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. D

    How Is EMF Induced in a Rotating Rectangular Loop?

    [SOLVED] Emf in a rectangular loop Homework Statement A rectangular loop (area = 0.15 m^2) turns in a uniform magnetic field, B = 0.16 T. When the angle between the field and the normal to the plane of the loop is pi/2 rad and increasing at 0.70 rad/s, what emf is induced in the loop...
  2. G

    Induced Emf in a Loop of Circular Wire

    Induced Emf For a loop of circular wire, there is a changing magnetic field running through it, but it is restricted to a region that is within the region of the loop of wire, so the field only runs through the loop of wire (it does not touch the loop of wire). Apparently, an emf is generated...
  3. V

    Back EMF (electromagnetic induction)

    1)When the current through the coil of a motor flows it cases the coil to spin. 2)The spinning motion causes the wires in the coil to cut through the magnetic field. 3)The current induced (back EMF) in the wires by this spinning motion opposes the change that causes it. (Lenz's Law)...
  4. T

    How Does the Changing Current in a Solenoid Affect a Coil Within It?

    [SOLVED] EMF problem - Current in Coil Homework Statement A small square coil is located inside an ideal solenoid at the center with its plane oriented perpendicular to the axis of the solenoid. The resistance of this coil is 2.00 \Omega and each side is 4.00 cm long. The solenoid has 125...
  5. S

    Induced Electric Fields and EMF

    Ok, this may borderline a homework problem, but I'm not quite grasping the concept of induced electric fields. So let's say that they exist a wire loop and solenoid. The switch in the circuit containing the solenoid is originally open and then closed. At this instant, we induce an emf in the...
  6. A

    Archived Calculating Back EMF of an Automobile Starter Motor

    Homework Statement An automobile starter motor draws a current of 1.9 A from a 5.5 V battery when operating at normal speed. A broken pulley locks the armature in position, and the current increases to 8.6 A. What was the back emf of the motor when operating normally? Answer in units of...
  7. G

    How Does Lenz's Law Apply to EMF in a Triaxially Bent Wire?

    Homework Statement A wire is bent into three circular segments each of radius r=9.2 cm. each segment is a quadrant of circle, ab lying in the xy plane, bc, in the yz plane, and ca lying in the zx plane. If a uniform magnetic field points in the positive x direction, what is in volts the...
  8. R

    Planning Lab to measure how emf varies with an angle

    A metal disc is placed in a uniform magnetic field of flux density,B,acting through the plane of the disc at and angle,\theta.When the dis is rotated an e.m.f. is induced between the centre and the rim of the disc. http://img158.imageshack.us/my.php?image=scanlc2.jpg Design an experiment...
  9. V

    How Does the Direction of a Magnetic Field Affect Induced EMF in a Loop?

    Homework Statement a) If B (magnetic field) is perpendicular to the plane of a circular loop, then is it true that when B increases the induced emf is clockwise and when the B decreases the emf is counterclockwise? Or do you need to know whether it is pointing into/out of the page? Or would...
  10. K

    Calculate Current Through Battery Connected to Resistor

    Homework Statement The voltmeter is removed (previous question was a voltmeter connected to a battery and it reads 12 V) and a resistor of 21 ohm is connected to the battery. What is the current (I) through the battery? Homework Equations emf=V I=emf/(R+r) The Attempt at a...
  11. R

    Archived Current of Coil, and Emf of Coil.

    Homework Statement A magnetic field is perpendicular to the plane of a single-turn circular coil. The magnitude of the field is changing, so that an emf of 0.91 V and a current of 3.2 A are induced in the coil. The wire is the re-formed into a single-turn square coil, which is used in the...
  12. R

    How Fast Can an MRI's Magnetic Field Safely Shut Off to Prevent Harm?

