Emf Definition and 939 Threads

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. 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...
  2. 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...
  3. R

    How Is EMF Induced in a Moving Conductor Calculated?

    Might have posted this in the wrong place before. A metal wire slides on a horseshoe-shaped metal loop of width 0.25 m. The loop has negligible resistance, but there is a 1Ω resistor in the circuit as well as a 6 volt battery. There is a uniform magnetic field directed into the plane of the...
  4. R

    What Is the Induced EMF in a Moving Conductor?

    A metal wire slides on a horseshoe-shaped metal loop of width 0.25 m. The loop has negligible resistance, but there is a 1Ω resistor in the circuit as well as a 6 volt battery. There is a uniform magnetic field directed into the plane of the page of magnitude 0.5 T. The slide wire is pushed...
  5. S

    What is the definition of EMF?

    Actually my sir asked me the definition of EMF so I just tell him that "Suppose a resistance(R) is connected across the terminals of a battery.A potential difference is developed across its ends.Current(or positive charge) flows from higher potential to lower potential across the resistance by...
  6. 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°...
  7. 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...
  8. 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...
  9. W

    Rate of emf decline and rate of change of internal resistance

    Hello everyone, I am trying to work on this topic as my research. i plan on using a potentiometer but i want to be able to use the resistor (across the test cell) as the load and still be able to obtain two balance points. one for emf, and another for calculating internal resistance as the cell...
  10. 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...
  11. C

    Neutralize EMF Radiation w/ Vibrational Energy Hologram

    A hologram with "vibrational energy" to neutralize EMF radiation within its radius? http://www.safespaceprotection.com/qanda.aspx How do the products work? Our products 'repattern' the electromagnetic field by creating a 'coherent polarizing field effect'. When the charged particles are...
  12. Y

    Exploring the EMF of Guitar Pickup Coils

    I have a discussion on how guitar pickup work, pickup is a coil with thousands of turns and magnet poles in the middle of the coil under each strings. My idea is the magnet poles magnetize a short portion of the string and when the string vibrates, it create a varying magnetic field. When the...
  13. G

    Calculate Emf in a magnetic field

    Homework Statement Calculate the EMF induced in a 5.0 cm by 2.5 cm rectancular coil containing 150 loops if it rotates at 28 Hz in a 0.014 T magnetic fieldHomework Equations Φ=BA ε=-N ΔΦ/Δt The Attempt at a Solution Φ=BA =.05m x .025m =1.25 x 10^-3 ε=-N ΔΦ/Δt =150 x (1.25x10^-3) /...
  14. A

    The emf of a string in a circle - magnetic fields

    Homework Statement there is a cylindrical area in xy plane with a magnetic field (into the plane) that changes with time like that: (dB/dt)=δ the magnetic field outside the area is 0. the radius of the cylinder is a. we pick a string in the circle with lentgh L which is smaller than the...
  15. F

    Determining Counter EMF in Magnetic Fields

    In the example diagram I have provided how can the counter EMF produced by the conductor moving through the magnetic field be determined? I am assuming the field is uniform and that the velocity and current in the wire is constant. My logic tells me that there is no net flux cut by the...
  16. P

    Electrochemistry, emf vs HCL concentration

    Homework Statement I just conducted a lab for pchem where we took electric potential measurements with a volt meter in a solution for the reaction AgCl(s) +1/2 H2 (g) <--> HCl(aq) + Ag(s) We took the measurements using different concentrations of HCl. We were told that the...
  17. 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...
  18. phosgene

    Changes in EMF due to a bar magnet falling through a loop of wire

    Homework Statement A bar magnet is dropped through a horizontal coil as shown below. An EMF is induced in the coil with the resultant voltage shown on the graph. Explain the shape of the graph, in particular the differences in the regions marked as A, B and C. Homework Equations...
  19. T

    Induction of emf in moving conductors in magnetic fields

    In preparation for an upcoming physics exam I have been going through some past paper questions. This question is from the multiple choice section of the paper, so while I can check the answer there is no source from which I can see it being worked through. Homework Statement An aircraft...
  20. R

    Back EMF: Explaining the Counter Voltage Induced by Change in Current

    Hi, I have a question about back emf So, my understanding of back emf (\epsilon_b=-L\frac{dI}{dt})is that its the counter voltage that's induced by change in current, whether it be from a battery or magnetic field (\epsilon_0). We can model this as an LR circuit where \epsilon_0 +\epsilon_b=IR...
  21. Ƒ

    Are EMFs Really Causing Your Poor Sleep? Separating Fact From Fiction

    Recently, someone I know has become convinced that the poor sleep I'm getting is the result of EMFs being emitted by anything near my bed that's electrical. I think this is complete ******** (Edit: look at that, the forum auto-censors). Although, if that was the end of the matter, I'd be...
  22. 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...
  23. 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?
  24. 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...
  25. 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...
  26. R

    Why Does Calculating EMF Use a Line Integral Instead of a Surface Integral?

    Hi, I'm currently reading griffiths and I'm a little bit confused in how it calculates emf. EMF is initially introduced as a path integral of force/unit charge around a closed loop. This makes sense, but then he presents us with an example problem (ex 7.4) Basically, a circuit resistor is...
  27. M

    Measuring Electromagnetic Radiation and Frequency: What Equipment Do I Need?

