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. P

    Inducing currents without change of flux linkage?

    Suppose I had cylindrically-symmetric rotating magnet surrounded by a plasma. I rotate it on its axis at a constant angular velocity, and so the electric field E produced is non-solenoidal and can be described as the negative gradient of some potential V(x,y,z). The electric field is induced...
  2. M

    What is the amplitude of induced EMF in a magnetic dipole antenna?

    1. A magnetic dipole antenna is used to detect an electromagnetic wave. The antenna is a coil of 50 turns with radius 5.0 cm. The EM wave has frequency 870 kHz, electric field amplitude 0.50 V/m, and magnetic field amplitude 1.7 X 10-9 T. (b) Assuming it is aligned correctly, what is the...
  3. B

    Why wouldn't rotating the handle 90 degrees induce an EMF?

    Homework Statement There is some current in the bottom coil. The question asks which would not be able to light the lightbulb - rotating the insulating handle 90 degrees - increasing resistance of variable resistor -reducing resistance of variable resistor - moving the insulating handle...
  4. Krushnaraj Pandya

    Calculating EMF of a Cell: Ni(s) | Ni2+ (0.1M) || Au3+ (1.0M) | Au (s)

    Homework Statement The EMF of the cell : Ni(s) | Ni2+ (0.1M) || Au 3+ (1.0M) | Au (s) is [ given E°(Ni2+/Ni)= -0.25V; E° (Au3+ / Au )= +1.50V ] Homework Equations E°cell=E°(cathode)-E°(anode) For SRP. Nernst equation The Attempt at a Solution SOP is given, therefore converting to SRP we...
  5. S

    Does Light Travel Through Fiber Optic Cable Generate EMF?

    Does light traveling through a fiber optic cable generate any sort of detectable electromagnetic field? Please forgive the stupid question. It’s something that popped into mind recently and google hasn’t adequately answered for me. I’m not a scientist or physicist. :blushing:
  6. F

    EMF Oppsition in Adjacent Wires +/- Swing

    Can someone please help with this thought experiment? I have sought advice elsewhere but it remains unclear to me. I have a straight piece of two conductor "figure eight" wire (like speaker wire or US power cord) laid out left to right. I connect the output from a push-pull type audio...
  7. thee qs

    Find the resistance and EMF in a circuit

    Homework Statement translation ; we know the values of R1 = 1 ohm R2= 2 ohm R3= 6 ohm R4= 2 ohm R5= 4 ohm R6= 4 ohm we can also see the voltmeter reading a potential difference value of 8 volts , and the...
  8. person123

    B How do you calculate induced EMF in an open loop with changing magnetic field?

    Let's say you have an open loop (like a section of a circle) in a changing magnetic field. I think there would be an induced EMF, but no current. What I can't figure out, though, is how to calculate the induced EMF. Using Faraday's law doesn't seem to help, as there's no enclosed area.
  9. F

    EMF induced in a coil encircling an ideal solenoid

    Homework Statement A current I(t)= (0,160 A s^{-3}) t^3 flows through an ideal solenoid with a turns density n = 9,00 \cdot 10^{-3} m^{-1} and a cross sectional area A_s=2,00\cdot 10^{-4} m^2 A single loop of wire has the same axis as the solenoid, but its radius is larger. That is: the loop...
  10. Yousuf Ejazi

    Calculate the induced EMF for angle α

    I have a question in my book and it’s confusing me a bit. I tried to search online for similar solved problems but couldn’t succeed. So here it goes:Calculate the induced EMF in a conductor loop when the angle between \vec{A} ~ and~ \vec{B} ~is~ changed ~from ~{0 °}~ to ~{α °} in 1 second...
  11. J

    Induced EMF and current in the circuit

    Homework Statement Homework EquationsThe Attempt at a Solution First I will label a few points . A is the point on the ring touching the rails , vertically below P . B is the point on the ring touching the rails , vertically below Q . T is the topmost point on the ring . M is the bottommost...
  12. P

    EMF of a battery and voltmeter

    Homework Statement We have to measure emf of a battery. We have to voltmeters. Each of them when connected alone across the battery, they read V1 = 0.9V and V2 = 0.6V and when both of them are connected to the battery simultaneously, they both read V3 = 0.45V . What is the emf of...
  13. A

    Is Solenoid Inductance Proportional to Turns Squared?

    Homework Statement In Solenoid, is emf E= N. d(phi)/dt, or is it simply d(phi)/dt? Some books contain the latter one. It was not a big deal until i gave a thought about it. Since increasing the N, we are actually increasing the value of n, thus increasing B= (mu) .n.I So, by placing N in front...
  14. EF17xx

    Electromagnetic induction emf question

    Homework Statement The correct answer is D Homework Equations The Attempt at a Solution The current is clockwise( in the first picture at least) due to right hand rule. The magnetic flux is changing, the direction of the induced emf depends on the direction of change (increase or...
  15. EF17xx

    Emf in a circuit (multiple choice question)

    Homework Statement Correct answer is Homework Equations emf = I(R + r) The Attempt at a Solution When current flows the voltage across the battery is less than the emf so it's not A I really don't understand why the answer is C because B, C, D all seem correct to me. Please help me with...
  16. E

    How is the emf greater than 9 V in this scenario?