    Homework Statement Magnetic resonance imaging (MRI) is a medical technique for producing "pictures" of the interior of the body. The patient is placed within a strong magnetic field. One safety concern is what would happen to the positively and negatively charged particles in the body fluids...
  13. B

    Solve Emf & Current for Changing Magnetic Field in Square Coil

    Homework Statement A magnetic field is perpendicular to the plane of a single-turn circular coil. The magnitude of the field is changing, so that an emf of 0.76 V and a current of 3.1 A are induced in the coil. The wire is the re-formed into a single-turn square coil, which is used in the same...
  14. R

    What is the induced emf in a radio antenna receiving an AM radio signal?

    Homework Statement A radio antenna that receives an AM radio station that emits at 800 kHz is constituted of an inductor of 120 turns with a radius of 0.6cm. The inductor is home to an iduced emf du to a magnetic field oscillating from the radio wave. The magnetic field is...
  15. B

    Solving for Resistance and EMF in a Circuit: A Guide

    Homework Statement What is the resistance R? (R1=15.13 Ω, I1=2.77 mA , I2=1.81 mA. Give answer in Ω.) What is the emf of the battery? Homework Equations I = E/R The Attempt at a Solution How do I solve this with the batteries EMF?
  16. P

    Question about Electric Generators (and Black emf)

    Please Help Me with this question. Thanks in advance! An ac generator with a maximum emf of 400 V is to be constructed from wire loops (radius 0.15 m). It will operate at a frequency of 60 Hz and use a magnetic field of 200 mT. How many loops will be needed? Given...
  17. P

    Induced emf: Faraday's Law and Lenz's Law Question

    [SOLVED] Induced emf: Faraday's Law and Lenz's Law Question The plane of a conductive loop with an area of 0.020 m^2 is perpendicular to a uniform magnetic field of 0.30 T. If the field drops to zero in 0.0045 s, what is the magnitude of the average emf induced in the loop...
  18. J

    How a potentiometer can be used to measure an unknown emf?

    Could someone perhaps explain how a potentiometer can be used to measure an unknown emf? My book gives a bad explanation...
  19. P

    Why the induced emf is always negative?

    Hi! I read this article about Faraday's law, which states, that the emf will be always negative, by the formula, is this http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html"
  20. D

    Fruit cell emf varying with temperature

    Homework Statement Hi all, In school, I tried to investigate how the emf (voltage) of a fruit cell varies with temperature. I took ~200ml of freshly squeezed orange juice, 2 copper electrodes, a voltmeter, heater and thermometer. I set up everything, and got an initial voltage of somewhere...
  21. T

    Induced EMF: Solenoid in Square Loop

    Homework Statement The solenoid has N turns of wire around a cylinder. The radius of the cylinder is "a" and length is "b." THe current of the solenoid varies as I(t)= I_{0}(1 - e^-\alphat) . I_{0} and \alpha are positive constants. Around the solenoid is a square loop of wire with side...
  22. D

    Finding the emf and internal resistance of a cell

    I hope someone can help me with my coursework by making sure my method is ok and is actually correct for my experiment. Note. I will not actually be doing this it is just a plan of what I could do! 1. I am provided with a cell of approcimate EMF 1.5V, an approximate internal resistance of 1Ω...
  23. S

    Electromagnetic waves & induced EMF

    Homework Statement A circular loop of wire can be used as a radio antenna. If an antenna with a diameter of 20.0 cm is located a distance of 2.40 km away from a from a source with a total power of 45.0 kW at a frequency of 101 MHz, what is the maximum emf induced in the loop? (Assume that the...
  24. M

    Induced emf in a current loop

    Homework Statement If the current is increasing at a rate of 0.1 A/sec what would the induced emf in a loop of 0.5 m be? Homework Equations I = \frac{|emf|}{R} emf = - \frac{d}{dt} B A cos(\theta) I'm obviously missing an equation or something. Some guidance would be much...
  25. C

    Induction and EMF: Homework Statement Solution

    Homework Statement In a ring of radius R, there is current I flowing in the counterclockwise direction as viewed from above. A small ring of radius r is on a common axis and is a hight z above the current carrying ring, where z>>R. The small ring moves up with velocity v. Calculate the emf in...
  26. A

    EMF and internal resistance

    Homework Statement Hi, my first post here. :smile: Anyway I am struggling with this homework, can someone help me out, show me what to do please. 1)a cell can supply a current of 1.2A through two 2ohm resistors connected in parallel. When they are connected in series the value of the...
  27. E

    EMF Shielding: Examining the Effects on Static Magnetic Fields

    Your opinion to the following will be appreciated Lets imagine a sphere made of a conductor (copper) placed within a static magnetic field. The field will penetrate the within the shier. Next we place inside the sphere a coil producing a variable electromagnetic field. The electromagnetic field...
  28. S

    How long can a 1.5 V battery light a flashlight bulb drawing 0.6A?

