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.

View More On Wikipedia.org
Recent contents Top users
  1. O

    How to Derive constant of total back/counter EMF for DC motors?

    Hi, I have been reading around about back EMF and their derivations for simple DC motors. However for some reason, the step between obtaining the total emf of the motor from summing the individual emf of the coils is not very clear. For example: Induced emf due to single coil: e = d∅_c/dt...
  2. K

    Determining the Direction of Induced Emf in an Aeroplane's Wing Tips

    Homework Statement An aeroplane in horizontal flight has a wing span of 50 m and is traveling due north at a speed of 500 kmph. Calculate the emf induced between the wing tips of the aeroplane. The value of the vertical component of the Earth’s magnetic field is 5.5 x 10-5 T. What...
  3. E

    Question in determing the direction of induced emf?

    How to determine the direction of emf induced in an alternator without the need to look at the graph. (i.e) using Lenz's Law or Faraday's Laws of determining the direction? Consider for example the instant when B and dA are perpendicular and no flux is present? Thanks for helping!
  4. L

    How can I derive the required expression for the induced EMF in this circuit?

    Homework Statement See attachment. Homework Equations The Attempt at a Solution For the first part: (B' and x' are B subscript 0 and x subscript 0 respectively.) \phi=BA=B'xl=B'l(L+x'cos(ωt)) EMF=-d\phi/dt=-B'ωlx'sin(ωt) Therefore V=-EMF=B'ωlx'sin(ωt) So...
  5. P

    Current in inductor used to measure EMF.

    Hi, Homework Statement This is not a formal HW question, yet I was wondering whether one of you might be willing to answer it nevertheless. According to Fraday's law, ε = -∂\phi/∂t. If I reversed the direction of the current in an inductor used to measure ε, would that have any effect on...
  6. K

    How Is EMF Induced in a Loop Within a Solenoid?

    Homework Statement A small loop with area 6.8 mm^2 is placed in a long solenoid with 854 turns/cm and a sinusoidally varying current w/ amplitude 1.28 A and angular frequency of 212 rad/s. What is the amplitude of the emf induced in the loop? Homework Equations \textbf{B}=\mu_0 in \Phi_B...
  7. B

    What Causes Induced EMF in an Incomplete Ring?

    Dear colleagues, My question here refers to the EMF induced in an incomplete ring that is stationary amidst a time dependent magnetic field. We know from Faraday's law that EMF is created upon changing the magnetic flux, either by changing the intensity of the field, or by changing the...
  8. M

    Magnetic Flux and Induced EMF at South Pole

    Homework Statement The magnetic field strength at the South Pole is around 66μT, and magnetic field lines point out of the Earth perpendicular to the surface. The sled is a metal square of side 0.95 m (with non-conducting wooden slats in between). What is the magnetic flux through the sled...
  9. S

    Find the max induced EMF in the loop by the changing loop

    Homework Statement I have attached the two questions. the first has a diagram. 1. Find the MAX Induced EMF (in volts) in the loop by the changing current? 2. What is the magnitude of the EMF induced in the loop by the changing current at t=.8 (seconds) answer in Volts? Homework...
  10. E

    Understanding Circuit Diagrams: Ideal Conductors and Physical Components

    Homework Statement In the circuit shown in the figure http://s3.amazonaws.com/answer-board-image/ffa3225e-0730-480c-8ed0-f417490d6b96.jpeg , the current in the 20.0-V battery is 5.00A in the direction shown and the voltage across the 8.00-Ω resistor is 16.0V , with the lower end of the...
  11. U

    Emf induced by swinging rod in magnetic field (I got 2 answers?)

