What is Emf: Definition and 939 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. gochaka

    Induced EMF in a rotating ring

    [Thread moved from the technical forums to the schoolwork forums] Let's suppose we have a ring with radias R and radius of the cross section r. We start rotating with an angular velosity w. We will there be an induced EMF in the ring and how can i find it's value?
  2. S

    Formula for the average EMF of generator

    I know the formula of emf of generator is ##\varepsilon=NBA\omega \sin (\omega t)##. If I draw the graph of emf against time, the graph will be sinusoidal and if I find the average, the average will be zero. How can the average emf of generator is ##\frac{4NBA}{T}##? Thanks
  3. L

    Introduce a battery to an LR circuit

    Let's say I have an LR circuit. I understand the mathematical derivation of this but would like to understand it conceptually. Okay, so a battery is introduced (let's say 12 V). The inductor hates the change, so it induces 12 V in the opposite direction, opposing the battery. So what happens...
  4. P

    Modelling the voltage measured across a dc motor

    Hi there, I have done an experiment measuring the voltage across the terminals of a dc motor, whilst running and then by holding it with my fingers to stop it. Done as demo for my high school class. The voltage reading was higher when the motor was running, which was against my expectations as...
  5. Annabel

    Can the human sense of touch detect differences in molecular charge?

    Kia ora, from New Zealand. A Google search found the topic "Spent batteries feel lighter" ... and for me, they do too, and for batteries that have a full charge, I can feel that too. Some people have higher sensitivities to EMF than others. Some people can sense molecular differences by touch...
  6. icemanhv

    Find the Retarding Force due to Eddy Currents

    TL;DR Summary: calculate retard force A linear conductor of length L moves at a velocity V over a fixed conductive plate at a distance R0. Current flows through the conductor, the direction of movement of the conductor is perpendicular to the direction of current in it. Find the retard force...
  7. S

    Correct energy transformation about circuit with EMF, a resistor and a motor

    My answer is C (although I am not sure if the correct one is C or D). I suppose since the 1-V battery is being charged by the 9-V battery, the electrical energy should be on the RHS of the energy transformation. Am I correct? Thanks
  8. vytautas_k

    I Faraday cage continuity question

    We have built a faraday cage out of 1,5mm thick galvanized steel sheets. Size is 1.5m*1.25m*1.5m. The steel got powder coated and was assembled on a frame. Each side of the cage is made from separate sheets that are joined using rivets. All of the separate sheets are connected with grounding...
  9. C

    B EMF Radiation detected from TV

    Hello. I have a quick question in regards to EMF radiation. In particular, I would like to know what quantity of microteslas a smart TV would emit. In particular, would it be unusual for a smart TV to emit in excess of 1000 microteslas even when turned off? Thanks.
  10. milkism

    Electromagnetism: Moving conductor and EMF

    For 1) I used $$V=Blv=Blwr$$, where $$w= \frac{4\pi rad}{sec}$$, $$l= 0.30m$$ and $$r=0.50m$$. I got 0.5 V. For 2) I used W=Vq=VIt, where $$q=It$$, where t=0.5 s, we get 1.125 J. For 3) I used P=IV, we get 2.25 W. Are these correct?
  11. D

    Calculate the induced EMF of a rotating loop

    there are a bunch of problems in this section that ask similar questions, but they ask the amplitude and this doesn't. this is an even problem so i do not have the answer, but my hunch is that it is not an amplitude question. i solved for the amplitude so i am guessing i got this one wrong...
  12. O

    Motional EMF in the presence of a time-varying magnetic field

    My question arises from the following problem: We have a uniform magnetic field into the page, decreasing at a constant rate dB/dt< 0, causing the bar to move to the right. Find the velocity of the bar as a function of the time, and in terms of the known parameters: the resistor R and the...
  13. A13579

    B Does a Faraday Cage trap EMF inside and increase exposure?

    Hi All, I’d like you to weigh in on a debate I’ve been having. I’ve been thinking about a Faraday Cage (FC from now on) but from the opposite perspective. Sure it stops EMF getting in. But does that also mean, if there are items emitting EMF in there, that gets trapped within the room. Taking...
  14. S

    EMF produced by generator of a car

    First I assumed the question asks about max e.m.f so I just used ratio: output voltage = (2500 / 1100) x 12.7 = 28.9 V, but the answer is wrong. Then I tried to use the ratio of the whole formula: $$\frac{\varepsilon_{1}}{\varepsilon_{2}}=\frac{NBA \omega_{1} \sin(\omega_{1}t)}{NBA \omega_{2}...
  15. S