    Homework Statement [/B] A battery has an emf of 9 V and an internal resistance of 2 Ω. If the potential difference across its terminals is greater than 9 V: A. it must be connected across a large external resistance B. it must be connected across a small external resistance C. the current must...
  17. L

    Calculating EMF and Load Resistance for a Motor

    Homework Statement A motor as an internal resistance of 0.5 ohms. It draws 20 amperes at full load from a 120-V supply. a) What is the emf of the motor? b) What is the load resistance? Homework Equations Potential difference between the terminals = emf - Ir (r = internal resistance) (P.D...
  18. Zam0la

    If the AC EMF is 0 at t=0, then how does current pass through a capacitor?

    Hi, I have a problem understanding the ac capacitive circuit, where Vc = Vm * sin(ωt), and, current leads the voltage, so at t = 0, v = 0 which means no emf, no pushing energy for the electrons, I think, then how current is max? what is the energy source that supplied the electrons with the...
  19. B

    Computing Alternator Back EMF as a function of load current

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  20. M

    What equation for induced EMF?

    I understand the direction compnent of this question. But I don't understand where the magnitude of the induced current came from? I'm assuming faradays law wasn't used as there wasn't a change in magnetic field?
  21. C

    Find the maximum amplitude of the induced EMF

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  22. negut constantin

    Weber induction force and EMF

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  23. J

    How Accurately Can Induced EMF Represent Batteries in Circuit Problems?

    Homework Statement Homework EquationsThe Attempt at a Solution Sorry for the unclear image . EMF induced in the left loop = 8 Total Resistance of the left loop = 6 Ohms Current will be induced in anticlockwise direction in the left loop of magnitude 4/3A. EMF induced in the right...
  24. J

    Current flowing in a cylinder and induced EMF

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  25. C

    Induced EMF in a coil outside a solenoid

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  26. F

    B What happens to virtual photons when an EMF is extinguished?

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  27. VSayantan

    Speed of a Magnet Falling through a Conducting Wire

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  28. F

    B What's the relationship between drop height and induced emf?

    In my experiment, I intended to find out how the change in the bar magnet drop height from solenoid affected the emf induced in the solenoid, however, I am unable to come up with an equation that shows a relationship between the two variables. I have thought of Biot-Savart law, but I do not...
  29. astrocytosis

    Induced current in a loop of wire when straight wire is cut

    Homework Statement A square loop, side a, resistance R, lies a distance s from an infinite straight wire that carries current I (pointing to the right). Now someone cuts the wire, so I drops to zero. In what direction does the induced current in the square loop flow, and what total charge...
  30. W

    Self-Induced Emf and Angular Frequency

    The context is an ac circuit with only one element - the inductor. My textbook says that the self-induced emf increases with increasing angular frequency, but I'm having trouble seeing this mathematically. If self-induced emf = ε = -L(dI/dt) and L = X/ω, then emf and ω are inversely related...
  31. P

    EMF between an axis and the surface of long wire

    A friend mine of gave me a problem : Calculate the Emf b/w the axis and surface of a long current carrying wire of radius ##r## and current density ##J##. I am not able to understand why there would be a potential difference between the axis and surface but i think that either of these could...
  32. C

    Why Do EMF Meters Measure ELF/VLF Differently?

    Why do EMF meters measure ELF and VLF in flux density units but RF in power density units? Is it because ELF/VLF have low power density and high flux density, and RF has high power density and low flux density? They are all EM waves, right? what's driving the difference in measurement units? thanks
  33. T

    Is current proportional to emf

    Faraday's law states that emf is proportional to change in flux. The scenario I am using is a magnet is moved towards a closed loop. As it moves towards the loop, flux through the loop is increased causing a emf to be induced. Does I also know that a current is induced. So does that mean current...
  34. I

    EMF and internal resistance laboratory work

    1. The problem statement I did a laboratory work in which I had to find power and internal resistance. I used an ordinary battery so it's closed circuit. I did all tasks except writing a hypothesis and conclusion I can't really think of it. I don't know what to write since I just measure using...
  35. A

    Induced EMF between two points

    Homework Statement Homework EquationsThe Attempt at a Solution First part is easy . Area of the triangular loop is A = (1/2)(2R)(R) = R2 Induced EMF in loop ABC = Rate of change of flux = dΦ/dt = AdB/dt = R2(dB/dt) which is correct . Now the second part has stumped me . I would really...
  36. V

    Calculating the RMS Value of Sinewave Voltage

    Homework Statement I keep telling myself that this problem should be easy, but I keep getting the wrong answer. Any help is appreciated. A generator has an armature with 500 turns, which cut a flux of 8.00 mWb during each rotation. Compute the back emf it develops when it runs as a motor at...
  37. Ryaners

    Unraveling the Mystery of EMF Internal Resistance

    I'm having a difficult time understanding why the internal resistance of the EMF source is disregarded in this problem. 1. Homework Statement You are asked to determine the resistance per meter of a long piece of wire. You have a battery, a voltmeter, and an ammeter. You put the leads from the...
  38. radji

    Solving for I2 in a Circuit: What Am I Doing Wrong?