    Homework Statement Homework Equations The Attempt at a Solution Their answer assumes that the B-field is perpendicular to the velocity at all times, which is clearly not the case. And in the next part of the question, won't the answer be zero? since now velocity is parallel to B-field? (So...
  12. B

    2 solenoids: emf and frequency

    I'm studying a circuit in AC. I use a function generator and set a waveform. I have a solenoid and I put a small solenoid inner it. Could you tell me if there is a relation between frequency and emf inducted on the second solenoid?
  13. NaOH

    Induced EMF (Faraday's law)

    Homework Statement It is often discussed that a changing magnetic field will create a non-conservative electric field, which would then induces an EMF and hence an induced current, but what happens when we connect voltmeters? Consider putting a circular loop of wire with some resistance in...
  14. AJKing

    EMF in a loop; non-constant magnetic field

    Homework Statement Refer to Figure attached. The current in the long straight wire is i = (4.5A/s2)t2-(10A/s)t Find the EMF in the square loop at t=3.0s.Homework Equations \xi = -\frac{d \Phi}{dt} And Biot-Savart law for straight wires of infinite length: B = \frac{\mu_0 i}{2 \pi R}The...
  15. L

    Find Induced Emf in this Coil

    A flat, rectangular coil of dimensions L and w is pulled with uniform speed v through a uniform magnetic field B with the plane of its area perpendicular to the field (the figure (Figure 1) ).How is the emf zero? My reasons as to why it's zero: the trig part forces the problem to be zero...
  16. J

    Calculate the emf of Battery w/ External & Internal Resistance

    Homework Statement What is the emf of the battery shown? there is an external resistance of 2.50 ohm and 4.0 A...The battery has an internal resistance of 0.50ohm Homework Equations V=Ir V= E +/- IR The Attempt at a Solution Since the current is going away the battery then i...
  17. M

    How to ground EMF shielding/clothing?

    For example if a hat is lined with nickel, silver, EMF paint, etc. , then how do you ensure the hat is grounded? I am a little unsure what grounding means relating to these forms of usage as I have seen EMF protective clothing advertised as grounded.By EMF I mean sufficient for all freq with...
  18. B

    Two Batteries In Series-Total EMF and Internal Resistance

    Two Batteries In Series--Total EMF and Internal Resistance Homework Statement Two 1.50-V batteries—with their positive terminals in the same direction—are inserted in series into the barrel of a flashlight. One battery has an internal resistance of 0.260 Ω, the other an internal resistance of...
  19. B

    Potential difference of an inductor equal to minus the induced emf?

    Hi, Two questions I'm finding it hard to get my head around with inductors... Say we have an increasing current, this is going to cause an increase in flux in one direction, which will induce an emf to drive a current which produces a magnetic field to oppose the change in flux. Than doing...
  20. M

    Find power of antenna from emf on coil

    I want to find power of the electromagnetic waves generated from a welding cable. I built my own coil antenna that is 6.5 inches in diameter (circular) with 50 turns and hooked up the ends to a digital oscilloscope to measure the voltage (the internal resistance is 1MΩ if it matters). I...
  21. T

    Exploring the Phenomenon of Counter EMF in Electric Motors

    Okay so, when a motor spins in an external magnetic field there will be a change in the magnetic flux threading... When this magnetic flux threading is changed there will also be an induced current that is produced but why is this induced current in the opposite direction of the current that is...
  22. H

    Back Emf same as battery emf?

    Homework Statement A coil of inductance 0.5 H is connected to a 18V battery. Calculate the rate of growth of current? Homework Equations E=-L(dI/dt) The Attempt at a Solution Actually I have the solution but the problem is that in the book they have assumed that the back emf that would develop...
  23. L

    What is the induced emf in one of the windings?

    1. A solenoid of length 3.64 cm and diameter 0.914 cm is wound with 170 turns per cm. If the current is decreasing at a rate of 36.3 A/s, what is the induced emf in one of the windings? this is what I did: Length of the solenoid l = 3.64cm = (3.64...
  24. J

    Calculating Back EMF: 200V DC Motor w/ 20A, 0.3Ω Resistance

    Homework Statement i have a 200v dc motor with 20A flowing through the armature with a resistance of 0.3Ω, how do i calculate the back emf? very confussed :( Homework Equations The Attempt at a Solution
  25. E

