What is Emf: Definition and 938 Discussions

EMF measurements are measurements of ambient (surrounding) electromagnetic fields that are performed using particular sensors or probes, such as EMF meters. These probes can be generally considered as antennas although with different characteristics. In fact, probes should not perturb the electromagnetic field and must prevent coupling and reflection as much as possible in order to obtain precise results. There are two main types of EMF measurements:

broadband measurements: performed using a broadband probe, that is a device which senses any signal across a wide range of frequencies and is usually made with three independent diode detectors;
frequency selective measurements: in which the measurement system consists of a field antenna and a frequency selective receiver or spectrum analyzer allowing to monitor the frequency range of interest.EMF probes may respond to fields only on one axis, or may be tri-axial, showing components of the field in three directions at once. Amplified, active, probes can improve measurement precision and sensitivity but their active components may limit their speed of response.

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  1. M

    Is there a delay of 1 or 2 seconds in generation of back emf in a dc motor

    Does the back emf in a dc motor start to appear after a slight delay(say 1 or 2 seconds) after starting up the motor? I have had this simple doubt for a long time.According to the "generator principle" both applied emf and back emf will always exist simultaneously.Also it would be in violation...
  2. F

    EMF induced in conducting wire

    We just went over Faraday's law in class and our teacher stressed how a changing flux is needed to induce an emf in some coil/loop/wire. I was just wondering then, how is an emf induced in a straight conducting wire moving at a constant velocity in a constant magnetic field? It seems as if...
  3. A

    Understanding the Flux Rule for Motional EMF?

    The flux rule for motional emf can be stated as: d\phi/dt = -\epsilon I have some questions regarding this. 1) Should I find this rule intuitive? And secondly a proof of this rule is given in my book, which I have attached. I don't really understand the idea of the proof - is the idea to...
  4. G

    EMF expression in electrical generator

    Hi all! I'm studying electrical generators, and while I was trying to come up to the expression of EMF induced in a conductor I got two different results following two different ways, and I can't find the error. With sinusoidal B, with e=bvl I get an RMS value E=2K_{f}f\phi, while with...
  5. 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...
  6. R

    How does back emf make energy conversion possible

    I've come across one thing about energy conversions: "To make an energy conversion possible, it is important that the supplied form of energy must be opposed by another force, otherwise this energy cannot be converted." Agreed! Once the motor starts generating torque, a back emf is produced...
  7. P

    EMF induced in a rotating coil

    Hi, So, I am learning about induced emf. I am having a hard time grasping the idea that we are interested in the angle between the normal to the plane of the coil and the magnetic field. In the standard illustration I actually understand it, but the following question confuses me. We are...
  8. B

    EMF induced in solenoid by current in a loop.

    Homework Statement A square loop with side-length a is positioned at the centre of a long thin solenoid, which has radius r (with r>a), length l and N turns. The plane of the loop is perpendicular to the axis of the solenoid, find V_{emf} induced in the solenoid Homework Equations...
  9. K

    How do I find the emf and internal resistor of this cell of this circuit?

    This is a potentiometer circuit. I don't have the diagram so I hope you can follow me. This is the main circuit with the unknown emf E with internal resistance r of the main cell and with two resistors P and Q connected in series at the bottom of this rectangular circuit, p being connected near...
  10. 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...
  11. G

    Short Circuit Current, EMF, Internal Resistance

    Okay so I have conducted an experiment to find the internal resistance of a battery pack by gradient of a graph of voltage against current. All so far okay. I went on to find the EMF also, again all so far okay. Now I need the short circuit current, and I can't find any information anywhere...
  12. alv

    EMF v time graph vs change in flux v time

    I'm actually a teacher and I can't explain this problem from the 2007 HSC. I've attached a copy of the question and the two graphs. The question shows a graph of change in magnetic flux vs time and asks you to choose which graph of induced emf best fits it. I don't understand why the induced emf...
  13. 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...
  14. 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...
  15. A

    Solving 2 Questions on Circuit Analysis: PD Across 3 Ohm & EMF of Batteries

    Homework Statement I have two questions. Both questions are in the picture below. Dont mind my workings lol http://img163.imageshack.us/img163/5680/scanpic0001qe.jpg Ans to the 1st qn: C The potential difference across the 3ohm resistor is zero. Thus, it can be considered that the 4ohm and...
  16. M

