What is Electromotive force: Definition and 56 Discussions

In electromagnetism and electronics, electromotive force (emf, denoted


{\displaystyle {\mathcal {E}}}
and measured in volts) is the electrical action produced by a non-electrical source. Devices (known as transducers) provide an emf by converting other forms of energy into electrical energy, such as batteries (which convert chemical energy) or generators (which convert mechanical energy). Sometimes an analogy to water pressure is used to describe electromotive force. (The word "force" in this case is not used to mean forces of interaction between bodies).
In electromagnetic induction, emf can be defined around a closed loop of conductor as the electromagnetic work that would be done on an electric charge (an electron in this instance) if it travels once around the loop. For a time-varying magnetic flux linking a loop, the electric potential's scalar field is not defined due to a circulating electric vector field, but an emf nevertheless does work that can be measured as a virtual electric potential around the loop.In the case of a two-terminal device (such as an electrochemical cell) which is modeled as a Thévenin's equivalent circuit, the equivalent emf can be measured as the open-circuit potential difference, or voltage, between the two terminals. This potential difference can drive an electric current if an external circuit is attached to the terminals, in which case the device becomes the voltage source of that circuit.

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  1. cianfa72

    I Confused about electromotive forces

    Hi, reading Griffiths - Introduction to Electrodynamics I'm confused about his claims in section 7.1 My point is that the job of electromotive force ##f## is actually produce the "movement/drift" of free charges against the electromagnetic field, so ##f## should not be given by the Lorentz...
  2. Z

    Why is source force + electrostatic force = 0 inside battery?

    If a battery is in a closed circuit, we have an electrostatic field ##\vec{E}## from the positive terminal to the negative terminal of the battery through the circuit outside the battery. This field generates current. Inside the battery, the electrostatic field lines go from positive to...
  3. Z

    Understanding the math of definition of electromotive force

    Does this mean we can write the following? $$\mathcal{E}=\oint_C \vec{E}\cdot d\vec{r}+\oint_C \vec{v}\times\vec{B}\cdot d\vec{r}\tag{3}$$ I haven't seen an equation like the above in my books and notes yet. What I have seen are two cases. In one case, we have a uniform magnetic field and we...
  4. Z

    How to obtain correct negative sign in calculation of motional emf?

    My issue is with the signs. We have $$\mathcal{E}=\oint \vec{v}\times\vec{B}\cdot d\vec{s}$$ $$=\oint v_0\hat{i}\times B_0(-\hat{k})\cdot dy\hat{j}$$ $$=\int_0^h v_0B_0\hat{j}\cdot dy \hat{j}$$ $$=v_0Bh$$ Now, apparently, the sign should be negative. Intuitively, the magnetic force on any...
  5. AmericaPacific42

    Algorithms for quadcopters to use back-EMF to detect obstacles close?

    Would it help to study Verilog (VHDL) or Field-Programmable-Gate-Arrays (FPGA) if self-crafted x64/aarch64-assembly would take too much power/be too slow?
  6. 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...
  7. D

    Problems about magnetic field, electromotive force, induced current

    Hi, I was practicing some problems on the magnetic field and the electromotive force, when I got stuck on these two exercises. Could you help me figure out how to proceed? In the first problem, I tried to find the magnetic field flux by multiplying the induced current for ∆t and R. Should I now...
  8. R

    Electromotive Force, Batteries, Resistors, and Potential Differences

    I have attached a small excerpt from my digital book where they start talking about emf. I am very confused. Let me explain what is confusing to me so that you can clear up what's bothering me. They start of by saying that an emf device pumps charges by maintaining a potential difference...
  9. Kaushik

    Intuition about electromotive force (EMF)

    My understanding of emf Let us consider 2 parallel plates with charges (opp. but equal in magnitude) stored on it. When we connect both the plates from the outer side, the electrons from the lower potential (i.e., negatively charged plate) moves to the higher potential (positively charged plate)...
  10. mcastillo356

    Electromotive force when current is alternating

    Hello, let's imagine a generator as shown in image. I just don't understand alternative current. -\epsilon shouldn't be always positive? Otherwise, how can it provide anything? Thanks
  11. mcastillo356

    B Electromotive force and trigonometry

    Faraday's law: \epsilon=-N\dfrac{\Delta{\phi}}{\Delta{t}}=-N\dfrac{\Delta{(BA\cos{\theta})}}{\Delta{t}}=-N\dfrac{\Delta{(BA\cos{(\omega t)})}}{\Delta{t}} Applying calculus \epsilon=NBA\omega\cos{(\omega t)} Shouldn't it be \epsilon=NBA\omega\sin{(\omega t)}, just if I apply limits? Thanks
  12. E

    Electromotive force (or E cell?) during charging and discharging

    The EMF of the cell is the potential of the cathode subtract that of the anode, consistent with ##\Delta G = -nF\varepsilon##. If the cell is running as a galvanic cell/discharging, the EMF should be positive. If it is charging, the cathode (the one gaining electrons) is now negative and at...
  13. K

