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. Pi314227

    Electromagnetism: Determining Induced EMF

  2. E

    Velocity of charges or bounding curve features in motional EMF?

    The motional EMF is$$\mathcal{E}_{\text{motional}} = \oint_{\partial \Sigma} (\vec{v} \times \vec{B}) \cdot d\vec{x} = \int_{\Sigma} \frac{\partial \vec{B}}{\partial t} \cdot d\vec{S} - \frac{d}{dt} \int_{\Sigma} \vec{B} \cdot d\vec{S}$$(that's because Maxwell III integrates to...
  3. S

    EMF induced in a straight current-carrying conductor

    An emf is induced in straight current carrying conductor as it moves at at right angles to a uniform and constant magnetic field. My textbook used direction 1 in the image shown to demonstrate this. I asked my teacher if direction two would be possible and he didn't understand me. So I want to...
  4. Kaushik

    Intuition about electromotive force (EMF)

    My understanding of emf Let us consider 2 parallel plates with charges (opp. but equal in magnitude) stored on it. When we connect both the plates from the outer side, the electrons from the lower potential (i.e., negatively charged plate) moves to the higher potential (positively charged plate)...
  5. Nomadyb

    Find EMF of ℰ on Wire: Speed v, Force, C, B, m, g, l & a

    a) the EMF of ℰ induced on The Wire, as a function of the speed 𝑣of the wire you can find it. b) flow induced in the wire𝑖, 𝐶, 𝐵, and are denominated in 𝑙 𝑎 (𝑎= the acceleration of the wire). c) find the magnetic force acting on the wire in𝐶,𝐵, 𝑙 and𝑎. d) 𝑎 acceleration, 𝑚, 𝑔, 𝑙, and are...
  6. KarlsShwarzschild

    Electromagnetic induction (finding ΔΦ).

  7. K

    DC generator: Induced EMF of a single coil armature

    Hi. I am reading a textbook about simple DC generator. The situation is if we are rotating a single armature coil within a constant field B, the induced emf will be (if we are to only think about the magnitude), emf= B*l*W*omegaB = magnetic field l = length of the coil W = width of the coil...
  8. F

    Motional EMF in the frame of a moving wire

    When a magnetic field is moving (moving magnet) while a wire isn’t---in the reference frame of the wire, there’s no known magnetic force/effect on the wire before the introduction of motional emf and relativity. If viewed in the reference frame of the magnetic field instead where the wire is...
  9. K

    EMF shielding using a conductor

    So I've been trying to figure out how EMF shielding works. More specifically, I've seen videos where placing a metal conductor in front of a circular coil (with AC running through at radio frequencies) apparently shielded anything behind it. After searching online, I repeatedly saw Eddy...
  10. F

    EMF below Power Transmission Lines

    Many years ago I was responsible for maintaining the integrity of a buried fiber optic cable that utilized a railroad right-of-way. Kinda like watching submarine races ! Along much of this routing, transmission towers / power lines would be adjacent to the railroad. Talking with railroad...
  11. Kaushik

    Understanding EMF and Potential Difference in Electrical Circuits

    Is emf the work done to move a positive charge from LOWER potential to HIGHER potential to maintain the potential difference or else the charges move from higher potential to lower potential and will reach a point where the potential is the same between the two points and the charge will stop...
  12. Kharrid

    Induced EMF in a moving Loop Conductor

    I am having trouble figuring out if the circular loop has an induced current. One explanation is ∫ E ds = -d Φ / dt. Since flux = B ⋅ A, a change in the magnetic field would require a change in the magnetic field, a change in the area, or change in direction of either vector. Since none of...
  13. P

    Background EMF levels abnormally high?

    Hello I am sitting in my home and I'm using 2 phone apps to measure my EMF levels (they both got the same number). I measured it about every 10 minutes. This is what I got 1)~46uT 2)`~189uT 3)~187uT 4)~48uT 5)~187uT 6)~50uT 7)~50uT It never reached under 40uT and I would say this morning, on...
  14. J

    Find EMF of a battery in this circuit

  15. A

    Universal motor with commutator, recovering the back EMF?

    So while thinking about motors this suddenly struck me, So as the universal series wound motor is spinning there is always some arcing going on around the place where the brushes contact the copper segments that slide past them, I assume this is at least partly because as each coil pair of the...
  16. F

    Induced EMF and current for a bar sliding on a circular rail

    On the left: my copy of the illustration in the problem. On the right: top view, with the angle. The problem gives the magnitude of the magnetic field, the radius of the rail, the resistance of the resistor, the initial rotational frequency of the bar. I am able to obtain the given solutions...
  17. E

    Clarification about the sign of EMF in batteries and electrochemistry

    In electrochemistry, we define ##E_{cell} = E_{cat} - E_{an}##, the difference between the electrode potentials of the cathode and the anode. This has the effect that if the reaction is spontaneous, we obtain a positive ##E_{cell}## and if it is not - i.e. we need an external driving voltage...
  18. B

    Solving Faraday's Law: 0.73V Induced EMF Help Needed

    I have a simple sketch of the diagram, and I know I must use the vertical component of the magnetic field of the Earth when doing this problem I got an induced emf of 0.73 volts but I do not know if I correctly substituted the right values into faraday's law equation? Any help will be really...
  19. funmi

    The EMF induced in straight current-carrying conductor

    Why is an emf induced in straight current carrying conductor as it moves at at right angles to a uniform and constant magnetic filed. By Faraday's law, this e.m.f. is equal to dΦ/dt but I do not understand how a wire cutting a uniform magnetic field experiences a change in magnetic flux . Its...
  20. funmi

    Why does a change of magnetic flux induce an EMF?