    EMF Production in Thermocouples: Causes & Effects

    1.what is the main cause for the emf produced in a thermocouple when their two junctions are at different temperatures? 2.why is it necessary that the emf produced causes the current to pass from the metal (which is at the top of thermoelectric series) to the other metal (which is at lower...
  17. C

    Inducing an EMF in an Aluminum Ring

    Homework Statement An aluminum ring of radius r1 = 5.00 cm and a resistance of 2.65 x 10^-4 Ω is placed around one end of a long air-core solenoid with 970 turns per meter and radius r2 = 3.00 cm as shown in the figure. Assume the axial component of the field produced by the solenoid is...
  18. R

    Emf of a Closed Loop of Wire In a Magnetic Field

    ]Homework Statement A closed loop of wire 7.2 x10^{}-3 m^{}2 is placed so that it is at an angle of 60degrees to a uniform magnetic field. The flux density is changing at 0.1 T/s. The emf, in V, induced in the loop of wire is A) 3.6x10^{}-4 b)3.6x10^{}-2 6.9 Homework Equations emf...
  19. L

    Magnets or EMF in car bumpers to protect from fender bender

    Hi I am new to the forums. I am sure I have found the right place for help with an idea I have been working on for a while. First off please forgive my lack of physics knowledge. What amount of magnetic force would I need to stop or dramaticallyjust deccelarate a standard size 5000lbs car going...
  20. T

    Understanding Potential Difference and EMF in Electric Circuits

    hello... potential difference between two points in a circuit is the work done by the electric field to get a charge from point to another and for the emf it's also the work done but my question is for a potential difference the work is negative( the electrons lose energy) and for the emf the...
  21. T

    Current, Polarity and EMF through circuit?

    Homework Statement In the circuit shown in the link, the 6.0 \ohm resistor is consuming energy at a rate of 24.0 J/s when the current through it flows as shown. A) Find the current through the ammeter A. B) What is the polarity and emf of the battery \epsilon assuming it has negligible...
  22. N

    EMF and voltage the same ? NO ? How ?

    Okay so are EMF and Potential Difference the same thing ? I know they are now but I am not able to understand how ? I came through this Derivation today if someone could explain it to me it would be great The figure shows a battery connected to a resistor V(A) - V(B) = V(b) = ε (emf)...
  23. D

    Does EM Wave induce emf in coil?

    Hi! Does electromagnetic waves induced emf or current in the coil or inductor due to faraday law as the magnetic field is continously changing in em wave?
  24. J

    Maximum EMF - parallel to field?

    for a rotating coil in a uniform magnetic field, why is the EMF maximum when the coil is PARALLEL to the field (i.e: flux linkage = 0)
  25. N

    Maximum and Minimum EMF in a generator

    Just a quick question, if the maximum induced EMF of a generator is say 200v, would the minimum EMF be -200v or 0v? No equations necessary I say.
  26. 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=...
  27. V

    The difference of Back EMF waveform shape of BLDC and brushless PM generator

    I am a bit confuse on the back EMF waveform shape of the brushless permanent magnet machine. As I know brushless DC permanent magnet motor has a trapezoidal shape back EMF waveform. If I use this motor as a generator, do I still get the same waveform shape or I will get a sinusoidal waveform?
  28. A

    Coils, Current & EMF: Exploring the Interaction

    I guess it does not produce any field in 2nd coil because there's no change in current in coil right?So there won't be any emf .. but coil 2 has magnetic field passing through it which results in current in coil(by forcing movement of electrons of coil 2 SINCE ELECTRONS ARE ALREADY IN MOTION...
  29. N

    Can Back EMF in Inductors and Motors be Eliminated or Reduced?

    Well I have been researching a lot about inductors and motors and so on and I just had a question that I can't seem to answer. Is it possible to get rid of or reduce the back emf generated in say a inductor or motor. I know you can decrease the inductance of a inductor in many ways, but is there...
  30. M

    Emf in a conducting rod moving away from a wire with current I

    Homework Statement A conducting rod of length l moves with a constant speed v perpendicular to an infinately long, straight wire carrying a current I. What is the emf generated between the ends of the rod when it is a perpendicular distance r from the wire? Homework Equations no idea...
  31. 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...
  32. U

    Find the EMF in the loop as a function of time.

