Emf Definition and 939 Threads

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. S

    EMF and induced current question

    The rectangular loop in the figure has 1.7×10^−2 ohm resistance. What is the induced at this instant? See picture below. Answer is in terms of I, like 5*I. emf = Bvl I= emf/R B= [\mu_0 / 2*pi] *I * ln(.03/.02) since the magnetic field goes through the loop from 2cm to 3cm then I used...
  2. B

    Two battery current & EMF problem

    The question was to find I1 I2 and E. I got those parts of the question correct. I1= 1 A I2 = 2A E = 15V. But the only reason I got these right is because the arrows for I1 & I2 were given in the attached figure. The directions of the current don't make sense to me. If the EMF from...
  3. M

    Ideal circuit with 4 resistances and one emf.

    Homework Statement In the figure below, R1 = 5.50 , R2 = 18.0 and the ideal battery has an emf E= 12.0 V. (a) What is the size and direction of current i1? (Take right to be positive.) (b) How much energy is dissipated by all four resistors in 1.00 min? Homework Equations I = E/R...
  4. I

    What Is the Maximum EMF Induced in a Vibrating Guitar String?

    Homework Statement Review problem. A flexible metallic wire with linear density 3.00 multiplied by 10-3 kg/m is stretched between two fixed clamps 64.0 cm apart and held under tension 301 N. A magnet is placed near the wire as shown in Figure P31.25. Assume that the magnet produces a uniform...
  5. I

    Calculating Induced EMF in a Coil

    Homework Statement A 32 turn circular coil of radius 4.60 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 to the expression B = 0.0100t + 0.0400t2, where t is in seconds and B is...
  6. K

    How Does the Right-Hand Rule Determine EMF Direction in a Non-Conducting Loop?

    A long, straight wire is in the same plane as a wooden, non-conducting loop. The wire carries an increasing current I in the direction shown in the figure (the current is pointing in the left direction). a. there will be no induced emf and no induced current b. there will be a...
  7. N

    Faraday's Law, EMF, Finding Current in Loop

    Homework Statement A 26.4 cm diameter coil consists of 23 turns of circular copper wire 1.90 mm in diameter. A uniform magnetic field, perpendicular to the plane of the coil, changes at a rate of 8.00E-3 T/s. Determine the current in the loop. Part Two: Determine the rate at which thermal...
  8. G

    Motional emf i thinkchange in area and magnetic field

    motional emf i think... change in area and magnetic field Homework Statement A rectangular loop is located in a uniform magnetic field of magnitude 0.29 T directed perpendicular to the plane of the coil. If the area of the coil increases at the rate of 5.2 10-3 m2/s, what is the magnitude...
  9. T

    Magnetic flux / induced emf problem

    Homework Statement A single coil with a radius of .02m is placed inside a 3m long solenoid that has 600 turns. The radius of the solenoid is 0.15m. The plane of the coil is perpendicular to the axis of the solenoid. The solenoid is connected to a battery, an ammeter and a switch. The ammeter...
  10. I

    How do I solve for current and emf in this circuit?

    Homework Statement Find I1.Find I2. FInd emf. emf is that E like symbol in the picture http://session.masteringphysics.com/problemAsset/1075283/3/32.P50.jpg Homework Equations I=V/R I=emf/R The Attempt at a Solution this problem is frustrating. i tried using kirchhoff's loop law
  11. C

    Total induced emf at different speeds

    Homework Statement Suppose you have a coil that is a certain distance from a magnet. Now you move the coil towards the magnet (until it is on top of the magnet) at a certain speed. Then you do the same thing only faster. Will the total induced emf be the same for both tries? Aka should the...
  12. G

    What is the Motional EMF in a Solenoid with a Square Loop Inside?

