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. 1

    Calculating emf in a Loop with Changing Magnetic Field

    1. In the figure the square loop of wire has sides of length 2 cm. A magnetic field points out of the page; its magnitude is given by B = 4.0 t^2y where B is in teslas, t is in seconds and y is in meters. Determine the emf around the square at t = 1.50 s...
  2. H

    Induced emf is proportional to the magnetic flux

    According to the faraday's law,the induced emf is proportional to the magnetic flux change linking the conductor.However,I wonder why there is induced emf when conductor is moving in uniform magnetic field,because i think that the magnetic flux linking to it is always the same as it move in...
  3. I

    How is the induced EMF calculated in a circuit with a swinging pendulum gate?

    Homework Statement I just took a test, and I am very unsure of the validity of how I approached the problem. Just looking for some feedback cause this is bugging me! There is a circuit in the xz-plane (vertical), and the circuit has on it a gate. The gate is a pendulum of length L, which...
  4. Z

    Induced EMF in a spinning wheel

    Faraday's law says that an induced voltage across a conductor is directly proportional to the rate of change of magnetic flux through the conductor, but I am having trouble getting my head round how it works in this situation. Imagine a perfectly circular thin copper disc, and that this disc...
  5. W

    Induced EMF - Does Voltage Change Matter?

    Suppose now i have connected a solenoid with a dc power supply of 10V When i increase the voltage to 20V the magnetic flux through the coil have increased will emf be induced shortly to oppose the change? what if i now change it to a 10V ac power supply so will there always be induced emf...
  6. I

    Induced emf in a square loop at rest

    Homework Statement Determine the emf induced in the square loop in the figure if the loop stays at rest and the current in the straight wire is given by I(t) = (15.0 A)sin(2500t) where t is in seconds. The distance a is 12.0 cm, and b is 15.0 cm. Homework Equations In the question...
  7. Y

    Relationship between potential and induced emf?

    Homework Statement [PLAIN]http://img220.imageshack.us/img220/2589/44045733.png Semicircular wire MN of diameter L moves with speed v perpendicularly through a uniform magnetic field of strength B. 1. Find the magnitude of the induced emf generated between M and N. 2. State whether M...
  8. C

    Why Does Swiping a Card in a Plastic Bag Help It Read at the Checkout?

    Sometimes when i swipe my card at walmart it doesn't read it , And the guy was like let's put it a plastic bag and then try it, and it works and I've heard a couple of other people doing this. So when i slide my card the magnetic strip induces a voltage and reads the card. So why does...
  9. G

    Calculating Induced EMF: 50 cm^2 to .005 M^2

    Homework Statement A flat coil of wire consisting of 20 turns, each with an area of 50 cm^2 is positioned so that the normal to its plane is at the angle of 30 degrees to a uniform magnetic field that increases its magnitude at a constant rate from 2.0T to 8.0T in 2.0 s. Homework Equations...
  10. A

    Induced EMF and a circular loop of wire

    Homework Statement A 25-turn circular loop of wire has a diameter of 1m. In 0.2 seconds it is flipped 180 degrees at a location where the magnitude of the Earth's magnetic field is 50 micro T. What is the emf generated in the loop?Homework Equations -(dB/dt)(A)=VThe Attempt at a Solution When I...
  11. L

    How Is EMF Induced in a Coil When a Magnetic Field Changes?

    1. A 100-turn, 2.0-cm-diameter coil is at rest in a horizontal plane. A uniform magnetic field 60 degrees away from vertical increases from 0.50 T to 1.50 T in 0.60 s. 2. EMF = N*d(phi)/dt phi = flux = ABcos(theta) 3. dB/dt = 1/0.6 EMF = 1000*(1/0.6)*pi*r2*0.5 = 0.262 V Is...
  12. S

    What is the induced emf in a coil with changing magnetic field?