    How and why does a changing magnetic flux induce an emf? Why doesn't a static one also produce one? How are the electric and magnetic forces related? Why do you move a wire through a magnetic field so that the wire, the motion, and the magnetic field are all mutually orthogonal in order to...
  21. O

    B Question about EMF and EEG emission

    1. how do you block or interfere with the emf(electromagnetic field) emission generated by your brain? 2. how do you block or interfere with an EEG taken from the mind(what factor influence EEG taken from the mind and can add noise to the signal?). 3. Regarding US Patent 3,951,134 "Apparatus...
  22. I

    Electromagnetism EMF induction calculations

    A square conducting loop of side length a is in a non-uniform magnetic field. The loop occupies the first quadrant of the xy plane, i.e. the space between the origin (x, y) = (0,0) and the point (x,y) = (a, a). The magnetic field is in the +z direction. Develop an expression for the magnitude of...
  23. Adesh

    I Calculating EMF in a Solenoid: Understanding the Relationship of Turns and EMF

    If I have a solenoid with N number of turns in total. And if I say that in each turn the EMF is equal to e then can I conclude that the total EMF in the solenoid i.e. from A to B is N \times e . I’m asking this because whenever a current I flows in each turn of the solenoid then we...
  24. garthenar

    Power related to the EMF of an adjustable coil in a magnetic field

    I'm already stuck on A. I'm hoping once I figure that out the rest will just fall into place but be prepared for this to take awhile. I understand how to use Faraday's law to get the current or voltage of the system based off the movement of the bar but I have no Idea how to relate the rate of...
  25. D

    Planning to measure voltage (back EMF) caused by opening a switch in series with an inductor

    i am planning to measure the back emf produced by inductor when you open a switch. i know it is very hard to predict the voltage. but is there any way to narrow the possibilities?
  26. D

    How to detect an inductor's back EMF?

    how do i detect an inductor's back emf on a breadboard? explain in detail. i am planning to connect an inductor to a switch. i have read when you open a switch, the inductor creates a back emf. how do i detect this back emf, when the switch is opened? explain in detail.
  27. C

    EMF induced in an off-axis solenoid due to a rotating magnet

    The red arrows correspond to the areas the solenoid will be moved too
  28. Luxucs

    Faraday's Law & Motional EMF Confusion

    Hi there! I have what I hope is a relatively straightforward question regarding Faraday's law and motional emf, but its been causing me to scratch my head for quite a while. Consider the diagram attached to this post (source is linked at the bottom). Assume that all of the wires and the rod are...
  29. D

    The Magnitude of Induced EMF in a Moving Rod?

    Homework Statement: A 25.0 cm long metal rod lies in the xy-plane and makes an angle of 36.9 with the positive x-axis and an angle of 53.1 with the positive y-axis. the rod is moving in the +x-direction with a speed of 6.80 m/s. the rod is in a uniform magnetic field B=...
  30. Physics lover

    EMF induced in a rotating rod having mass

    I first calculated induced emf and then calculated torque about O.But what will i do further.I think i need to find current and then fidn emf=IR. Help please.
  31. TOUHID11

    Calculate Induced EMF in Circular Loop: Cylindrical Coordinates

    In order to calculate for the curl of the induced electric field for a loop moving in a uniform magnetic field, and using the cylindrical coordinate system for a curl, it's my understanding that since the B field is in the 𝑧̂ direction, then so is the partial time derivative of B, and therefore...
  32. alv

    B What would happen if there were no back EMF in an electric motor?

    I know that as per Lenz's Law, back EMF moves in a direction so as to counter the flow of the changing magnetic flux which induced it in the first place. And it does so because if it flowed in the same direction it would contravene the law of conservation of energy as more and more energy would...
  33. b1llso

    Find the EMF generated by a guitar string in the pickup coils

    I tried using the general EMF equation but don't know how to apply it with the equation from the problem
  34. T

    Finding the induced EMF in a solenoid-wire system

    I know that ##B = \mu n I## and ##\phi = B \pi R^2##. So with have ##d\phi / dt = \mu n \alpha \pi R^2##. But I don't know what to do with this? is this the answer? I don't think so but I don't know what to do after this.
  35. K