    Homework Statement A square, single-turn wire loop 1.00 cm on a side is placed inside a solenoid that has a circular cross section of radius 3.00 cm, as shown in the figure below. The solenoid is 16.0 cm long and wound with 95 turns of wire. (a) If the current in the solenoid is 3.00 A...
  13. S

    Emf and internal resistance of a battery

    When switch S in the figure is open, the voltmeter V of the battery reads 3.07 V. When the switch is closed, the voltmeter reading drops to 2.95 V, and the ammeter A reads 1.70 A. Assume that the two meters are ideal, so they do not affect the circuit. Find the value of the emf. (see picture...
  14. J

    Induced emf in secondary coil - help

    induced emf in secondary coil - help ASAP Please Homework Statement A very long, straight solenoid with a cross-sectional area of 6.00 cm2 is wound with 40.0 turns of wire per centimeter, and the windings carry a current of 0.250 A. A secondary winding of 2.00 turns encircles the solenoid...
  15. R

    How Much Power to Generate 400,000 Gauss EMF?

    Hello all, first time post here. I was curious, hypothetically, how much juice would you need to generate a 400,000 Gauss magnetic field?
  16. S

    How Do You Calculate Current Drawn from a Series-Parallel Battery Circuit?

    Homework Statement A battery consisting of six cells in series, each of e.m.f 1.5V and internal resistance 0.5 Ohms is connected to two 3 Ohm resistors which are in parallel. Calculate: a) the total parallel resistance of the combination [1.67] b) write down the internal resistance of the...
  17. S

    How to Calculate the Internal Resistance of a Battery?

    Homework Statement A torch bulb is marked 2.5V and 0.3A. When it is connected to a dry battery of e.m.f 3.0V the correct p.d of 2.5V is produced across it. Calculate the internal resistance if the battery.Homework Equations I=V/R?The Attempt at a Solution It's on paper - sorry!Answer = 1.67 -...
  18. A

    Calculate induced emf across loop antenna by another loop antenna

    Homework Statement i need to find emf across 50 ohm load attached to square loop antenna of single turn with each side of 10 cm. this antenna is placed 10 cm away from a similar antenna which is driven by a sinusoidal source of 10 v amplitude and with 1mW power. Homework Equations i...
  19. L

    Does a Decreasing Magnetic Field Induce Conventional Current?

    \xi = - \frac{dB}{dt} Is the emf induced by a changing flux. Is the current created by this emf conventional current? For example. Say there is a square loop in the plane of this page, with a decreasing magnetic field going INTO this page. This creates a clockwise current to battle the change...
  20. O

    How Do You Calculate the EMF of a Cell in a Circuit?

    Finding the EMF of the cell... first post =) Homework Statement Find the EMF of the cell. (diagram has been uploaded) Homework Equations Resistors is series : Rs= R1 + R2 + R3 Resistors in parallel: 1/Rp = 1/R1 + 1/R2 + 1/r3 V=IR The Attempt at a Solution Firstly i re-drew...
  21. J

    Can a Faraday Cage Protect Against EMF Radiation from Neighbors?

    Couple of months ago Deany100 posted a note on microwave radiation which he suspected was his neighbour tracking his movements. I've got a similar problem, a home bathed in emf picked up by a device (15 pounds sterling) that doesn't tell me what the frequency is. Great disbelief from everyone...
  22. R

    How back emf helps in energy conversion in a motor?

    How back emf helps in energy conversion in a motor? hello, I am an engineering student and during the past few days i was searching on the net to clarify my doubts on "motors".So i came across an article and on reading one part of it a doubt arosed in my mind. Q1) "To take a practical...
  23. B

    Electrical current without an EMF?

    Homework Statement Can there be an electric current without an e.m.f.? (i noticed my mistake in the title, it should be e.m.f. not EMF)I was given this question and I don't know how to answer it, any help would be greatly appreciated
  24. C

    Induced EMF Problem: Calculate Magnetic Flux & Initial EMF

    An elastic conducting material is stretched into a horizontal circular loop of radius 20 cm and placed in a magnetic field of strength 0.5 T, directed vertically downwards. a. What is the magnetic flux through the loop? b. When released, the radius of the loop decreases at an initial rate...
  25. G

    Effects of Increasing Supply Voltage on DC Motor Back EMF

    If i have a series DC motor and at say X rpm it has a back emf of 11.5v and supply voltage of 12v and i increased the supply voltage (keeping rpm the same) would the back emf stay the same? and if not would the diffrence between back emf and supply increase? Thank you.
  26. M

    How Does a Diode Affect Capacitor Voltage in an AC-DC Conversion Circuit?