    Homework Statement You'll have to forgive me... I've got some rather poor Physics books with which to consult, so this may seem rather simple. I think I understand the concepts but I don't think I have all of the information from my sources to answer this problem. A 1.5 V battery stores...
  29. J

    Solving Induced EMF Problem with Jordan's Help

    Hi guys - Here's the problem I'm stuck on: In a physics laboratory experiment, a coil with 160 turns enclosing an area of 13.7 (cm^2) is rotated during the time interval 4.60×10−2(s) from a position in which its plane is perpendicular to Earth's magnetic field to one in which its plane...
  30. D

    Calculating EMF from a Rotating Coil: Max & Avg Values

    On a particular dynamo, the rotating coil has 500 loops, each of area 2.5x10-4 m2. The magnetic field can assumed to uniform and of magnitude 0.400T. When the bicycle is ridden at a particular speed, the coil rotates with a frequency of 8Hz. What is the maximum emf produced? What is the average...
  31. H

    Why Does a Battery's EMF Seem Constant Over Time?

    just a simple question, why is it that in almost all circuits where a battery is the source of DC , the EMF is considered to be constant over time while in reality is it not ? The EMF is a decreasing function w.r.t time considering the battery is operating.
  32. P

    Exploring Faraday's Law and Induced EMF in a Moving Loop Configuration

    well iwas going through the standard example that is there is a uniform magnetic field in aregion going into the palne and ther is a rectangular 'loop' with one side which can slide on metallic rods with constant speed v and the the relation \epsilon=BLv well the example is (almost) here well...
  33. K

    Find the maximum induced emf in the large coil solenoid

    Homework Statement Given: u_0 = 1.25664e-6 A 2 m long large coil with a radius of 14.6 cm and 320 turns surrounds a 4.2 m long solenoid with a radius of 4.6 cm and 7700 turns. The current in the solenoid changes as I = I_0 sin (2pi*f*t) where I_0= 30 A and f=60 Hz.Inside solenoid has...
  34. S

    Emf of coil outside a solenoid

    Homework Statement A coil of 120 turns and radius of1.8 cm and resistance of 5.3 ohm is placed outside a solenoid. If the current in the solenoid is changed, what current appears in the coil while the solenoid is being changed? Homework Equations emf= -dflux/dt = A*dB/dt emf=iR...
  35. S

    Induced emf in a square loop

    Homework Statement "A square loop of wire is held in a uniform magnetic field of .24 T directed perpendicular to the plane of the loop. The length of each side of the square is decreasing at a constant ratee of 5.0 cm/s. What is the emf induced in the loop when the length 12 cm...
  36. I

    Explaining motional emf in terms of the hall effect?

    im thinking of the classic airplane problem. the one where the flying airplane develops a voltage across its wingspan. now this was posed to me as a motional emf problem and i bought it then but simultaneously thought that it seemed like a hall effect problem. so two questions now that i think...
  37. V

    EMF, curled or straight antenna

    hmm just a topic for discussion..im revising for an exam and i found an old paper asking to find rms values of electric field and magnetic field from power...and that's easy, using the poynting vector. Then it asks, given a piece of wire of length l in these fields, you can either use it as a...
  38. S

    Faraday's Law of Induction and EMF

    Homework Statement A coil of 15 turns and radius 10cm surrounds a long solenoid of radius 2cm and 1x10^3 turns/m. The current in the solenoid changes as I = (5A)sin(120t). Find the induced emf in the 15-turn coil as a function of time. Homework Equations emf = -Nd[flux]/dt d[flux]...
  39. B

    Induced EMF and long steel beam

    A 11.8m long steel beam is accidentally dropped by a construction crane from a height of 8.62m. The horizontal component of the Earth's magnetic field over the region is 16.7μT. What is the induced emf in the beam just before impact with the Earth, assuming its long dimension remains in a...
  40. mbrmbrg

    What is the Maximum Induced emf of a Rotating Loop?