    Find ∆V while sliding a card through a card reader with magnetic field

    emf = dΦ/dt = (B*A)*d/dt = B(dA/dt), dA/dt= L*d/dt(vt) = L*v, emf = B*L*v per coil Since there are 25 loops the total emf= 25(vBL) This is where I'm am stuck. Would I assume that B is 24 uT, the velocity as 3m/s , and the length as 1mm? If so I would get ∆V as 1.8*10^-6.
  36. D

    Need someone to check my equations and work on this induced EMF problem

    a magnetic field has a magnitude at time t of 4.5x10^-5 T +(2.6x10^-6 T/sec)t. the field points in the positive z-direction. a rectangular wire loop, which is 0.3 meters by 0.4 meters, lies in the xy-plane. Note: If the z axis points out of the paper. a.) What is the induced emf in the loop...
  37. christang_1023

    Solve the differential equation of motional emf

    . Above is the figure of the problem. I am trying to solve x(t) and differentiate it to obtain v(t); however, I have difficulty solving the differential equation shown below. $$ v(t)=\int a(t)dt=\int \frac{B(\varepsilon-Blv)d}{Rm}dt \Rightarrow \frac{dx}{dt}=\frac{B\varepsilon...
  38. cianfa72

    Galvanic cell - open circuit voltage and EMF

    Hi, having not a deep knowledge of electrochemistry I've some doubts about processes involved in a galvanic cell. Take for instance a Zn/Cu Daniell cell for which E0cell is 1,10V. That means emf for it is 1,10V. Starting to read from how battery works I had a first understanding of how...
  39. Fleuryette

    Electromotive force (EMF) induced in a rectangular loop

  40. L

    Exploring EMF and Faraday's Law in LR Circuits

    1) Take a non-steady circuit such as an LR circuit. Why does Kirchoff's voltage law work when analyzing such a circuit? Is it because we're assuming that dI/dt and thus dB/dt are approximately zero thus meaning that curl E is approximately zero? 2) ε, the electromotive force, is the line...
  41. A

    Calculate EMF & Resistance of Solar Cell

    A solar cell generates a potential difference of .1v when a 500ohm resistor is connected across it, and a potential difference of .15v when a 1000ohm resistor is substituted. What is A.) the internal resistance of the solar cell and B.) the emf of the solar cell I am a bit confused because...
  42. L

    Find the magnetic field given I, t, Hall EMF, and G_s

    Hi! I had this problem for homework (Mastering Physics), and I'm not sure what I'm doing incorrectly. Is there something fundamental that I'm misunderstanding? Each time I do this, I get 0.717136.. T as my solution 1. Homework Statement I = 12.6 A w = 0.0142 m t = 0.00122 m E_H = 1.82 *...
  43. J

    This method may be more intuitive for some people.

    Homework Statement Question - Calculate the rate of energy transferred to the 1.5V cell during charging if the potential between X and Y is 1.9V Homework Equations P = VI The Attempt at a Solution I think I understand how to get the answer. If XY has 1.9V then the resistor will get 0.4V...
  44. OmarbinNaeem

    B EMF Induction in a Loop of Wire - Does It Work?

    an emf can be induced in a coil if the magnetic flux through the coil is changed my question is does all magnetic field induces an emf in a loop of wire?
  45. S

    EMF induced in rotating rod inside uniform magnetic field

    Homework Statement A 40 cm rod is rotated about its centre inside a region of uniform magnetic field of 6.4 T. Given that the speed of rotation is 15 rad/s, find potential difference between the centre and either end of the rod Homework Equations emf = - ΔΦ / Δt ω = 2π / T The Attempt at a...
  46. Celso

    Induced EMF — generator coil area calculation

    Homework Statement What is the necessary area for a generator that produces an emf of ##\mathcal{E} = 150V## when it spins at a ratio of 60 revolutions per second, in a magnetic field of ##B = 0.5 T##? Homework Equations ##\oint_{c} E \cdot dl = \mathcal{E} = -\frac{d}{dt}\iint_{s} B \cdot dS...
  47. Krushnaraj Pandya

    EMF and potential in a circuit

    Homework Statement Four identical cells each having emf E and internal resistance r are connected in series to form a loop abcd as shown in figure (picture 1). Find potential difference across ab and ac. 2. Homework Equations V=IR equations for combination of cells and resistances The...
  48. G

    Confusion about the direction of the vectors: motional EMF

    Homework Statement I'm working through an example with motional EMF and I'm having trouble understanding the directions of vectors so that I can apply induction law. The magnetic circuit seems complex because the circuit is used to analyze other situations but the air gap 3, the coil 3 and the...
  49. MPavsic

    Induced EMF in closed loops around bar magnet

    I wonder if there is a way to calculate induced EMF in closed loops around bar magnet, which is traveling with constant velocity v to the right as depicted?
  50. K

    The speed of a metal wire on two rails with a magnetic field

    Homework Statement I tried to understand the problem b) and c).[/B] Homework Equations Faraday's law: ∇xE = - ∂B/∂t emf ε = Bdv Force : F =ma, Lorenz's force F=q(vxB) ==> ma = IdB Power : power of battery = εI, mechanical power of the wire = Fv The Attempt at a Solution I think I solved...
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