    Homework Statement There is a circuit which consists of a capacitor with capacity C, a diode and a source of changing EMF such that during time \frac{1}{2}T EMF equals E and during another half of T it is -E (current flows in the opposite direction). What will be the voltage on the capacitor...
  27. K

    Induced Emf, Contributing factors

    Homework Statement A generator is constructed by rotating a coil of N turns in a magnetic field B at a frequency f. The internal resistance of the coil is R and the cross sectional area of the coil is A. Which of the following statements are true? (Give ALL correct answers, i.e., B, AC...
  28. V

    How Do Cells with Different EMFs Combine in a Circuit?

    Homework Statement CELLS OF EMFS E1 ,E2,E3...HAVING INTERNAL RESISTANCES r1,r2,r3...are connected to an external resistance R.What is the resultant emf? Homework Equations The Attempt at a Solution
  29. X

    The Carrier of an EMF: Resistance & Quantum Mechanics

    The Carrier of an Emf is in many definite/indefinite parts the net of the circuits Total Resistance (Rt). These definite/Indefinite parts are the SMALLEST degree expressed to that of a single atom, the basic Resistive properties or qualities of Itself, and the electron structure as pertaining to...
  30. D

    What Direction Does the Coil Rotate in a Magnetic Field?

    Homework Statement A circular coil, with radius of 10 cm, and 25 turns, rotates in a constant magnetic field of strength 2.4 T, with the axis of rotation perpendicular to the direction of the magnetic field. (a question about finding the induced voltage- 5.92 V) part C. If the...
  31. M

    Understanding Back EMF: Effects on Parallel and Series Electrical Circuits

    Ex:At home, you plug an electric drill into an electrical outlet and turn it on. At the moment you turn on the drill, the lights in the room(wired in parallel to the drill) flicker and dim briefly, but then return to normal after a moment. notice that the example is wired in parallel, but in...
  32. M

    Slide Wire on a V-Shaped Track: Calculating EMF, Current & Area

    A slide wire (resistance R) travels without friction along the track shown below. It starts from position x0 (non-zero) and velocity v0 , to the right as shown. Assume that the v shaped rails meet at an angle of q. Use the coordinate system shown. The magnitude of the external magnetic...
  33. S

    Why is there no induced emf in electric motor?

    Hello everyone :smile: I got a small question. In an electric motor when the coil rotates isn't this a change of magnetic flux. So wouldn't there be an induced emf in the coil? Any help would be appreciated. Thanks :smile:
  34. P

    Induced emf in a Rectangular Coil

    Homework Statement A rectangular coil of 150 turns has dimensions of 23 x 70 cm and is located in a uniform 3 T magnetic field. In 1.2 s, the plane of the coil is rotated from a position where it makes an angle of 15° with the magnetic field to a position where it makes an angle of 72°...
  35. R

    Solving for Total Magnetic Flux and Induced EMF

    Homework Statement http://img164.imageshack.us/img164/3313/qqqqo3.jpg Find the total magnetic flux through the coil at t=0 Given that the rate of change of flux at any time,t, is equal to 2 \pi f \phi sin 2 \pi ft, find the maximum instanteous value of the induced emf at any time & the...
  36. N

    Missing minus sign - Motional EMF

    Homework Statement An infinitely long conductor carrying a current I in the positive z-direction lies on the z-axis. A conducting triangle of wires lies in the yz-plane, which moves with a speed v in the positive y-direction. The following figure shows the situation on time t = 0: a)...
  37. S

    Magnetic fields and induced EMF

    Homework Statement An coil has 4000 turns of wire and a mean area of cross-section 25cm^2. A magnetic field of flux density 0.16T passes through it. When the magnetic field is reduced steadily to zero in 0.0050s, emf is induced across the terminals of the coil. Calculate the value of this...
  38. L

    How Does Internal Resistance Affect Terminal Voltage in a Circuit?