    Homework Statement electromagnetic field Homework Equations E=hv The Attempt at a Solution I know that t is the variable and h1 and h2 I and I2 are considered given all we are doing is doing the algebra set up but not sure how to show this exactly.?
  33. 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...
  34. L

    Confused about how inductors work and produce a back EMF?

    Homework Statement A coil with zero resistance has its ends labeled a and b. The potential at a is higher than at b. Explain why the two following statements are consistent/correct with the situation. a) The current is increasing and is directed from a to b. b) The current is decreasing...
  35. T

    Motional emf, Gaussian flux, and lenz's law help

    Homework Statement Two bars, each 30cm long, and each having a resistance of 2-ohms, are connected to a 1,500 volt battery. The bars are attached to each other with 3 insulating springs, each having a spring constant of 9N/m. The two bars are initially at rest, 4cm apart. The switch is...
  36. M

    Is This a Case of Motional emf?

    If a straight length of wire moves in a suitable direction through the magnetic field of a magnet, there will be a motional emf in the wire. If the magnet moves toward the wire, is there an emf in the wire, motional or otherwise?
  37. M

    What self-induced emf appears in that coil?

    Homework Statement The Attempt at a Solution I now know the units are nWb. I'm just getting a little confused by answer. ((6.1mH)(7.3mA))/224turnes=.198795) (mH*mA)/turnes So would that equal= 198.795 nWb?
  38. E

    EMF Induced on Secondary Coil of a Transformer

    Homework Statement http://www.xtremepapers.com/CIE/International%20A%20And%20AS%20Level/9702%20-%20Physics/9702_s02_qp_4.pdf <------ CLICK ON THE LINK The Attempt at a Solution Is the above diagram the correct answer ? Please note that the diagram in the question paper is for COIL...
  39. O

    The back EMF of a DC motor spinning at full speed

    Homework Statement The resistance of the armature windings of a DC motor is 5Ω. The motor is connected to a 120V line. When the motor is spinning at full speed, it draws a current of 4A. The back EMF at full speed is: Homework Equations Pav=\frac{1}{2}I2peakR Pav=\frac{1}{2}εpeakIpeak...
  40. 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...
  41. E

    Variation in EMF of a magnet moving through a Coil

    NOTE : My Current question and source of debate is in post number 10. Lets say I have a square coil. I accelerate it in a direction such that it is perpendicular to a magnetic field directed into the page. As it enters the field and before it is completely in the field an...
  42. E

    Transformers - Variation in Current, EMF and Magnetic FLux

    This question about the variation of the aforementioned quantities in a transformer. Now according to me and my knowledge of electricity this is how I feel it should play out : If the input current were a sine curve that varies with time then : Input voltage = cosine curve i.e. it is...
  43. T

    Resistance in emf - Faraday's Law

    Homework Statement A rectangular wire loop measures 5.00 cm by 8.00 cm, and lies in the plane of the page. It has a resistance of 3.00 Ω. In this region, there exists a uniform magnetic field pointing into the page. Its magnitude, in Tesla, varies according to the equation B(t) = 10.0 −...
  44. E

    Average Back EMF Induced in Coil in Loudspeaker

    Hi, I would appreciate any help with the following: The coil in a loudspeaker has an inductance of L = 112 microH. To produce a sound of frequency 40kHz, the current must oscillate between peak values of + and - 4.4 A in half a period. What average back emf is induced in the coil during this...
  45. S

    Does Higher emf Affect Electron Flow?

    Hello experts! Higher the emf of the battery implies that the higher the drift speed of the electrons or increase the flow time of electrons from the conductor? Which one is true and how??
  46. S

    EMF: Does It Have a Direction?

    is the a direction in emf ?? Does electromotive force have a direction? In some circuits it is showed as in the image above.. Please tell me that it having a direction ?
  47. 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...
  48. M

    How to Calculate Induced EMF Using Graphical Analysis?

    If i am given a graph of B(magnetic field) vs Time,how can i find the induce emf from this graph?? I know that the emf=-Nd∅/dt and ∅=BAcosθ but somehow i don't quiete arive to the right answer
  49. K

    Redox Reactions and standard emf calculations

    What is the equation I should be using for the following question: At 298K a cell reaction has a standard emf of +0.17V. The equilibrium constant for the cell reaction is 5.5 X 10^5. What is the value of n for the cell reaction. Grateful for anyone's help!
  50. 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...
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