What is Emf: Definition and 937 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. cnh1995

    Faraday's law and motional EMF paradox

    Consider the diagram below (sorry for the quality:frown:). The circle is the front view of a cylindrical iron rotor (highly ferromagnetic, very low reluctance). The orange part is a single turn of a conductor (very high reluctance) wound around the rotor body. The grey lines are magnetic field...
  2. T

    I Non-EMF Wireless Communication

    Is there any theoretical room for wireless communication that does not require EMF and would be unhindered by RF blocking materials or Faraday cages?
  3. JohnBM

    Low Level EMF and Helmholtz coil design

    The problem statement: I want to produce an LL-EMF of a specific amplitude and frequency in a pair of Helmholtz coils of pre determined radius. The frequency and amplitude are derived from the following equations from this publication...
  4. M

    Why Is My DIY Grounding Mat Triggering the Volt Meter Alarm?

    Overview: I've made a grounding mat using a piece of copper speaker wire attached to the grounding prong on a wall plug (I removed the other two prongs). The other end of my copper wire is attached to a sheet of aluminum which I set on the floor like a mat. I checked to make sure that my wall...
  5. O

    How is there not an infinite regress in the EMF of a motor?

    Say we have a coil connected to a battery in a uniform magnetic field, perpendicular to the magnetic moment of the coil. This is a simple motor. There is a torque on the coil that varies with the angle θ between the field and the moment. Clearly, the angular velocity ω is not constant. Here is...
  6. 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...
  7. S

    I Is internal battery "emf" a conservative force?

    First, the electric/Coulomb force set up by a battery across its terminal is conservative, and its potential is given by the well-known V. I also understand the conventional usage of emf is as a voltage potential. However, a battery does more than just set up the electric field and its...
  8. P

    What is the equation for working out emf frequency

    Hi there My understanding is that any charge will produce some emf when it is accelerated..is this always true? What is this law called? Is there a simple equation to relate charge size, acceleration rate and resulting emf frequency? Many thanks
  9. S

    Makeshift DC Motor Radiating EMF

    My brother is a high school student tasked with making a DC motor. It is finished and works however I thought for the sake of learning and the additional cool factor (I'm relatively new to this field), I could make an optical RPM sensor which I could tack on. So I made this...
  10. H

    Magnetism in Coils: Why PD Changes Direction?

    This is a graph for a magnet falling through a coil, it shows the p.d induced in the coil (the induced emf) when the magnet enters and then shows the induced emf in the coil when the magnet leaves. My question is why does the p.d appear to change direction? (i.e why the positive and negative...
  11. ahmed963u2

    Exploring Fleming's Right Hand Rule for EMF in a Straight Wire

    Homework Statement If the wire moved perpendicular to a magnetic field as in the fig. then the volt of point a is ( bigger than -smaller than-equal ) the volt of the point b Homework Equations please see the attached files The Attempt at a Solution
  12. S

    B Why does accelerating a loop through a constant magnetic field induce an EMF?

    So from Faraday's law, a change in the magnetic flux will induce an emf on a loop of wire. { \phi }_{ B }\quad =\quad BA\\ { \varepsilon }_{ ind }\quad =\quad \frac { d{ \phi }_{ B } }{ dt } (in this case a perpendicular field) From these equations, it looks like the only way to induce a...
  13. V

    EMF Induced in a Rotating Disk: Is BvR the Correct Formula?

    Homework Statement Homework EquationsThe Attempt at a Solution O is the center of the disk Part a) The emf induced across PS can be found by thinking as if a rod PS is rotating and translating . The component of velocity of rod perpendicular to its length is vsin45° = v/√2 . The length PS...
  14. CAH

    How is emf constant of flux linkage is increasing?

    how is this true? I get that it's the rate of change of flux, and if you have a graph of flux linkage against time, the gradient with a constant gradient then emf is constant since it IS the gradient... But how? If the flux linkage is increasing, why doesn't the emf?
  15. A

    EMF about a loop moving toward a wire with current

    Homework Statement A picture of a related problem is attached below. My problem is the same, except that v is toward the wire, not away from it. I, r, v, b and a are given. "Find the emf about the loop without assuming that B changes at any point" My problem is that this question isn't...
  16. P