    Homework Statement A torch battery has an emf of 6.0V and an internal resistance of 1 Ohms. What is the voltage measured by a voltmeter connected across the terminals of the battery, when the battery is connected to a bulb that draws 0.5 A from the battery? Anyways thanks I solved it.
  39. T

    Different emf across the same resistance

    Hi, I'm having some difficulties understanding some elementary physics concept. When different emf is supplied to a resistor of the same resistance, the p.d. across changes in different scenarios. By definition, p.d. is the amount of electrical energy converted to other forms of energy...
  40. D

    How Do You Calculate the EMF of a Cell in a Circuit?

    http://img.photobucket.com/albums/v345/crimsonflames/ddsa.jpg thats the circuit. isn't the equation emf = I(R+r) 0.032 * 900? isn't right and i know it isn't, so i don't know what to do! =(
  41. S

    Photodiode & EMF: Is This Correct?

    Just thought i would ask, when a photodiode is acting in reverse bias, it allows current to pass but does not generate voltage. In fact, it has a voltage drop. When in forward bias, it allows current to pass but also generates an EMF. Is this correct, any response would be appreciated.
  42. Redbelly98

    Exploring Superconductors: Understanding Induced EMF in Magnetic Flux Changes

    After following the thread "ideal conductor vs superconductor" a while, I am wondering about something that must have a simple answer. If you change the magnetic flux through a superconducting ring, what happens to induced emf?
  43. A

    Help a fiction writer with EMF field removal

    Hello! I am a writer currently working on a children's science fiction novel. I need help with a problem of plot that hinges on my main character emitting an EMF field similar to an appliance like a hair dryer. This character is surrounded by this field and needs to get rid of it in a simple...
  44. N

    Confused about Polarity of Induced EMF (Lenz Law)

    Hello, I am currently working through Faraday's Law and I am confused when my two textbooks don't explain how they set the polarity of V_{emf}, particularly when they say "the sign convention for V_{emf} is arbitrary in that it depends on the assumed direction for dS". How is the polarity...
  45. E

    Calculating EMF from Rotating Spherical Shell in a Magnetic Field

    Homework Statement A perfectly conducting spherical shell of radius 'a' rotates about the z-axis with angular velocity \omega in a uniform magnetic field \mathbf{B} = B_0 \hat{\mathbf{z}}. Calculate the emf developed between the "north pole" and the equator.Homework Equations The Attempt at a...
  46. S

    A coil with turns given magnetic field, time find the emf induced in the coil

    A coil with 240 turns enclosing an area of 13.7 cm^2 is rotated in a time interval of 3.70×10^-2 s from a position where its plane is perpendicular to the Earth's magnetic field to one where its plane is parallel to the field. The Earth's magnetic field at the lab location is 5.9×10−5 T ...
  47. B

    Conceptual question on induced EMF

    Homework Statement A circular loop of wire is rotated at constant angular speed around an axis whose direction can be varied. In a region where a uniform magnetic field points straight down, what must be the orientation of the loop's axis of rotation if the induced emf is to be zero...
  48. S

    Are Electromagnetic Fields from Laptops Causing Discomfort?

    My background is in CS, so I apologize if I sound a bit stupid wrt the topic at hand. My friend and I have been having a heated debate about the causes of "discomfort" that I've been experiencing with my Macbook Pro. After having it on my lap for several minutes, even on top of surfaces that...
  49. G

    Back EMF in a Motor - Does It Alternate?

    Say you had a circuit with a battery and a loop of wire somewhere in the middle of the circuit. The loop of wire is positioned so that it is free to rotate in a uniform magnetic field, and since a current is running through it, a torque will be supplied by the magnetic force allowing it to...
  50. G

    How does an induced emf occur in a wire without contact with the magnetic field?

    A circular loop of wire has a changing magnetic field running through it...however, the magnetic field is not touching the wire; its contained so that it simply runs through the loop without touching the wire. Apparently, there is still an emf through the wire even though the magnetic field is...
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