What is Emf: Definition and 938 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. K

    Finding Induced EMF in antenna

    A car with a vertical radio antenna 75 cm long drives due east at 29 m/s. The Earth's magnetic field at this location has a magnitude of 5.9* 10^-5 T and points northward, 72° below the horizontal. (b) Find the induced emf between the ends of the antenna. Relevant Equations: E = BVL...
  2. P

    Max EMF Induced in AC Generator?

    Homework Statement The armature of an AC generator is rotating at a constant speed of 30 revolutions/second in a horizontal field of flux density 1.0Wb/m^2. The diameter of the cylindrical armature is 24cm and its length is 40cm. What is the maximum emf induced in the armature having 30 turns...
  3. J

    EMF canceling in toroidal coil setup

    Hello All, My question concerns induced emf in an air-core toroid setup. This is simply a thought experiment and I think I know the answer to this but thought I would run it past you all anyway. Picture a non-magnetic toroidal core with two one-layer coils wound on it 180 degrees apart...
  4. M

    Calculating EMF Induced Current in a Circular Loop of Wire

    Homework Statement The magnetic field perpendicular to a circular loop of wire 23 cm in diameter is changed from +0.52 T to -0.45 T in 175 ms, where + means the field points away from an observer and - toward the observer. Homework Equations (pardon me with the equations, I am LaTeX...
  5. S

    Calculating EMF Induced by a Moving Wire in a Magnetic Field

    Can i get some help with this homework problem?? A physicist wraps a long copper wire around a toilet paper roll and places the roll in a uniform magnetic field. The axis of the roll is aligned with the field. The radius of the roll is r = 6 cm and the strength of the field is B = 0.1 T. If the...
  6. S

    Magnetic Induction: Exploring Induced EMF & Faraday's Law

    induced emf! Hello, I am studying magnetic induction, induced emf, and faraday's law, that sort of thing. My book gives an example where the magnitude of a B-field is increasing at constant rate through an axis of a circular coil with N number of turns and a certain radius r, find the...
  7. S

    Calculating Induced EMF in a Coil with Uniform Magnetic Field

    I have a homework problem where there is a long copper wire around a toilet paper roll that is placed in a uniform magnetic field. The axis of the roll is aligned with the field. I am given the radius of the roll, strength of the field, and a velocity at which the wire is unrolled. How do i...
  8. A

    Detect EMF with an EMF Meter: How to Use, Effects & Errors

    http://www.electronics-lab.com/projects/science/006/emmeter.gif" i want to ask 1. is it used to detect emf ? is it an emf meter 2. how to detect 3. how does am-plifer work ? transistor used for?? diode use for? 4 if i change other value of capacitor? inductor? resistor...
  9. L

    Calculating EMF from Coils in Rotating Magnetic Field

    To find the emf generated by two ten turn coils, the planes of which are at 60 degrees in a radial magnetic field B=Bcos(theta)sin(omega*t) in the direction Ar, that rotates with omega rad/sec at the instant when the coil A1A2 makes an angle alpha with the plane of the maximum flux density...
  10. D

    EMF, Voltage, Current, Potential Difference etc.

    Hey. At school, we are currently studying electricity, and i am having some trouble understanding some of the concepts. From what i understand, EMF is the energy given to each unit of charge, which allows it to flow around a circuit. Why do the charges need energy? Is the energy they receive...
  11. A

    An emf self-induced in a solenoid of inductance L changes. .

    An emf self-induced in a solenoid of inductance L changes in time as E=E_0e^(-kt) Find the total charge that passes through the solenoid, if the charge is finite. (Use E_0 for 0, k, and L as necessary.) Now I know that the induced emf is E_L=-L(dI/dt) I'm having a hard time on this...
  12. E

    Magnetic Flux and Induced EMF in a Coil

    In a physics laboratory experiment, a coil with 210 turns enclosing an area of 12.7 cm^2 is rotated during the time interval 3.70 \cdot 10^{−2} sfrom a position in which its plane is perpendicular to Earth's magnetic field to one in which its plane is parallel to the field. The magnitude of...
  13. J

    Inrush Current and back EMF spikes

    Hello, I was just wondering if someone could explain to me why there is a huge spike of current & voltage in a wire when you close and open an electrical circuit. I've noticed these 'kicks' or 'spikes' in wires when runing square waves through them (doesn't seem to occur with sine waves). I...
  14. M

    Can the Slope of a Graph Determine EMF and Internal Resistance?

