What is Motional emf: Definition and 58 Discussions

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  1. M

    I Contradiction in formula for motional EMF

    The formula for motional EMF is $$\oint({\bf{v}}\times{\bf{B}})d{\bf{l}}=-\frac{d}{dt}\int{{\bf{B}}\cdot{\bf{\hat{n}}}da}$$However applying this for a rotating disk of radius a there seems to be a sign contradiction $${\bf{v}}\times{\bf{B}}=\omega s{\bf{\hat{\varphi}}}\times...
  2. O

    Motional EMF in the presence of a time-varying magnetic field

    My question arises from the following problem: We have a uniform magnetic field into the page, decreasing at a constant rate dB/dt< 0, causing the bar to move to the right. Find the velocity of the bar as a function of the time, and in terms of the known parameters: the resistor R and the...
  3. Ahmed1029

    I Is electromotive force always equal to potential difference?

    In the case motional emf, there is a static magnetic field and a rectulgular loop that goes into the field region, then current is produced. There is no electric field, but there is an emf. However, Griffiths states that emf is equal to the potential difference between the source endpoints. But...
  4. SpectraPhy09

    Solve Motional EMF Problem: Total Resistance, V(ind), I(ind), and Fnet Explained

    I request please go through my solution Total Resistance = R + 2λx V(ind) = vBd I(ind)= vBd/(R + 2λx ) v = i(R + 2λx )/Bd If force on the wire due to induced current is iBl then Fnet = iBd - F ...(i) or Fnet = F - iBd ...(ii) Fnet = v(dv/dx) v(dv/dx) = iBd - F By solving...
  5. B

    Motional EMF for loops of wire vs conducting plates

    I'm in an intro E&M class, and I'm trying to distinguish between Motional EMF for loops of wire and conducting plates. This question might be kind of silly, but are Eddy currents pretty much the same thing as induced currents in a loop of wire? More specifically, what I am trying to ask is if...
  6. R

    Electric Fields with Motional EMF

    Suppose I have a wire loop that I am moving away from a very long wire which carries a current upward and I want to find the induced current in the loop. The way I know how to approach this is with either Faraday's Law or motional EMF. My question concerns the motional EMF approach. My...
  7. E

    Velocity of charges or bounding curve features in motional EMF?

    The motional EMF is$$\mathcal{E}_{\text{motional}} = \oint_{\partial \Sigma} (\vec{v} \times \vec{B}) \cdot d\vec{x} = \int_{\Sigma} \frac{\partial \vec{B}}{\partial t} \cdot d\vec{S} - \frac{d}{dt} \int_{\Sigma} \vec{B} \cdot d\vec{S}$$(that's because Maxwell III integrates to...
  8. F

    Motional EMF in the frame of a moving wire

    When a magnetic field is moving (moving magnet) while a wire isn’t---in the reference frame of the wire, there’s no known magnetic force/effect on the wire before the introduction of motional emf and relativity. If viewed in the reference frame of the magnetic field instead where the wire is...
  9. Luxucs

    Faraday's Law & Motional EMF Confusion

    Hi there! I have what I hope is a relatively straightforward question regarding Faraday's law and motional emf, but its been causing me to scratch my head for quite a while. Consider the diagram attached to this post (source is linked at the bottom). Assume that all of the wires and the rod are...
  10. christang_1023

    Solve the differential equation of motional emf

    . Above is the figure of the problem. I am trying to solve x(t) and differentiate it to obtain v(t); however, I have difficulty solving the differential equation shown below. $$ v(t)=\int a(t)dt=\int \frac{B(\varepsilon-Blv)d}{Rm}dt \Rightarrow \frac{dx}{dt}=\frac{B\varepsilon...
  11. G

    Confusion about the direction of the vectors: motional EMF

    Homework Statement I'm working through an example with motional EMF and I'm having trouble understanding the directions of vectors so that I can apply induction law. The magnetic circuit seems complex because the circuit is used to analyze other situations but the air gap 3, the coil 3 and the...
  12. 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.
  13. 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...
  14. 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...
  15. Pushoam

    Faraday's law of electromagnetic induction -- Motional EMF

    There is magnetic field in B = B ##\hat z## in a region from x=0 to x=l. There is a metal rectangular wire loop with length l and width w in x- y plane with coordinates of four corners as (0,0),(0,w),(l,0),(l,w). This loop is moved with velocity v=v##\hat x##. Now according to Faraday's law...
  16. cnh1995

    Faraday's law and motional EMF paradox

    Consider the diagram below (sorry for the quality:frown:). The circle is the front view of a cylindrical iron rotor (highly ferromagnetic, very low reluctance). The orange part is a single turn of a conductor (very high reluctance) wound around the rotor body. The grey lines are magnetic field...
  17. A

    EMF about a loop moving toward a wire with current

    Homework Statement A picture of a related problem is attached below. My problem is the same, except that v is toward the wire, not away from it. I, r, v, b and a are given. "Find the emf about the loop without assuming that B changes at any point" My problem is that this question isn't...
  18. kostoglotov

    Lenz's Law applied to a non-circuit conductor?