    Ok so I'm no pysichs buff, but could someone describe what the differences between Electromagnetic Radiation (Is this just a name for radiation?) and Electromagnetic Frequency (Do microwaves fall in this category aswell)? I basically would like to know the generic name of the devices (not brand...
  28. 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...
  29. 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...
  30. 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?
  31. S

    Exploring the Effects of Self-Inductance and DC Excitation on Coils

    Hi, When coil is excited with ac because of varying nature of current ,flux of varying nature will be developed in coil and because of varying flux emf will induce in coil. what will happen if coil is excited with dc? intially cureent will increase from zero to maximum and is...
  32. E

    Induced motional emf conceptual question

    A magnetic field points into the page. A conductive bar is moved to the right (perpendicular to the magnetic field). Why do the electrons move downwards toward the bottom of the bar? The book doesn't offer a good explanation of this... using right hand rule I would say that the electrons are...
  33. 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...
  34. T

    Emf induced on an eccentric rotating bar

    Suppose that a conducting bar is rotating with a speed ω, around an axis a distance a from one end, and b from the other. Suppose further that it is immersed in a uniform magnetic field B parallel to the axis of rotation, and that the measured difference of potential between both ends is V. I...
  35. U

    Finding pd given r, R, and emf?

    Homework Statement What potential difference is measured across a 23 ohm load resistor when it is connected across a battery of emf 7.99 V and internal resistance 0.555 ohms Answer in units of V Homework Equations V=IR V=emf - Ir The Attempt at a Solution V=IR emf-Ir=IR...
  36. P

    Finding peak Emf, pretty quick answer

    I attached the link to the problem. Shouldn't the Vlmax be 330V? I tried entering that and it wasn't accepting my answer.
  37. 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...
  38. S

    Motional EMF (direction of moving bar)

    Homework Statement Can someone please explain to me how to determine the direction of the veloicty of a bar that is conducting and inducing a current. Fb=q(VxB)?
  39. S

    What is the role of motional emf in opposing motion?

    Homework Statement Hello all, If we have a bar that is perpendicular to a magnetic field, and that bar is being pushed to the left via an applied force, all the while a current (due to a voltage source and resistor) runs down through the wire. Therefore, there would be a total force comprised...
  40. S

    Find unknown EMF magnitude in circuit

    Homework Statement A DC circuit which includes Emfx of unkown magnitude and orientation, at the location indicated is shown below. Note the 'ground' (V=0) in the circuit. The potential between 'b' and 'a', Vb-Va, is measured to be 2.00 V. a) The magnitude of Emfx is: b) Calculate...
  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

    What is causing the input voltage to be attenuated on the oscilloscope?

    Hello all, I'm not quite sure if this is a physics question or an EE question because it came up in an electronics design course for physicists with mostly engineering motivation, anyways here goes: I have an RLC band pass filter with a sinusoidal AC voltage input (Agilent function...
  43. X

    Magnetic Induction: EMF Function of Time

    Hi, I've been thinking about the shape of the voltage waveform induced by a magnet falling through a coil. I know (both intuitively and from empirical experience) that the voltage should become increasingly positive as the magnet approaches the coil, then it should decrease rapidly...
  44. B

    Faraday's Law and EMF calculation

    Hi, I found this site with a calculation for EMF: http://www.6pie.com/faradayslaw.php. I just want to see if I have this right. Magnet Surface: Height (0.05 m) * Width (0.004 m) = Area 0.0002 m2. Coil windings: 6 Rotations per second: 5 NdFeB Magnet in N42 (1.33 Tesla) (I will reduce this...
  45. B

    Calculating EMF for Magnetic Generator Design

    Hi, I hope someone can help me. Looking at a magnetic generator, we will have a ring of magnets which will rotate, and then we have the copper wire static. I understand that if I have a ring of alternating magnets, all placed right next to each other, thereby creating a solid magnet surface...
  46. H

    Terminal PD vs EMF: When is PD Greater?

    when is terminal pd greater than emf ?
  47. B

    Calculating Kg Force for Generating 1 Watt of EMF in Magnetic Generators

    Hi, I wonder if someone can help me. Looking at magnetic generators, we spin the magnets around which will induce an EMF. This EMF will create a magnetic field opposite that of the magnets, thereby creating a drag. If I had to generate 1 watt for 1 second, how much kg force will I need to...
  48. M

    How Is EMF Generated in a Coil Flipped in a Magnetic Field?

    A 25-turn circular coil of wire has diameter 1.00 m. It is placed with its axis along the direction of the Earth’s magnetic field of 50.0 /T, and then in 0.200 s it is flipped 180°. An average emf of what magnitude is generated in the coil? This is a question in my book and the answer was...
  49. A

    Back E.M.F Protection: Selecting Diodes & Surge Suppression

    Help me. I want to use diode for back E.M.F protection.Please help me to select diode which is suitable ,like short circuit recovery time is 9 nano second and ifsm should be 30-90 amp. please also send me surge suppression protection circuit or surge protection device.
  50. B

    Calculating the EMF of batteries

    Hi all, I'm a total noob when it comes to chemistry, and would need your help to understand the calculation of EMF for batteries. I have a couple of questions about the calculation for Zinc-Carbon batteries in Wikipedia: http://en.wikipedia.org/wiki/Zinc%E2%80%93carbon_battery I can't...
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