    I'm doing a few practicals at home this summer, and the one that is bugging me is about EMF. I have measured voltage and current at different resistances across a 4,5 V battery, and made a graph. But to find the emf, am I supposed to take the slope of the graph, or the gradient? Or is that the...
  15. S

    Calculating Energy Dissipated in Copper Wire Loop Due to Magnetic Field Increase

    A piece of copper wire is formed into a single circular loop of radius 9 cm. A magnetic field is oriented parallel to the normal to the loop, and it increases from 0 to 0.45 T in a time of 0.45 s. The wire has a resistance per unit length of 3.3 10-2 /m. What is the average electrical energy...
  16. S

    Average emf is it same as induced emf?

    average emf is it same as induced emf?? Q: a 25-turn circular coil of wire has diameter 1.00m. it is placed with its axis along the direction of the earth'smagneticfield of 50.o uT, and then in 0.200s it is flipped 180 degrees. an average emf of what magnitude is generated in the coil? N =...
  17. L

    Circuits & EMF: Understanding Internal Resistance

    Hello Iam stuck on a topic (have been for a while !) about the electro motive force around a circuit (emf). Iam also confused about how internal resistance plays a role in this. I do vaguely remember a teacher telling me that max current is at max internal resistance (or something around...
  18. N

    Solving a Difficult Physics Problem: Induced EMF

    This problem is difficult to describe, so I'll post a picture. http://img71.imageshack.us/my.php?image=pic1ik.gif The figure above shows a rod of length L caused to move at a constant speed v along horizontal conducting rails. The magnetic field B (the magnitude and direction of which are...
  19. S

    Calculating Charge, Current, and EMF in a Reversing Magnetic Field"

    Hey lads, if anyone can, please check If I am correct... Thanks :) [i]A 100 turn circular coil has a diameter of 2 cm and a resistance of 50 Ohms. The plane of the coil is perpendicular to a uniform magnetic field of magnitude 1 T. The direction of the field is suddenly reversed. (a) Find...
  20. P

    How Do You Calculate Maximum Induced EMF in a Coil System?

    A 1 m long large coil with a radius of 15.2 cm and 280 turns surrounds a 7.6 m long solenoid with a radius of 5.2 cm and 3200 turns. The current in the solenoid changes as I=I_0 sin (2\pi f t) where I_0= 30 A and f=60 Hz.Inside solenoid has 3200 turns and outside coil has 3200 turns. The...
  21. P

    Induced EMF and long steel beam

    A 5.97 m long steel beam is accidentally dropped by a construction crane from a height of 6.82 m. The horizontal component of the Earth's magnetic field over the region is 12.2 \mu T. The acceleration of gravity is 9.8 m/s^2. What is the induced emf in the beam just before impact with the Earth...
  22. T

    Emf + Internal Resistance of Battery

    These circuit questions seem to give me so many problems, this question in particular. What are the emf and internal resistance of the battery in the figure below? I have the answers, that emf is 9V and internal resistance is 0.5 ohms, however I'm having problems getting there. What...
  23. T

    RC Circuit - Current, Emf, Etc

    Doin a question based around RC circuits and was in a bit of a bind, lookin for some help. The capacitor in the figure below begins to charge after the switch closes at t = 0s. a) what is deltaVc when t >> 0 after switch is closed? b) what is Qmax in terms of emf, R and C? c) in this...
  24. W

    Induced EMF in N=181 Coil, A=.01955^2 at t=2.50, 7.50, 15.0, 25.0ms

    A magnetic field with the time dependence shown in Figure 23-38 is at right angles to a 181 turn circular coil with a diameter of 3.91 cm. What is the induced emf in the coil at each of the following times? (a) t = 2.50 ms 0 V (b) t = 7.50 ms (c) t = 15.0 ms...
  25. T