    Homework Statement A coil is wrapped with 247 turns of wire on the perimeter of a square frame of sides 27.3 cm. Each turn has the same area, equal to that of the frame, and the total resistance of the coil is 2.53. A uniform magnetic field is turnedon perpendicular to the plane of the coil...
  13. Q

    Induced EMF in Rectangular Loop in B-Field

    ok, if i have a rectangular loop parallel to the screen and in a uniform constant magnetic field pointing perpendicularly outwards of the screen, if i move the loop to the right of the screen, will there be an induced emf in the loop? my understanding is that there will be? because we have a...
  14. D

    How Is Current Induced in a Multi-Turn Coil with Changing Magnetic Field?

    Homework Statement A coil with 253 turns of wire, a total resistance of 28 Ohms, and a cross-section area of 0.36m^2 is positioned in a plane perpendicular to the field of a powerful electromagnet. What average current is induced in the coil during the 0.33s that the magnetic field drops...
  15. S

    Not really HW Just want to fully understand induced emf

    Ok... so there really isn't a homework problem that goes directly with what I am asking... I just want to understand how/why induction (magnet through a coil) gave certain results in our Science Workshop program... Here is what we did in our lab: We dropped a magnet (south end in first)...
  16. M

    Calculating Average Induced EMF of 100-Turn Coil in 0.36s

    Homework Statement The area of a 100 turn coil oriented with its plane perpendicular to a 0.28 T magnetic field is 4.3×10−2 m^2. Find the average induced emf in this coil if the magnetic field reverses its direction in 0.36 s. Homework Equations E = -N (flux - flux initial)/(t-to)...
  17. A

    Induced EMF in a Pair of Circular Wires with Changing Magnetic Field

    Homework Statement The setup is a pair of two circular wires, a small one of radius b and a large one with radius 2b, both sharing the same center and located on the same plane. There is a magnetic field of strength B within the smaller circle that comes out of the paper towards the observer...
  18. fluidistic

    Induced emf in a wire hard mathematically

    Homework Statement See picture for clarity. There are 2 infinite wires carrying a current i, one wire does it in the upward direction and the other wire in the downward direction. They are separated by a distance L+2d. Between them lies a circuit such that the part that moves with a velocity v...
  19. A

    -ve sign in expression of induced emf

    -ve sign in expression of "induced emf" What is the concept behind putting negative sign in: e=-[rate of change of flux] Which direction does it signify?? How does direction affect the equation? How and where can we use negative sign in other expressions of induced emf??
  20. F

    Maximum induced emf with 2 Solenoids

    Homework Statement A 3 m long large coil with a radius of 15.7 cm and 130 turns surrounds a 7.2 m long solenoid with a radius of 5.7 cm and 4700 turns. The current in the solenoid changes as I=I_0 sin (2\pi f t) where I0= 30 A and f=60 Hz.Inside solenoid has 4700 turns and outside coil has...
  21. F

    Max Induced Emf: Homework Statement & Equations

    Homework Statement A flat loop of wire of area 15.7 cm2 and 1.09 turns is perpendicular to a magnetic field whose magnitude decays in time according to B = 0.5 e−t/7. What is the maximum induced emf? Answer in units of V. Homework Equations \epsilon= \Delta\PhiB/\Deltat \epsilon=...
  22. P

    Calculating the induced emf in a solenoid, have worked on it, can't get answer

    Homework Statement A solenoid of length 3.7 cm and diameter 0.95 cm is wound with 160 turns per cm. The current is decreasing at a rate of 38.0 A/s. (a) What is the induced emf in one of the windings? (b) What is the induced emf in the entire solenoid? Homework Equations emf =...
  23. L

    Induced emf at a specific time

    Homework Statement A circular coil with 35 turns of wire and a diameter of 19 cm is placed in a magnetic field given by B(t) = 6 t^2 + 6 t + 2 T where t is time in seconds. The field is perpendicular to the plane of the coil. What is the induced EMF at t = 5 s? Homework EquationsE=-d(magnetic...
  24. S

    Induced EMF Problem: Find Emf & Current in Square Coil

    Homework Statement A magnetic field is perpendicular to the plane of a single turn circular coil. The magnitude of the field is changing so that the emf of 0.80 V and a current of 3.2 A are induced in the coil. The wire is then re-formed into a single turn square coil, which is used in the...
  25. K