    I was reading through my College Physics text, and it talked about how NASA once swung a 20km long conductor through the Earth's magnetic field and setup a 3500V potential difference along the wire. I understand that there's no conventional current in that wire, it's not a circuit. But for a...
  19. L

    Why is the negative sign in the equation for motional EMF often discarded?

    I'm trying to understand how electric generators work on a very basic level. I understand the heart of this is the application of the Lorentz force on a conductor moving at a velocity ##v## in the presence of a magnetic field ##B##. I also understand how it can be shown that the emf is equal to...
  20. Gerry Rzeppa

    How Does Motional EMF Affect Electron Distribution in a Conductor?

    Here is a typical description of motional EMF: "The figure below shows a conducting rod of length L being moved with a velocity v in a uniform magnetic field B: The magnetic force acting on a free electron in the rod will be directed upwards. As a result, electrons will start to accumulate...
  21. Kerrigoth

    Are induced currents created by motion or change in flux?

    The book I am working through is beginning to dive into induced currents and Lenz's Law. When a conductor is moving perpendicular to a magnetic field, the charge carriers inside will experience a magnetic force; This causes the charges to move. Eventually, the electric field from the charge...
  22. fricke

    Exploring the Work Behind EMF Generation

    I understand that EMF is generated by moving a closed loop of wire through a constant magnetic field. I am confused about one thing, who is actually doing work here? (of course it is not magnetic field).
  23. patrickbotros

    Direction of Current in a Sliding Bar

    I'm studying for the Physics SAT Subject Test using The Princeton Review (2011-2012). There's this motional EMF problem about a rod sliding to the right along a u-shaped wire in a uniform magnetic field that's going into the page and the current moving counter clockwise. Problem is: I can't...
  24. E

    Quick question on direction of dl in motional emf integral

    Homework Statement u = velocity B = magnetic flux density Homework Equations ∫ (uxB)⋅dl The Attempt at a Solution From my understanding, the direction of dl depends on the resulting direction of (uxB). How will I know if my dl is in positive or negative?
  25. Y

    Faraday's Law and Motional EMF: Exploring Causes

    is "motional emf "the real explanation for the reason why emf is induced in faraday law?? thanks...
  26. PhiowPhi

    Misconception of motional EMF?

    For motional EMF: $$ \epsilon = -vBL$$ If a conductor moves in a constant magnetic field, there isn't induced EMF correct? Only when existing/entering the field there is induced EMF? I've had this misconception, that any motion(fast/slow) in a magnetic field would instantly induced EMF, but I...
  27. PhiowPhi

    Confused, velocity in motional EMF?

    Motional EMF is defined as E = -vBL Where B is the magnetic field in Tesla, L is the length in meters and ,v is the velocity. Lets say we have a distance(1m) where half of it(0.5m)the magnetic field is present. So, there is a wire that moves with constant acceleration throughout that whole...
  28. R

    Why does current flow from lower to higher potential in motional EMF?

    I understand how motional EMF is produced, the magnetic force separates the charges in the rod to create an EMF. I don't understand though, why the current would flow from a lower potential to a higher potential. I'm sorry I've made like 3 threads in the past 2 days, trying to contribute where...
  29. D

    Understanding Motional EMF: Why Can We Use the Formula for a Spinning Bar?

    I was reading this: http://web.mit.edu/8.02t/www/materials/StudyGuide/guide10.pdf (specifically page 10-6), and came across the formula; ## \mathcal{E} = -Blv## which is used for motional emf. Next, I was trying to solve a problem on page 10-33 (bottom problem) with a spinning bar...
  30. 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...
  31. 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...
  32. A

    Understanding the Flux Rule for Motional EMF?

    The flux rule for motional emf can be stated as: d\phi/dt = -\epsilon I have some questions regarding this. 1) Should I find this rule intuitive? And secondly a proof of this rule is given in my book, which I have attached. I don't really understand the idea of the proof - is the idea to...
  33. T

    Motional emf, Gaussian flux, and lenz's law help

    Homework Statement Two bars, each 30cm long, and each having a resistance of 2-ohms, are connected to a 1,500 volt battery. The bars are attached to each other with 3 insulating springs, each having a spring constant of 9N/m. The two bars are initially at rest, 4cm apart. The switch is...
  34. M

    Is This a Case of Motional emf?