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    Here is the question i am working on: An AM radio station broadcasts with average power 4000 W in all directions. A dipole receiving antenna 65 cm long is at a location 4 miles from the transmitter. Compute the amplitude of the emp that is induced be this signal between the ends of the...
  26. M

    Effects of Doubling Primary Coil Current on Secondary Coil Induced EMF

    What would be the change of the induced emf in the secondary coil of a two coil system ( without ferromagnetic core ) if both the amplitude and the frequency of the current in the primary coil would double?: emf would stay the same emf would be halved emf would be 4 times larger...
  27. D

    Emf, current, electric fields what direction is everything moving?

    emf, current, electric fields...what direction is everything moving? This is a very simple question but I can't seem to figure out the actual direction that things are moving in a simple circuit. Suppose you have a simple circuit consisting of a battery and a resistor: The electrons inside the...
  28. Reshma

    Find the Induced EMF in a Rotating Spherical Shell in a Magnetic Field

    1] A square loop of wire, with sides of length 'a' lies in the first quadrant of the xy-plane, with one corner at the origin. In this region there is a non-uniform time-dependent magnetic field \vec B (y,t) = ky^3t^2\hat z. Find the induced emf in the loop.I applied the flux rule here...
  29. P

    Calculating Induced EMF for Moving Loop in Uniform Magnetic Field

    If we have a loop with radius 12 cm that lies in a uniform magnetic field of 1.5T. The magnetic field is in the positive z direction and the loop lies in the horizontal xy-plane. Now if the loop is removed from the field in a time interval of 2.0x10^-3 s what will the average induced emf be...
  30. K

    Faraday's Induced Emf: Help & Solutions Needed

    I'm reposting this because my homework is due in an hour and I still have not received help on how to solve this. I understand now that the the loop is equal to one. But could someone be generous enough to explain to me what the area is on problem 2 and 3 and why .503 is wrong for question 1...
  31. K

    Induced EMF ~ Faraday's Law/Lenz's Law

    I'm having a lot of trouble with the following questions. I was wondering if someone could steer me in the right direction and tell me what I'm doing wrong. I suppose my biggest problem is I don't understand how to do these without knowing the number of turns in the coil. Here's what I've done...
  32. M

    At what rate must the current be to changed to produce 40V emf

    A 20 H inductor carries a steady current of 4.0 A. At what rate must the current be changed to produce a 40 V emf in the inductor? I odn't get this one because it gives you all the information! The formula is: EMF = -L di/dt they want u to find di/dt, but they already give you it has a steady...
  33. M

    What is the emf induced in the looop?

    An elastic conducting material is stretched into a circular loop of 11.0 cm radius. It is placed with its plane perpendicular to a uniform 0.900 T magnetic field. When released, the radius of the loop starts to shrink at an instantaneous rate of 80.0 cm/s. What emf is induced in the loop at that...
  34. M

    Calculating Max Emf from Electric Generator Coil

    Hello everyone, I'm confused on how to do this problem, it says: An electric generator contains a coil of 120 turns of wire formed into a rectangular loop 40.0 cm by 20.0 cm, placed entirely in a uniform magnetic field with magnitude B = 3.50 T and with B initially perpendicular to the coil's...
  35. I

    What is the EMF generated in the loop?

    " A circular loop of wire 10 cm in diameter is placed with its normal making an angle of 30 degrees with the direction of a uniform 0.50T magnetic field. The loop is "wobbled" so that its normal rotates in a cone about the field direction at the constant rate of 100rev/min; the angle between the...
  36. J

    Inductance of a coil for a given emf and current change

    Hi, My Question: What is the inductance of a coil if the coil produces an emf of 2.50V when the current in it changes form -28.0 mA to 31.0 mA in 12.0 ms? For this I know I will need to use this equation: E=-M(delta I)/(delta t) My problem is that I was wondering if coming up...
  37. L

    EMF related question, I'm totally stuck

    Hey, I got this question in a homework from a teacher of mine: "A car battery with a 15.0 V emf and an internal resistance of 0.0430 is being charged with a current of 62.5 A. What is the potential difference across its terminals?" I've tried nearly every formula I know on it. EMF =...
  38. P