    Induced EMF on a primary coil by a secondary coil

    [b]1. Derive an equation to find the induced EMF on an inner solenoid by a changing current on an outer solenoid. Homework Equations We're given the equation EoRMS=(No*Ai*(2*pi*f)*(permeability constant)*IiRMS)/Li for the induced EMF on the outer solenoid by a changing current in the...
  26. B

    Calculate Speed & Induced EMF of Spacecraft

    Homework Statement A spacecraft is in a circular orbit of radius 3.1 X 10^4 km around a 1.9 X 10^30 kg pulsar. The magnetic field of the pulsar at that radial distance is 1.4 X 10^2 T directed perpendicular to the velocity of the spacecraft . The spacecraft is 0.20 km long with a radius...
  27. V

    Positively charged side with induced emf

    Homework Statement A solid conductor travels at 150 m/s across a uniform .045T magnetic field. Which side is positively charged and what is the emf across this block? Homework Equations I actually solved for the emf no problem. I am having difficulty determining which side is...
  28. J

    What is the difference between an induced emf and a back emf?

    What is the difference between an induced emf and a back emf? (i) i thought they were the same thing? . . .. .
  29. K

    Does Faraday's Law Account for Varying Induced Flux in a Loop?

    Faraday's law says induced emf = - d(flux)/dt If this is applied to a loop where induced emf causes currents, and thus flux itself, do we have to consider that flux (of course we don't if it's constant)? If the external flux has a nonzero second derivative, then the induced emf is...
  30. H

    Induced emf in triangle with changing area

    Two straight conducting rails form a right angle where their ends are joined. A conducting bar in contact with the rails starts at the vertex at time t = 0 and moves with a constant velocity of 7.00 m/s along them, as shown in Fig 31-44. A magnetic field with B = 0.350 T is directed out of the...
  31. S

    Finding Magnitude induced EMF in Coil

    Homework Statement Homework Equations e=d/dt(BAN) The Attempt at a Solution
  32. T

    Magnetic field due to coil (induced emf)

    Homework Statement Suppose you wrap wire onto the core from a roll Nof cellophane tape to make a coil. Describe how you can use a bar magnet to produce an induced voltage in the coil. What is the order of magnitude of the emf you generate? State the quantities you take as data and their...
  33. N

    Finding induced emf of solenoid

    Homework Statement A 800-turn solenoid, 18 cm long, has a diameter of 2.9 cm. A 15-turn coil is wound tightly around the center of the solenoid. If the current in the solenoid increases uniformly from 0 to 3.1 A in 0.28 s, what will be the induced emf in the short coil during this time...
  34. T

    How Does a Changing Magnetic Field Induce EMF in a Circular Loop?

    Homework Statement Resistance = R Radius = r Angle = theta B = 2cos(2t) Circular wire loop where magnetic field acts at theta degrees with respect to the normal of the wire loop. Find induced current and emf in wire loop. Find induced electric field at radial distance d from center of wire...
  35. K

    Faraday's law and induced emf in a loop

    (As magnitudes) Faraday's law states that induced emf in a conducting loop by a changing flux, say x, is: emf = dx/dt Now if we have a circuit with the loop in series with wires (which are then tied together), how do I picture the voltages/currents in this circuit? I mean, the actual...
  36. I

    Calculating Induced EMF in a Coil

    Homework Statement A 32 turn circular coil of radius 4.60 cm and resistance 1.00 is placed in a magnetic field directed perpendicular to the plane of the coil. The magnitude of the magnetic field varies in time according to the expression B = 0.0100t + 0.0400t2, where t is in seconds and B is...
  37. K

    How Does the Right-Hand Rule Determine EMF Direction in a Non-Conducting Loop?