    If a straight length of wire moves in a suitable direction through the magnetic field of a magnet, there will be a motional emf in the wire. If the magnet moves toward the wire, is there an emf in the wire, motional or otherwise?
  35. E

    Induced motional emf conceptual question

    A magnetic field points into the page. A conductive bar is moved to the right (perpendicular to the magnetic field). Why do the electrons move downwards toward the bottom of the bar? The book doesn't offer a good explanation of this... using right hand rule I would say that the electrons are...
  36. S

    Motional EMF (direction of moving bar)

    Homework Statement Can someone please explain to me how to determine the direction of the veloicty of a bar that is conducting and inducing a current. Fb=q(VxB)?
  37. S

    What is the role of motional emf in opposing motion?

    Homework Statement Hello all, If we have a bar that is perpendicular to a magnetic field, and that bar is being pushed to the left via an applied force, all the while a current (due to a voltage source and resistor) runs down through the wire. Therefore, there would be a total force comprised...
  38. F

    Solving for the unknown variable in Motional EMF problem | Physics Homework

    Homework Statement http://img854.imageshack.us/img854/3980/33288207.th.png Uploaded with ImageShack.us The Attempt at a Solution http://img813.imageshack.us/img813/1065/60582408.th.png Uploaded with ImageShack.us \varepsilon = IR \varepsilon = -\frac{d \Phi}{dt}...
  39. H

    What is the motional emf of the loop at time t = 0.032 s?

    Homework Statement A conducting loop is made in the form of two squares of sides s1 = 3.8cm and s2 = 6 cm as shown. At time t = 0, the loop enters a region of length L = 18.6 cm that contains a uniform magnetic field B = 1.2 T, directed in the positive z-direction. The loop continues through...
  40. K

    Motional Emf: Solving for Speed with E=vBL

    Homework Statement A 1.2 kg rod that has a length of 1m and a resistance of 5 ohms slides with constant speed down a pair of frictionless vertical conducting rails that are joined at the bottom. Other than the rod, the rest of the circuit is resistance-less. A uniform magnetic field of...
  41. X

    Understanding the Time-Varying Induced EMF in a Spring-Magnetic Field System

    A Rectangular wire frame, is placed in a uniform magnetic field directed upward and normal to the screen. A side AB is connected to the spring . The spring is stretched and released when the part has come to a point A'B'(t=0). How would the induced emf vary with time? - Graph & Equation...
  42. A

    Energy changes during motional emf

    I had already posted this question in the homework category but since i did not get any replies i thought maybe i should post it here again... A U shaped conducting tube is placed in a constant magnetic field in the downward direction. On the left of the tube is a resistance R.All other rods...
  43. A

    Energy changes during motional emf

    Homework Statement A U shaped conducting tube is placed in a constant magnetic field in the downward direction. On the left of the tube is a resistance R.All other rods are assumed resistance less.On this tube a conducting rod is moving with constant velocity towards right with the help of an...
  44. L

    Calculating speed needed to create motional EMF of given magnitude

    1. How fast would you have to drive your car to create a 3.0 V motional emf along your 1.2 m-long radio antenna? Assume that the motion of the antenna is perpendicular to B. 2. d(phi)/dt = EMF dphi = B*A v = dx/dt 3. I'm really not sure here. I think the length of the antenna...
  45. S

    Motional EMF through a circular magnetic field

    Homework Statement A circuit consists of three fixed sides shaped like a |_| and a sliding rod of length 2R. The rod slides at constant speed v into a region of nonzero field B coming perpendicularly out of the paper and limited to a circle of diameter 2R. What is the magnitude and sense...
  46. D

    Motional emf with gravity as the pulling force

    Homework Statement A square loop of mass m resistance R and side l] is halfway inside a magnetic field pointing into the page. It is then released from rest and gravity pulls it out of the magnetic field. a) Calculate the induced emf and the current as functions of the velocity. b) Calculate...
  47. M

    How Does a Falling Metal Bar Generate Electricity in a Magnetic Field?

    1. A 50 g horizontal metal bar, 12 cm long, is free to slide up and down between two tall, vertical metal rods. A 0.060 T magnetic field is directed perpendicular to the plane of the rods. The bar is raised to near the top of the rods, and a 1.0 Ohm resistor is connected across the two rods at...
  48. Y

    Motional emf problem on inclined plane

    Homework Statement a straight horizontal rod slides along parallel conducting rails at an angle with the horizontal of 30 degrees (inclined plane). The rails are connected at the bottom by a horizontal rail so that the rod and rails form a closed rectangular loop. A uniform vertical field...
  49. V

    Calculating Motional EMF when v not perpendicular to B

    Hi, I'm trying to figure out how you can calculate motional EMF when the velocity of the object in question is not perpendicular to the magnetic field. There are two equations in my textbook, but the text describing them is not very helpful. The first is \varepsilon = vBL, which can be...