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. 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...
  2. Blitzmeister

    Induced emf of Sliding Wire Down Rails

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  3. 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...
  4. AdityaDev

    Measuring resistance using potentiometer

    Homework Statement Homework Equations For R, current I=E/R and for X, I=E/X... both are separate cases. The Attempt at a Solution [/B] Why does the balancing change? The potential difference across each resistor is E Itself right? I tried by assuming an arbitary internal resistance r...
  5. A

    Inductor Role in Time Constant of a DC Circuit

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  6. I

    EMF radiation shielding and dirty electricity

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  7. D

    Back EMF in Average Loudspeaker Coil: Compare to 18V

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  8. D

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  9. PhiowPhi

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  10. W

    Equation for the EMF generated by a spinning magnet?

    I have found an equation which gives the EMF generated in a coil when a magnet is spun near it, the equation is given as: E = FNAω × 10-8 volts. Where F is the field of the sample perpendicular to the axis of roation, N is the number of turns, A is the effective area of the coil and ω is the...
  11. G

    How should I think about the emf?

    I was under the impression that an emf is the work done per charge by any source that is not electrostatic. However, there are such things such as emfs produced by varying magnetic fields. Being that magnetic fields never do work, what is an appropriate way of thinking of an emf?
  12. hideelo

    What happens in a simple circuit with an EMF and a resistor?

    Imagine a simple circuit, an EMF connected to a resistor which connects back to the EMF. This is how I'm thinking about it I know there is a current which means that there is charge moving around the loop which means that there must be some field because otherwise the charges wouldve never...
  13. arpon

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

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  14. carllacan

    Where is the EMF located in a circuit under induction?

    Suppose we have a closed circuit with a time-varying magnetic flux. An EMF is inducted on the circuit. Where would we put the battery representing the EMF? For example, suppose we have a resistance in the circuit and we want to know the intensity through it. Would we just set the voltage...
  15. B

    Question about the RLC circuit

    In the direct current circuit, when you have a switch and you turn it on, you get the coil to generate a current to the opposite direction but the magnitude isn't big enough to offset the main one. But in the case of alternating currents, the coil seems to generate enough current up to the...
  16. G

    How do the magnetic fields produce emfs?

    I should add context to this question that what I know so far of the emf is that it is defined as ##\mathscr E=\oint f \cdot d\ell## where ##f## is the sum of the forces on the charges. In the textbook I'm reading it gives an example of where a person pulls a rectangular wire with a velocity...
  17. 2

    Magnetic fields in transformers and back emf?

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  18. S

    Why is ABCDEFA considered a closed loop even with the presence of conductor BE?

    Homework Statement So this is not really a 'solve this problem' question, but, rather, I am trying to understand how they can call ABCDEFA a 'closed loop' even though there is another connection (BE) inside it, and why the equations work. Keeping in mind that I am still in high school...
  19. 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...
  20. Coffee_

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  21. Samurai44

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  22. Y

    Faraday's Law and Motional EMF: Exploring Causes

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  23. PhiowPhi

    Misconception of motional EMF?

    For motional EMF: $$ \epsilon = -vBL$$ If a conductor moves in a constant magnetic field, there isn't induced EMF correct? Only when existing/entering the field there is induced EMF? I've had this misconception, that any motion(fast/slow) in a magnetic field would instantly induced EMF, but I...
  24. I

    Internal Resistance of a Battery homework

    Homework Statement Students are given some resistors, a battery with internal resistance, and an ammeter. They are asked to determine the emf e and internal resistance of the battery using just this equipment. Working with the circuit shown at right, they insert each resistor into the circuit...
  25. Just_some_guy

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  26. H

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  27. Ahmad Kishki

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  28. N

    Does a high emf excite electrons?

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  29. K

    Question re: emf waves distance & speed

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  30. C

    Power wasted/delivered to a resistor

    Homework Statement two 8 ohm resistors and a battery of emf 12.0V and internal resistance of 8ohms are connected in series. Calculate the power delivered to each external resistor and then calculate the power wasted due to internal resistance. I keep coming up with the answer 18 but apparently...
  31. 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...
  32. 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...
  33. M

    Circuit analysis - two sources of emf

    Hello all! I'm confused about an explanation that my textbook gives, and I'm wondering if someone can help me understand it. The passage is as follows: A circuit consists of a 12 V battery with negligible internal resistance, in parallel with a charging unit with an emf of 15 V and internal...
  34. 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?
  35. R