    EMF produced across one side of a coil

    I want to ask about part c (iii). This is what I did: max e.m.f produced = NBAω = 32 x 0.074 x 4.8 x 10-2 x 2.4 x 10-2 x 2π x 9 = 0.15 V But the answer is 0.077 V so my answer is off by factor of two. Is my mistake related to "emf produced across the side of AC of the coil"? Thanks
  16. guyvsdcsniper

    EMF when a loop is pulled away from a current-carrying wire

    Setting up the integral to find the flux that is giving me trouble. I know that I will have to break up this integral into 2 parts, the first part account for when the start of the loop is increasing in area, and another right when I pass the halfway mark of the loop and the area begins to...
  17. M

    I Maxwell-Faraday Law & Stealth Magnets: Exploring EMF Induction

    Consider a closed path consisting of a loop of wire with a nonconducting gap that completes the closed path. The wire is threaded through a toroidal permanent magnet, magnetized around the toroid (what I call a stealth magnet). The magnetic flux is considered to be confined to the magnet. The...
  18. P

    Induced EMF due to motion of a wire perpendicular to a magnetic field

    This question appeared in a university entrance exam.Basically, if magnetic flux passing through a surface of a loop changes over time ,only then e.m.f will be induced to that loop.But here only a straight line is used and there's no chance of forming any area.So by definition there's no chance...
  19. LCSphysicist

    EMF induced in a wire loop rotating in a magnetic field

    To solve this problem, we need to evaluate the following integral: $$\epsilon = \int_{P}^{C} (\vec v \times \vec B) \vec dl$$ The main problem is, in fact, how do we arrive at it! I can't see why a Electric field arises at the configuration here. The magnetic field of the rotating sphere is...
  20. J

    I Does an inductor always create a back EMF?

    Let’s use a solenoid for example. Let’s say you have a current that passes through a solenoid clockwise. Due to Faraday’s law of induction, the induced current from the magnetic field from an increasing current opposes the initial direction of current causing resistance. If the initial current...
  21. J

    EMF generated in a blood cell by an oscillating magnetic field

    At first I tried plugging everything in with 60Hz in the numerator but that did not work. I was told to think about sinusoidal waves and derivates but I'm not sure how that works. Any ideas? Thanks a lot
  22. L_ucifer

    Using Faraday's laws to find the induced EMF

    Here is the question: We know the equation \epsilon = \frac{d\phi }{dt} = BAcos(\theta ). This means that the only way we can create an induced voltage is if we change the magnetic field, change the area of the loop in the magnetic field, or change the angle between the normal vector to the...
  23. billyt_

    Drop height of a magnet vs. induced EMF in a solenoid

    I am currently working on an experiment that involves dropping a magnet from varying heights and measuring how the induced emf of a solenoid changes as a result. I am currently somewhat struggling with a derivation for a relationship between the two variables, however, this is what I have been...
  24. kalm1307

    Resistor value given capacitance and EMF with a resistance R after 3ms

    T=R/C So: 0.003s=R/100μC R=3*10^-7Ω I am really confused with the equations I have to use
  25. C

    Going From EMF to Faraday's Law to find E-field in my HW problem

    Ok, so I understand how to find dphi/dt that is integral of -d/dt(B "dot" da). In this case I find a Phi that is a constant in space in time which causes me confusion in next step. Edit: dphi/dt is constant... Grithff's then says E field same as a Mag field above center of circular current. He...
  26. J

    EMF radiation in electric cars

    What is power of EMF radiation inside of electric cars? What is frequency range, ELF?
  27. SpectraPhy09

    Solve Motional EMF Problem: Total Resistance, V(ind), I(ind), and Fnet Explained

    I request please go through my solution Total Resistance = R + 2λx V(ind) = vBd I(ind)= vBd/(R + 2λx ) v = i(R + 2λx )/Bd If force on the wire due to induced current is iBl then Fnet = iBd - F ...(i) or Fnet = F - iBd ...(ii) Fnet = v(dv/dx) v(dv/dx) = iBd - F By solving...
  28. Benjamin_harsh

    Back EMF vs Reverse Currents: What's the Difference?

    Is it Back EMF, Reverse Current, Reverse Saturation current, Backfeeding? If not, tell the difference.
  29. guyvsdcsniper

    EMF and how it's related to Potential Difference

    So Kirchoff's Loop law states that, The sum of all the potential differences encountered while moving around a loop or closed path is zero. Ok so that is basically a statement of energy conservation. So I see why in the TYPED solution, they related all voltages in the circuit equal to zero. I...
  30. np115

    How to solve for EMF and internal resistance in a circuit?