    Thermoelectric Emf: Investigating Temperature Dependency

    Why the thermoelectric emf doesn't depend on the lower temperature of the thermocouples... And why does the emf start to decrease after a certain increase of temperature?
  26. K

    Conflict: why synchronous machine doesn't produce induced EMF in the DC field wires

    conflict: why synchronous machine doesn't produce induced EMF in the DC field wires Isn't it current path through winding which produce magnetic field (rotating) the must produce EMF in the dc winding of field (same theory of induction) but why isn't true in synchronous
  27. K

    Induced emf and flux in a transformer, conceptual question

    Hey Folks, I am a third year electrical engineering student and just want to clarify a concept involving electromagnetics/transformers. When supplying magnetization current to a transformer (assume sinusoidal), this induces a time changing magnetic flux in the core. The time changing magnetic...
  28. P

    Working out EMF and internal resistance

    Homework Statement Power supply of EMF = E and internal resistance of R. Need E and R. This is connected to resistor of 10'000 Ohm. The current drawn from the power supply is 0.91A. A voltmeter of internal resistance 10'000 Ohm is connected in parallel to the resistor. The voltmeter reads...
  29. C

    What causes the change in direction of the magnetic field in motional emf?

    Motional emf -easy question(re-edited) Hi guys ,: i'm learning about the motional emf , because i will have to take an exam soon, but I've found something really confusin for me well : they say that when a conductor is moving in a direction perpendicular to the field with constant velocity...
  30. C

    How do you shield electronics from EMF?

    Hello all, I have an issue which I believe all you smart electrical engineers can help me with: How do you shield electronics from emf? I have a tablet pc with an active digitizer. The digitizer becomes inaccurate towards the peripheries of the screen --the closer to the bezel you are--...
  31. M

    Engineering Solving a University Level EMF Problem: Need Help!

    Hello, I am trying to solve a problem in university level electromagnetic fields course. The problem is in the attachment. My answer is simple, that for the magnetic flux B\rightarrow (vector field B\rightarrow), the Amperian loop will have a current equivalent to 0 because current through...
  32. X

    True or false? a conductor through a uniform magnetic field produces NO emf

    Many sources on the web claim that when moving a conductor through a magnetic field, an emf is produced. However, some sources claim, that the intensity of the magnetic field also needs to change in order to produce a voltage over the conductor, for example...
  33. I

    Find the emf induced in a metal ring rotating in a magnetic field

    First off, sorry if this is a simple question, I'm very bad at electromagnetism. Homework Statement A metal ring of radius R rotates with constant angular velocity ω about a diameter. Perpendicular to the rotation axis is a constant magnetic induction field \underline{B}. Find the EMF...
  34. P

    EMF generated by a magnetic field

    Homework Statement An automobile with a vertical radio antenna of length 75 cm travels at 80 km/h eastward. The Earth’s magnetic field has magnitude of 7x10-5 T and is directed 52o downward with respect to due North. What EMF is generated between two ends of the antenna? Comment on the...
  35. P

    Electricity and EMF Question

    Hi, I learned that, when a field does work on something, like when an electric field does work on an electron, it loses potential energy -- much like when a ball falls from a height (gravity is doing work on it) it loses potential energy. Hence, as an electron flows from the negative to...
  36. T

    Transformer, flux linkage and emf short question

    http://www.xtremepapers.com/papers/CIE/Cambridge%20International%20A%20and%20AS%20Level/Physics%20%289702%29/9702_s05_qp_4.pdfHomework Statement 6b Homework Equations flux linkage = BA B=F/iL The Attempt at a Solution from my understanding, if B=F/iL , if the current is zero...
  37. C

    Calculating EMF in Windings of a Cylinder (60cm x 5cm) for 6mV

    How many windings you have to reel up on a cylinder (length is 60cm and the diameter is 5cm) so that a Emf of 6mV gets induced if ΔI/Δt=1A/s? This is not my homework. I was doing this as a preparation. I know the formula: ε=-L*ΔI/Δt L is unknown. How to relate it to the windings, the length...
  38. M