    Understanding Internal Resistance in Batteries

    A battery has an Emf 6 Volts. It is completely discharged. It is charged by maintaining a potential difference of 9 Volts across it. If the internal resistance of the discharged battery is 10 ohms, find the current through the battery, just after the connections are made. My textbook says that...
  17. Toughnuke

    Electromotive force (EMF) of a circular loop wire

    Hi, so, I got help from this homework problem by posting it on chegg, but I want clarification. A circular loop of wire of radius 12.0 cm is placed in a magnetic field directed perpendicular to the plane of the loop and into the page. The field is decreasing at a constant rate of 0.050 T per...
  18. 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...
  19. ElPimiento

    Calculating a Homopolar Generator's EMF

    Homework Statement Homework Equations ##\omega = v/r## ##q\vec v \times \vec B\ =\ \vec F_B## ##|\vec F|d\ =\ |\vec E|## ##q \Delta V\ =\ E## The Attempt at a Solution (So, my answer is twice and large as the correct answer. This is the second time this has happened on a question like this ...
  20. 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(...
  21. S

    About electricity and the EMF -- Multiple choice question

    Homework Statement The cell in the circuit has an emf of 2.0 V. When the variable resistor has a resistance of 4.0 Ω, the potential difference (pd) across the terminals of the cell is 1.0 V. What is the pd across the terminals of the cell when the resistance of the variable resistor is 12...
  22. 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...
  23. 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...
  24. H

    Calculate the EMF induced in the square?

    Homework Statement A square of wire with side length 1.4 m rotates about an axis through one corner that is perpendicular to the plane of the square with an angular speed of 3 rev/s. There is a uniform magnetic field of 1.2 T perpendicular to the square. Calculate the EMF induced in the...
  25. 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...
  26. A

    What constant p.d. should be used in a potentiometer?

    Homework Statement I am trying to use a slide wire potentiometer and a galvanometer to find the electromotive force of an unknown cell. I have heard that the balance point at which the galvanometer needle has zero deflection should be towards the centre of the slide wire as this reduces...
  27. Syed Taha ahmed

    I Why does back EMF in DC machines deviate from a pure sine wave?

    Why voltages out of the rotating loop is constant positive and negative **Why nOT varying like sinusoidal **. Like it has a wave form of O to 2*vBL and then it maintain its value there for some time before failing to zero and changing direction. I have attach the image file from my book...
  28. Stinson

    EMF Pinewood Derby: A Practical Physics Project

    I'm taking physics 2 and was told to create a practical project utilizing electromagnetic force. Starting thinking about Lenz's law (which I just read in a textbook for the first time a week ago) and I thought it would be fun to send a pinewood derby car uphill instead of down. The sprinkler...
  29. G

    Experiment to find out emf of the battery

    I conducted an experiment to find out the emf of a battery, the result turned out to be 1.35 volts , however on the battery, it says the battery is 1.5 voltage. Does it mean that my method is wrong or less accurate. But I used three different methods:1) measuring the terminal voltage across...
  30. G

    Measuring Battery EMF: Is it Accurate?

    Guys, is it accurate to measure the battery's emf by putting voltmeter across the battery( it is not connected in circuit). I think this is accurate because there is no current flowing, this means that there is no voltage across the internal resistance.
  31. Amr Elsayed

    Why Does Back EMF Decrease as Current Derivative Decreases in an LR Circuit?

    Good evening everyone. I have a misconception regarding self inductance. I know that back emf induced in an inductor starts with the same value of the original voltage source and then keeps decreasing to reach zero. I also know how this is related to the derivative of current with respect to...
  32. 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...
  33. C

    B-field to create average emf in rotating coil

    Homework Statement A 0.300 m radius, 487 turn coil is rotated one-fourth of a revolution in 4.17 ms, originally having its plane perpendicular to a uniform magnetic field. (This is 60 rev/s.) Find the magnetic field strength needed to induce an average emf of 10,000 V.[/B]Homework Equations E...
  34. PhysicsBoyMan

    How does emf relate to current, elec potential, & resistance

    Homework Statement How does emf relate to current, electrical potential, and resistance? Specifically, in the context of the equations I have posted below. Homework Equations V = IR [/B]where V is electric potential, I is current, and R is resistance. V = emf - IR How can V = emf - IR...
  35. B