    I know the answers are 9 V and .5 ohms but I have no idea how to get there. Originally I solved for Delta V in the open circuit through (1.636)(5) which gave me 8.18V. Then, I solved for Delta V in the closed circuit through (1.565)(1/((1/5)+(1/10)) which gave me 5.51 V. I used these voltages...
  31. S

    EMF due to Lorentz electric and magnetic forces

    Example of emf due to Lorentz magnetic force is motional emf. When rod PQ moves to the left, there will be downwards magnetic force acting on the positive charge in the rod PQ so point Q is at higher potential compared to point P so there will be potential difference (emf) between P and Q The...
  32. K

    Electric field in a rotating rod in a magnetic field

    The first part of the problem seems easy enough, the free electrons in the wire would move in a circle owing to an electric field that would be induced in the rod which would provide the centripetal force for the same (Please correct me if I am wrong). So we have $$eE=mω^2x$$, where e is the...
  33. I

    Is it possible to reduce back EMF in DC motors using shorting rings?

    Shorting rings (also called Faraday rings) are commonly used in loudspeakers to reduce the back emf that is induced in their voice coils. Could something similar, i.e. shorted turns of wire that are placed in the same plane as each of the windings in a DC motor, be used to reduce the back emf...
  34. S

    Why is the value of EMF generated by a generator not zero at t = 0 s?

    Does this mean that initially there will be emf induced on the coil at t = 0 s? Initially, at t = 0 s, the coil has not been rotated yet so based on Faraday's Law, there won't be any change in magnetic flux so why the value of emf is not zero at t = 0 s? Thanks
  35. F

    Question about a simple model for for back EMF

    Summary:: I've found an exercise about sparks caused by back emf. The exercise is based on a mathematical model of the current flowing in the circuit around the time when the switch is closed. I'm wondering to what extend that model is realistic The exercise I'm referring to proposes this...
  36. P

    I Unveiling the Proof of Induced EMF Formula: E = ∫ (v x B) · dl

    Why is ##E = \int (\vec v \times \vec B) \cdot d \vec l##? This seems to be a general formula, and I would like to know its proof. Thanks for all the help.
  37. PhysicsTest

    Checking the Back EMF of a Motor

    I want to check the back emf of motor as shown in the article below My question is does a normal probe will be sufficient or do I need a differential probe to check the back - EMF waveform?
  38. B

    Induced EMF in a small square loop of wire at center of an AC Circuit

    I used the voltage of the power supply and resistance to solve for the current in the larger circuit (20V/5ohms=4 amps). I am not sure if the equation listed above is the correct one I should be using, but I tried it using the following numbers. For omega, I used 2*pi*frequency. N should...
  39. R

    What Are Some Options for High Frequency EMF Meters?

    Hello all, I don't know if this is the correct place to post, but I'm looking to detect all electromagnetic waves around me that have a frequency that is under the visible spectrum. The reason is to try and get some kind of idea of what kind of radiation is around me, so that I can be more...
  40. AN630078

    Root-mean squared values and a.c. emf

    1. We are given the root-mean squared value for the voltage at 230V. Therefore by rearranging the equation Vrms=V0/√2 one can find the value of the peak voltage; ie. V0= Vrms *√2 V0=230*√2 V0=325.269... ~ 325 V (which is the peak voltage of mains electricity in the UK). To find ω use the formula...
  41. Z

    EMF induced by a magnet falling through a coil

    I've been told that if you drop a magnet through a coil the induced emf and flux graphs would look like this: I understand that when the bar magnet is in the middle of the coil the emf induced is zero as flux change in top and bottom is in opposite directions but why is effective flux maximum...
  42. H

    What is the magnitude of the induced EMF?

    I did the question in this way as attached. I got the crrect answer, but I feel unsure if it is the right way of doing it.
  43. alan123hk

    B What Is the Correct Calculation for Induced EMF in This Circuit Scenario?

    I think the answer to the first question is 3V and the answer to the second question is 6V Does anyone have a different answer?
  44. greg_rack

    Exploring EMF in Moving Conductive Bars

    Hi guys, I have barely started studying EMI, and already have a question about the nature of the current induced in a bar of conductive material traveling through a magnetic field. The moving charges inside the bar will experience a Lorentz's force, pushing the negatives on one side of the bar...
  45. B

    Motional EMF for loops of wire vs conducting plates

    I'm in an intro E&M class, and I'm trying to distinguish between Motional EMF for loops of wire and conducting plates. This question might be kind of silly, but are Eddy currents pretty much the same thing as induced currents in a loop of wire? More specifically, what I am trying to ask is if...