    EMF, Current and Electric/Magnetic fields

    Hi, I've been trying to understand electricity... and series of questions came into my mind.. 1. I know if i move a conductor in an external magnetic field, it induces EMF, but I don't really understand INDEPTH how CHANGE in MFlux induces EMF? I think something to do with EF and MF... 2...
  39. A

    Difference between induced emf and counter emf

    What is the difference between induced electromotive force and counter electromotive force? The way they are generated seems the same to me. Can someone tell me the difference and how they are different?
  40. 1

    Find induced emf (among other things) in this current + motion

    Homework Statement Attached Homework Equations The Attempt at a Solution I'm pretty confused about this one. First, I thought that maybe the magnetic field's non uniformity didn't matter since the rate of change of flux was going to be the rate of change of area, but that's...
  41. N

    Motional EMF in Transportation

    Homework Statement Airplanes and trains move through the Earth's magnetic field at rather high speeds, so it is reasonable to wonder whether this field can have a substantial effect on them. We shall use a typical value of 0.50 G for the Earth's field. A) The French TGV train and...
  42. S

    How to explain alternating emf?

    Homework Statement Explaining alternating current is simple as the direction of current will change accordingly. For example: a magnet is pushed through a solenoid. Explain why is there an induced alternating current. As it is pushed forward, the solenoid experiences an increase in magnetic...
  43. M

    Max EMF in a Model AC Generator Coil

    Homework Statement In a model ac generator, a 500 turn rectangular coil, 8.0 cm by 20 cm, rotates at 120 rev/min in a uniform magnetic field of 0.60 T. (a) What is the maximum emf induced in the coil? (b) What is the instantaneous value of the emf in the coil at t = (∏/32)s? Assume that...
  44. O

    Direction of emf, Flemings left & right hand rules

    Homework Statement A metal strip moving to the right in a B field which is going into the plane of the page. Can this problem be considered using both Fleming's right and left hand rules? Homework Equations The Attempt at a Solution If I consider Fleming's left hand rule: the negative...
  45. I

    EMF induced via change in magnetic flux vs. velocity in magnetic field

    Homework Statement (See attached diagram)--Note this problem was taken from Barron's SAT Physics Practice Test 1 #58 As the loop rotates, the induced voltage will be maximum at... a) 0 degrees and 90 degrees b) 0 degrees and 180 degrees c) 90 degrees and 270 degrees d) 180 degrees and 270...
  46. S

    Type of induced/back emf in dc motor's armature coil

    Actually this not a question, I want to justify whether my concept is right or wrong. So let's start. Faraday's law of electromagnetic induction e=Nd\varphi/dt explains the way of electromagnetic energy conversions. The induced voltage is always ac because the rate of change of flux is...
  47. S

    Induced emf when dropping a magnet into a solenoid

    I am having some trouble understanding why and how the emf changes when a bar magnet is dropped into a solenoid. As the magnet moves down (have not entered the solenoid yet) it experences an increase in magnetic field. As it enters the magnet it experiences an increase in magnetic still until...
  48. J

    Current problem with emf and internal resistance

    Homework Statement Assuming each battery has internal resistance r = 1.0 Ω, determine the magnitudes and directions of the currents through each resistor shown in the following figure. The batteries have emfs of 1 = 9.0 V and 2 = 12.0 V and the resistors have values of R1 = 15 Ω, R2 = 20 Ω...
  49. N

    Computing the emf induced in a coil

    Homework Statement The following is Problem 1-13 on page 23 from Electrical Engineering Fundamentals, 2nd ed., by Vincent del Toro: "In the configuration shown in Fig. P1-13 the coil has 100 turns and is attached to the rotating member which revolves at 25 \frac{rev}{s}. The magnetic...
  50. M

    Is there a delay of 1 or 2 seconds in generation of back emf in a dc motor

    Does the back emf in a dc motor start to appear after a slight delay(say 1 or 2 seconds) after starting up the motor? I have had this simple doubt for a long time.According to the "generator principle" both applied emf and back emf will always exist simultaneously.Also it would be in violation...
Back
Top