    Calculating EMF Induced by Moving Rectangular Loop

    Homework Statement A rectangular loop of wire, of sides 5 and 3 cm moves away from a a long straight wire at 4ms-1 carrying a current of 2 amps. What is the EMF induced in the moving loop, when the near side of the loop is 3cm away from the wire? Homework Equations B = μ0I/2πr EMF = -dΦ/dt...
  36. K

    EMF Radiation Measurement: 15 Milli Volts vs Milli Volts/Meter

    I measure the Electromagnetic Field Radiation at a point for example x distance from a source, and i am reading for ex. 15 milli volts per meter. Are these 15 millivolts per meter, equals to 15 milli volts ? I mean, if i do the opposite procedure, and receive a reading of 15 milli volts, at a...
  37. A

    Electromotive Force: E=Vab=IR Explained

    The potential difference between the ends of a wire is given by Vab = IR. Combing that with E = Vab we have E = Vab = IR (ideal source of emf). " That is, when a positive charge flows around the circuit, the potential rise as it passes through the ideal source is numerically equal to the...
  38. adjacent

    Definition of Electro Motive Force

    Homework Statement The title. I am confused with the definition as my textbook does not have a good definition for this. P.S I have my exams tomorrow so I can't ask my teacher now.(Mid night) Homework Equations $$V=\frac{E}{Q}$$The Attempt at a Solution E.M.F is the energy given to each...
  39. PhiowPhi

    Confused, velocity in motional EMF?

    Motional EMF is defined as E = -vBL Where B is the magnetic field in Tesla, L is the length in meters and ,v is the velocity. Lets say we have a distance(1m) where half of it(0.5m)the magnetic field is present. So, there is a wire that moves with constant acceleration throughout that whole...
  40. J

    How Does Back EMF Cause a Voltage Drop in Household Circuits?

    Hi pf, I have been reading about back EMF and just had a few questions. I found this article on wiki (http://en.wikipedia.org/wiki/Counter-electromotive_force) and the last paragraph talks about the effects of back emf when you plug a vacuum cleaner into your wall. It says that you can notice an...
  41. almarpa

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    Hello all. I am currently studying electromagnetism with Griffiths' books, and I have already donde electrostatic and magnetostatics. Now I am reviewing Ohm's law en emf concepts, but I have a doubt: In griffths book, when explaining ohm's law and emf, it seems to me that he assumes...
  42. gaba

    EMF generator from audio speaker

    Hello everybody, I am trying to figure out how to build an electro magnetic generator. I was wondering if an audio transducer such a speaker could act as a EMF generator, when the paper woofer is removed and the device connected to a function generator and an amplifier, in order to play the...
  43. gaba

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    Hello everybody, I am trying to figure out how to build an electro magnetic generator in order to stimulate my home and my brain with beneficial frequencies such as the Schumann resonance and doing perceptive experiments on all the surrounding frequencies. I was wondering if an audio transducer...
  44. I

    Find EMF - Loop inside a loop generating a magnetic field

    Homework Statement A loop of wire with an area 1cm^2 is centered within a larger loop of area 1m^2. Apower supply is generating current through the larger loop described by the function: I(t) = 40A * cos(2t/s * 2pi/5) Calculate EMF at time t = 3sHomework Equations Magnetic flux =...
  45. C

    Diamond Circuit, Three EMF Sources -- Parallel?

    Hi, Is the circuit in the attached image essentially one with three parallel branches, each with a different source of emf and two resistors? ie, ε1, R1, R2 on one branch, ε2, R3, R4 on another, and similarly for the last one? Thanks!
  46. A

    Question involving EMF and internal resistance

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  47. Solibelus

    Difficulty understanding EMF due to a changing magnetic flux

    Hello everybody, I'm studying classical electrodynamics and I'm having a difficulty grasping the concept of EMF. I'll try to explain what causes my confusion. Say I've got a conducting loop, an electrical wire, with some resistance R which contracts (or expands) with time. Let's put that loop...
  48. D

    Magnitude of EMF when the armature coil is in different positions?

    I am currently studying for a Magnetism and Electricity exam coming up. I was just wondering, within the topic of Electromagnetic induction; my textbook states that if an armature coil of some sort -either from an AC or DC generator was placed horizontally into a magnetic field that is also...
  49. F

    Elevated EMF / AC magnetic fields in home

    Apologize in advance for the long post! We are in this particular situation and, being a novice with electrical- related issues, would greatly appreciate opinions here. After a year long search, we found this nice home in Texas meeting all requirements my wife and I were looking for, and at...
  50. 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...
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