    Homework Statement A rectangular loop of wire has area A. It is placed perpendicular to a uniform magnetic field B and then spun around one of its sides at frequency f. The maximum induced emf is: BAf BAf 4(pi)BAf 2(pi)BAf 2BAf Note: No, it's not my mistake that BAf appears...
  41. mbrmbrg

    Induced emf generation problem

    Homework Statement In Figure 30-52 (see attached), a metal rod is forced to move with constant velocity v along two parallel metal rails, connected with a strip of metal at one end. A magnetic field B = 0.370 T points out of the page. (a) If the rails are separated by 26.0 cm and the...
  42. K

    Average induced emf in coil

    A circular conducting coil with radius 2.61 cm is placed in a uniform magnetic field of 0.827 T with the plane of the coil perpendicular to the magnetic field. The coil is rotated 180° about the axis in 0.222 s. (a) What is the average induced emf in the coil during this rotation? (b) If the...
  43. G

    What is the relationship between magnetic field and induced emf?

    Homework Statement http://www.jyu.fi/kastdk/olympiads/2005/Th2.pdf http://www.jyu.fi/kastdk/olympiads/2005/Th2%20Solution.pdf (Question 3) My first doubt is why the magnetic field at the center of the coil is: |\vec B|=\frac{\mu_0 NI}{2a} and not |\vec B|=\mu_0 nI, where n=N/l...
  44. M

    Induced emf between theh ends of an antenna

    Homework Statement A car with a vertical radio antenna 85cm long drives due east 25 m/s. The Earth's magnetic field at this location has a magnitude of 5.9 * 10-5 T and points northward, 72 degrees below the horizontal. a) Is the top or the bottom of the antenna at the higher potential? b)...
  45. J

    How Does Resistance and EMF Affect Energy Transfer in a Circuit?

    Homework Statement A wire of resistance 5.0 Ω is connected to a battery whose emf ε is 2.0 V and whose internal resistance is 1.0 Ω. In 2.0 min, how much energy is (a) transferred from chemical to electrical form in the battery, (b) dissipated as thermal energy in the wire, and (c)...
  46. A

    How Fast Should the Smaller Solenoid Move to Induce 2.3V in the Larger Solenoid?

    Homework Statement Assume the field inside the larger coil (.075m Radius, 1200 loops), reaches the maximum value when the smaller solenoid, (.05m radius, 1200 loops), is halfway inserted. with what constant velocity must the smaller solenoid be inserted to induce 2.3V in the larger solenoid...
  47. J

    Magnitude of induced emf in current loop

    Homework Statement The 2-cm (radius) current loop shown in the figure is changing from θ1 = 11 degrees to θ2 = 66 degrees in a magnetic field 2 T in 0.5 seconds. What is the magnitude of the induced emf to the nearest hundredth of a millivolt. The wire in Problem 2 has a thickness of 1.04...
  48. C

    Print ViewInduced EMF and Current in a Shrinking Loop

    Induced EMF and Current in a Shrinking Loop Homework Statement Shrinking Loop. A circular loop of flexible iron wire has an initial circumference of 170 cm, but its circumference is decreasing at a constant rate of 13.0 cm/s due to a tangential pull on the wire. The loop is in a constant...
  49. M

    Find the induced emf between the ends of the antenna

    Homework Statement A car with a vertical radio antenna 75 cm long drives due west at 23 m/s. The Earth's magnetic field at this location has a magnitude of 5.9 10-5 T and points northward, 74° below the horizontal. Find the induced emf between the ends of the antenna. Homework Equations...
  50. 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...
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