    Calculate Amplitude of EMF Induced by Radio Station

    Sry, i think i put this in the wrong forum last time. Here is the problem i have been working on. An AM radio station broadcasts isotropically (equally in all directions) with an average power of 3.20 kW. A dipole receiving antenna 70.0 cm long is at a location 4.00 miles from the...
  39. D

    EMF Crisis: Understanding the Dangers of Electromagnetic Fields | EMF-Health.com

    http://www.emf-health.com/ What do you think?
  40. P

    Solving the EMF Problem: Cross Sectional Area, Number of Turns & Revolutions/s

    Here is the question. A coil rotates at 200 revoltuions per second in teh Earth's field. If the coil has a cross sectional area of 20cm^2 and has 1000 turns, approximately what emf is induced in the coil as a function of time? I know emf= int of E*dL and that the voltage proudces across a...
  41. G

    Induced EMF in a Wooden Ring with Time Varying Magnetic Field

    Suppose i have a wooden ring and a time varying magnetic field is passed through it. Then will there be induced emf.
  42. P

    A few faraday law questions and emf,

    I got a few problems i am working on and getting stuck on. Here is what i got: Problem 1 A 34 turn circular coil of radius 3.20 cm and resistance 1.00 is placed in a magnetic field directed perpendicular to the plane of the coil. The magnitude of the magnetic field varies in time according...
  43. S

    How to Prepare Notes on Corrosion, EMF Series, and Fossil Fuels?

    Hi Guys! im a student of electronics engineering (1st year). I have to prepare notes for my exams about the following topics. Please recommend me some websites from where i can prepare these notes. 1) Corrosion (Theories, Inhibition and Prevention) 2) EMF Series 3) Types of Fuels...
  44. T

    Measuring unknown emf by potentiometer

    in measuring unknown emf by potentiometer, why must the stable current source(working battery) have a voltage greater than any voltage to be measured? i can't understand why.. in our exercise in physics, we used a slide-wire potentiometer with the assumption that 1 meter of the slide-wire...
  45. J

    Magnetic field and induced EMF

    Here is the problem: A coil containing 590 turns with radius 3.85 cm, is placed in a uniform magnetic field that varies with time according to B=( 1.20×10−2 T\s)t+( 3.40×10−5 T\s^4)t^4. The coil is connected to a resistor of resistance 690 Ohms, and its plane is perpendicular to the magnetic...
  46. V

    High Rez EMF Mapping: Seeking Advice

    Greetings, This was posted in the Classic Physics section, reposted here in Quantum section in case someone here may have some experience in EMF mapping. I am currently working on an interesting experiment on crystalized metal and I need to find a way to get a detailed map of an...
  47. B

    How Do You Calculate EMF for a Custom Generator Design?

    EMF - need urgent help - physics hey, i have a physics quiestion which i am stuck on any help would be much appreciated you are asked to design a generator to produce the same 50Hz, 240V(rms) as found in a domestic voltage outlet The generator is made of a single flat coilbeing made to...
  48. B

    Calculating Internal Resistance and EMF of a Voltage Source

    Hello everyone! I have a question and any help would be appreciated! (a) What is the internal resistance of a voltage source if its terminal voltage drops by 2.00 V when the current supplied increases by 5.00 A? (b) Can the emf of the voltage source be found with the information supplied?
  49. R

    Induced EMF: Find Flux for Rectangular Loop in z-y Plane

    Can someone check my equation for flux. Thanks A rectangular loop in the z-y plane is situated at t = 0 at the points (x=1,z=0), (x=1,z=5), (x=4,z=5), and (x=4,z=0). The rod of the loop with end points (x=4,z=0), and (x=4,z=5) is moving in the x-direction with a velocity v=5\vec{i}m/s while...
  50. A

    EMF wave are form due to changing of current which result in electric

    i once read a book which says that EMF wave are form due to changing of current which result in electric fields and result in changing magnetic field and repeated in infinite loop :smile: i also heard that visible light (which is also a EMF radiation) is produced due to the changing orbit of...
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