    Half-Reactions: Find Strongest Oxidizing Agent

    Homework Statement Based only on the half-reactions in the table, which of the following is the strongest oxidizing agent? (See attachment) Homework EquationsThe Attempt at a Solution I noticed in this table that not all elements are in the reduction form. I flipped them around and changed...
  36. 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...
  37. J

    Power Output of Radiation from a Solenoid with Linearly Decreasing Current

    Homework Statement A solenoid contains 610 cycles, with a radius of 0.136m and a length of 0.87m. The current of the solenoid decreases linearly from the maximum value I0 to zero through the equation I(t)=I0−bt, where I0=0.31 A and b=0.21 A/s. What is the power output of the electromagnetic...
  38. F

    Where does the emf radiation from an elliptical come from?

    Where does the emf radiation from an elliptical come from? Just the console? I just read an article about dirty "electric pollution," and I was wondering.
  39. L

    Average back emf induced in coil

    Homework Statement The coil in a loudspeaker has an inductance of L = 56uH (or 5.6 x 10^-5 H). To produce a sound frequency of 20 kHz, the current must oscillate between peak values of +2.2 A and -2.2 A in one half of a period. What average back emf is induced in the coil during this variation...
  40. S

    Electromotive force in a moving bar

    Hi everyone, I've seen in some lecture notes that a moving conducting bar (length of L) in a uniform and constant magnetic field develops an electromotive force (emf) between the 2 edges of the bar. Lets assume that the bar moves to the right (with velocity v) and the magnetic field (B) points...
  41. B

    Generating EMF in a Coil with a Bar Magnet - 65 Characters

    Exactly what and how generate EMF in a coil when I move a bar magnet in it? How the changing magnetic field makes electrons to move? (I mean quantum mechanically.) As I read somewhere when I close the circuit (put a load, a LED for example on the coil) the generated current in the coil will be...
  42. C

    Current Algebra: Find Current in Electric Circuit w/ 3 Resisters of R

    Homework Statement An electric circuit consists of 3 identical resistors of resistance R connected to a cell of emf E and negligible internal resistance. What is the magnitude of the current in the cell? (in the diagram two of the resistors are in parallel with each other then the other in...
  43. P

    Find emf and the inner resistor of the battery

    Homework Statement When the resistor is R1, in the circle there is the intensity I1 and when the resistor is R2, the intensity becomes I2.Find emf and the inner resistance.[/B] Homework Equations I=emf/R+r The Attempt at a Solution I1=efm/R1+r I2=efm/R2+r efm is equal I1*(R1+r)=I2(R2+r)...
  44. Amr Elsayed

    Understanding Back EMF: What is it & What Does it Do?

    Hello all, I have got a hard time thinking of what back E.M.F is. Does it appear only at the moment of switching and closing the circuit ? why it's even there and what is its relation with E.M.F . If some one can provide me with a brief and good source that elegantly explains the whole thing I...
  45. kostoglotov

    Energy dissipated by a loop of muscle in an MRI machine

    edit: turned out to be a calculator typo...mods feel free to delete this thread if you wish, I won't complain. Homework Statement [/B] Having issues with part a) imgur link: http://i.imgur.com/4hzLyhb.jpg Homework Equations Resistivity of muscle (from table in text): \rho \approx 13...
  46. F

    Can anyone explain this anomaly?

    Homework Statement Hi, so during an experiment, I spun a magnet inside a coil of wire in order to induce an emf, and measure the input power and output power. One of the things I found was that the slower the magnet spun (ie the smaller the input voltage to the motor) the larger the induced...
  47. cnh1995

    Rotor EMF in squirrel cage induction motor

    In a 3 phase induction motor,when stator rmf rotates, it cuts the rotor bars and induces an emf in them. Is this emf sinusoidal? I mean shouldn't it be a square wave since the rotor "bars" are being cut by the flux, like in case of concentrated stator windings of a synchronous generator? If the...
  48. 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...
  49. L

    Average back emf induced in coil

    Homework Statement hey! I saw you've answered this question before... 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...
  50. electric jake

    Why does Back EMF produce so much voltage?

    I apologize if this question is a bit confusing, but I am studying electrical engineering and what keeps baffling me is why when you run current through an inductor then place an open in the circuit how come the Back EMF current is able to cause a spark across the open. I get that the inductor...
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