magnetic field such that the direction of its motion is perpendicular to the magnetic field, then no change of flux occurs. Then according to faraday, no emf is induced... But I was thinking that can the emf be produced in it due to the deflection force?
  39. B

    Understanding Induced EMF: Basic Concepts and Doubts

    Basic question on induced emf - ATTACHMENT CORRECTED PLEASE RE-READ Hi Please read the attached doc which is excerpted from the text University Physics by Young & Freedman.The discussion is on the emf induced in a loop made of a superconducting material when a magnet is moved towards it...
  40. Y

    Equivalent circuit of induced EMF of coil.

    I want to model a coil circuit with induced EMF. I want to verify that my model is correct in a circuit. That the voltage developed across the load is calculated correctly. From the equivalent circuit, you can see the current depend on the jX_L and it acts as a low pass filter in the complete...
  41. C

    Estimate induced emf in the coil

    [Solved] Estimate induced emf in the coil Homework Statement Consider a single-loop coil whose magnetic flux is given by this image: Estimate the induced emf in the coil at times near t = 0.3s, t = 0.4s and t = 0.5s This is for algebra-based physics. Homework Equations emf = -N*(change in...
  42. M

    Does the direction of winding affect the induced emf in a an inductor coil?

    Hi, What if the inductor coil is wound one way rather than the other. Then the circular flow of current will be clockwise in one case and anticlockwise in the other. And if the inductor is connected to an ac source, then the current will vary. Now in both the orientations, the variation...
  43. F

    Induced emf as a magnet passes through a solenoid

    I'm doing a lab write-up for physics 2. The experiment is about the title, a bar magnet being dropped through a solenoid. I have to explain four graphs that plot the change in emf (y axis) vs. time (x) axis. There are incoming and outgoing peaks on these graphs. I have to tie these results...
  44. F

    Induced Emf: Magnet Passes through a Coil

    I'm doing a lab write-up for physics 2. The experiment is about the title, a bar magnet being dropped through a solenoid. I have to explain four graphs that plot the change in emf (y axis) vs. time (x) axis. There are incoming and outgoing peaks on these graphs. I have to tie these results...
  45. G

    Induced emf when accelerates in uniform magnetic field

    Hi everyone, This question is bothering me for days and would hope to receive some help in this forum. This question is with regards to Faraday's law on EMI. Will an emf be induced when a copper rod is accelerated between a uniform magnetic field? My answer would be no if the rod has...
  46. A

    Induced EMF in Non-Uniform Magnetic Field

    Homework Statement A square wire loop of dimensions L X L (L = 15.5 cm) oriented in the xy-plane and centered on the x-axis enters a region where the magnetic field (B = 0.855 T) is first oriented in the +z- and then in the -z-direction as shown in the diagram below. (Note: W = 15.501 cm and...
  47. E

    Induced Emf in Rectangular Loop Problem

    Homework Statement "You push a rectangular loop of width .2m, length .8 m, and resistance R=200\Omega into a magnetic field (out of page) Bout=.4 T and a speed v=.2 m/s. The long side of the rectangle is parallel to the x axis. What is the current flowing in the loop?" Homework Equations I...
  48. F

    Induced emf in a rotating conductor

    Homework Statement Find the induced emf in a rotating disc of radius (a) in a magnetic field (B), rotating with uniform angular velocity (w). If the disc is replaced by a rotating conductor of length (a) fixed at one end find the induced emf in the conductor. 2. The attempt at a solution...
  49. B

    Induced EMF and angular velocity

    Homework Statement A circular coil of area A with N turns rotates at an angular velocity w about an axis perpendicular to its own axis and to a uniform magnetic field B. Find an expression for the induced EMF in the coil in terms of B, N, A, w and time t. What is the orientation of the coil...
  50. L

    Induced Voltage Equation: Solving for Constant Magnetic Field

    I know the equation for emf: -dflux/dt= -dB/dt*A . In my case everything is constant accept the magnetic field and I am unable to find the equation for that. I know that is what dB is for but that simply does not work in this case. What is the equation for induced voltage, when everything is...
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