    A long, straight wire is in the same plane as a wooden, non-conducting loop. The wire carries an increasing current I in the direction shown in the figure (the current is pointing in the left direction). a. there will be no induced emf and no induced current b. there will be a...
  38. T

    Magnetic flux / induced emf problem

    Homework Statement A single coil with a radius of .02m is placed inside a 3m long solenoid that has 600 turns. The radius of the solenoid is 0.15m. The plane of the coil is perpendicular to the axis of the solenoid. The solenoid is connected to a battery, an ammeter and a switch. The ammeter...
  39. C

    Total induced emf at different speeds

    Homework Statement Suppose you have a coil that is a certain distance from a magnet. Now you move the coil towards the magnet (until it is on top of the magnet) at a certain speed. Then you do the same thing only faster. Will the total induced emf be the same for both tries? Aka should the...
  40. J

    Induced emf in secondary coil - help

    induced emf in secondary coil - help ASAP Please Homework Statement A very long, straight solenoid with a cross-sectional area of 6.00 cm2 is wound with 40.0 turns of wire per centimeter, and the windings carry a current of 0.250 A. A secondary winding of 2.00 turns encircles the solenoid...
  41. A

    Calculate induced emf across loop antenna by another loop antenna

    Homework Statement i need to find emf across 50 ohm load attached to square loop antenna of single turn with each side of 10 cm. this antenna is placed 10 cm away from a similar antenna which is driven by a sinusoidal source of 10 v amplitude and with 1mW power. Homework Equations i...
  42. C

    Induced EMF Problem: Calculate Magnetic Flux & Initial EMF

    An elastic conducting material is stretched into a horizontal circular loop of radius 20 cm and placed in a magnetic field of strength 0.5 T, directed vertically downwards. a. What is the magnetic flux through the loop? b. When released, the radius of the loop decreases at an initial rate...
  43. K

    Induced Emf, Contributing factors

    Homework Statement A generator is constructed by rotating a coil of N turns in a magnetic field B at a frequency f. The internal resistance of the coil is R and the cross sectional area of the coil is A. Which of the following statements are true? (Give ALL correct answers, i.e., B, AC...
  44. D

    What Direction Does the Coil Rotate in a Magnetic Field?

    Homework Statement A circular coil, with radius of 10 cm, and 25 turns, rotates in a constant magnetic field of strength 2.4 T, with the axis of rotation perpendicular to the direction of the magnetic field. (a question about finding the induced voltage- 5.92 V) part C. If the...
  45. S

    Why is there no induced emf in electric motor?

    Hello everyone :smile: I got a small question. In an electric motor when the coil rotates isn't this a change of magnetic flux. So wouldn't there be an induced emf in the coil? Any help would be appreciated. Thanks :smile:
  46. P

    Induced emf in a Rectangular Coil

    Homework Statement A rectangular coil of 150 turns has dimensions of 23 x 70 cm and is located in a uniform 3 T magnetic field. In 1.2 s, the plane of the coil is rotated from a position where it makes an angle of 15° with the magnetic field to a position where it makes an angle of 72°...
  47. R

    Solving for Total Magnetic Flux and Induced EMF

    Homework Statement http://img164.imageshack.us/img164/3313/qqqqo3.jpg Find the total magnetic flux through the coil at t=0 Given that the rate of change of flux at any time,t, is equal to 2 \pi f \phi sin 2 \pi ft, find the maximum instanteous value of the induced emf at any time & the...
  48. S

    Magnetic fields and induced EMF

    Homework Statement An coil has 4000 turns of wire and a mean area of cross-section 25cm^2. A magnetic field of flux density 0.16T passes through it. When the magnetic field is reduced steadily to zero in 0.0050s, emf is induced across the terminals of the coil. Calculate the value of this...
  49. N

    Confused about Polarity of Induced EMF (Lenz Law)

    Hello, I am currently working through Faraday's Law and I am confused when my two textbooks don't explain how they set the polarity of V_{emf}, particularly when they say "the sign convention for V_{emf} is arbitrary in that it depends on the assumed direction for dS". How is the polarity...
  50. B

    Conceptual question on induced EMF

    Homework Statement A circular loop of wire is rotated at constant angular speed around an axis whose direction can be varied. In a region where a uniform magnetic field points straight down, what must be the orientation of the loop's axis of rotation if the induced emf is to be zero...
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