    Homework Statement I am attempting to solve the circuit for three currents (I1, I2, I3). I am starting with I1 in terms of I2, then using a third equation to solve for I2 in terms of E and R. I wrote my loops at the bottom of the page. I am using Loop 1 @ B, Loop 2 @ B, and Junction at B to...
  39. Pushoam

    EMF in bigger loops kept on both sides of a small loop

    Homework Statement Modification: Take two big loops kept on both sides of the small loop at a distance a from the corresponding end of the small loop. Homework EquationsThe Attempt at a Solution The flux through the bigger loop is equal to the flux through the smaller loop, provided that the...
  40. Pushoam

    EMF across a loop with one end in a parallel plate capacitor

    Homework Statement Homework EquationsThe Attempt at a Solution Assuming the current is steady, I = JA, where J is the current density and A is the cross- sectional area ##E = J/\sigma## Can I take ##\sigma ## uniform here ?
  41. S

    What EMF is induced in the loop at t=0?

    Homework Statement A current of 10A is switched off at time t = 0 in a long cable (picture in attachments). It powers a device, with the current return wire having 5 mm separation. A wire loop of dimension 5 by 5 cm is located at a distance 3 cm from the cable. 1) What Emf is induced in the...
  42. R

    Fairly hard induced EMF problem

    Homework Statement (ignore the suggested problems for test 3.) If for whatever reason the image doesn't load the given's are that: The area is 25pi(.125m)^2 (A circular coil with 25 turns and a radius of 12.5cm.) The magnetic field varies with time and is...
  43. bachir1994

    Emf without load and load voltage of a generator

    HelloLast week I was confronted with a situation where an industrial equipment, with a large equipotential mass (metal casing) that was not grounded, had a ground tension, measured with Fluke, of 382 vac. I put a 220 Vac bulb and nothing happens, the bulb remains off and is not burned out. By...
  44. T

    Potential difference of an emf with internal resistance

    Homework Statement A battery of emf 2V and internal resistance 0.1ohm is being charged with a current of 5 ampere. In what direction will the current flow inside the battery?What is the potential difference between the two terminals of the battery? Homework Equations If a battery of emf E and...
  45. Asmaa Mohammad

    The average EMF in a coil rotating in a magnetic field

    Homework Statement [/B] Homework EquationsThe Attempt at a Solution Solution for Q1: Solution for Q2: Solution for Q3: Are they correct?
  46. Pushoam

    Motional EMF -- problem 7.7 in Griffith

    Homework Statement I need help in solving (c) and (d). Homework EquationsThe Attempt at a Solution Since the force acting on the bar is Magnetic force and magnetic force does no work, the speed of the bar will remain same i.e. v0. Question (d) is not making sense to me.
  47. Pushoam

    What is the Resistance of a Loop in the Presence of Motional EMF and Gravity?

    Homework Statement Homework Equations Flux rule: ξ= -dΦ/dt' F= ma The Attempt at a Solution Let's assume that the loop is going down with speed 'v <0'. Using Flux rule, ξ = - Bva Current will move in clockwise direction to increase Φ. The effect of magnetic force on AC gets canceled by the...
  48. Asmaa Mohammad

    The magnetic flux and the induce emf in the dynamo.

    Homework Statement Multiple-choice question: Homework EquationsThe Attempt at a Solution I know that the induced emf is maximum when the coil is parallel to the direction of the magnetic flux, so I should choose either B or C, and I have read the the rate by which the coil cut the magnetic...
  49. esha

    How Does EMF Work? Exploring Force, Terminals & Charge Carriers

    i know an emf device maintains a potential difference across a circuit. it does so by doing work on charge Carriers. this means it somehow maintains a force in the circuit. I also know there are two terminals present in an EMF device. are these terminals charged so as to maintain an...
  50. Pushoam

    Magnetic flux rule for calculating motional EMF

    Homework Statement Griffith ( Introduction to electrodynamics , 3 ed.) says in Problem 7.9: An infinite number of different surfaces can be fit to a given boundary line, and yet, in defining the magnetic flux through a loop, Φ = ∫B.da, I never specified the particular surface to be used...
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