    Calculate the current in all parts of this electric circuit

    In the circuit below, the output is 23 W across the resistor with the resistance 6 Ohm. Calculate the amount of current in all parts of the circuit as well as the polarity and EMF ε of the unknown battery. Circuit: My attempt: I get 6 unknowns with 5 equations. I don't know how to find the...
  14. Alefan_

    Average current around a magnetic loop that changes its shape

    To find out what the induced loop current was i used the formula: V=R×I To find out what the value of V was i used the formula that links electromotive force (fem) to angular speed: Fem=ω×B×A The only thing that's missing is the loops area but considering that it's a semicircle and that the...
  15. D

    Electromotive force induced in a conducting rod

    i don't understand why in the solution of this exercise, the induced electromotive force / current is counterclockwise. Shouldn't it be clockwise? ##emf=-\frac {d \Phi_B} {dt} ## According to the picture the rod is moving upward, the magnetic flux (entering the page) is decreasing due to a...
  16. G

    Energy in a rotating square loop

    Homework Statement A square circuit of resistance R=20Ω and side ℓ = 0,2 m spins 100 times per second around an horizontal axis that splits it in two. There is an uniform magnetic field B=1T perpendicular to the position ocupied by the circuit at t=0s. Calculate (1) the magnetic flux, (2) the...
  17. X

    Find the induced electromotive force (epsilon)

    it is something like this but instead of the rectalve contour iswith the angle 37 degree!https://www.physicsforums.com/attachments/114624 1. Homework Statement the speed of the slide is 0.16 m/s the length of the slide is 0.14 m the angle of the contour is 37 degree Homework Equations...
  18. G

    Why is EMF (ElectroMotive Force) equal to a voltage?

    Hello. Let's say a current i(t) flows through an inductor of solenoid type. Time-varying i(t) generates a magnetic field B(t) inside the magnetic core of the inductor. As B(t) is time-varying, EMF, which is the line-integral of the induced electric field E(t) along a coil (which carries i(t))...
  19. G

    Faraday's law for constant velocity, area and B-field

    Hi. If a planar wire loop is moved through a homogeneous magnetic field (field lines perpendicular to the loop plane) with constant velocity and no rotation, Lorentz force will move some electrons to one side of the loop, creating a potential difference. But how does this work with Faraday's...
  20. P

    Induced Magnetic Moment (vector) vs. Induced EMF (scalar)

    When I induce magnetic flux through a closed loop, I should expect the lines of flux produced by current in that loop to oppose the change of flux through that loop. But what happens when that loop, say a rectangular loop, is curved into the shape of the letter J (like a candy cane) and my flux...
  21. Aafia

    What is induced magnetic field?

    What is emf in the coil? As far as I know about emf is that it is potential difference between two terminals of a source but what if it is induced in coil by changing flux linkage. What exactly happen when emf is induced?
  22. M

    How Electromotive Force is Produced by Electric Generator?

    at 5:42 - What is Electromotive Force? Why does it make electrons move (make electric current) in one direction?
  23. Toughnuke

    Electromotive force (EMF) of a circular loop wire

    Hi, so, I got help from this homework problem by posting it on chegg, but I want clarification. A circular loop of wire of radius 12.0 cm is placed in a magnetic field directed perpendicular to the plane of the loop and into the page. The field is decreasing at a constant rate of 0.050 T per...
  24. E

    Solve Homework 2: Electromotive Force in a Shrinking Loop

    Homework Statement A little stuck on the second question it basically boils down to not quite understanding the question. Was hoping someone here could push me in the right direction. The Question is two part, already completed the first half but wouldn't mind if someone looked over it while...
  25. S

    Electromotive force in a moving bar

    Hi everyone, I've seen in some lecture notes that a moving conducting bar (length of L) in a uniform and constant magnetic field develops an electromotive force (emf) between the 2 edges of the bar. Lets assume that the bar moves to the right (with velocity v) and the magnetic field (B) points...
  26. B

    Calculate the electromotive force E

    hi, How can I demonstrate the relationship of the electromotive force of a motor AC thank you
  27. H

    Understanding Back EMF in DC Motors

    Homework Statement Hello, i decided to sign up here to ask one question: A dc motor with load has verry small resistance. We can almost say the resistance is only that of the copper. Without load the motor is accelerating as much as it can per given voltage and it generates strong back emf...
  28. Caio Graco

    How does a changing magnetic flux induce an electromotive force in a loop?

    Consider a loop which is subjected to a magnetic flux variation. Among the points that turn there will be the electromotive force induced?
  29. L

    A question on Electromotive Force

    May i know if electromotive force (emf) moves from the positive terminal to the negative terminal or a battery through a circuit or is it from the negative terminal to positive terminal?
  30. Y

    Understanding Electromotive Force and Internal Resistance in Batteries

    sorry for the drawing.Hope you guys can bear with it. The electromotive force can be calculated by opening the switch and the voltmeter will show the exact voltage of the battery. When the switch is closed,there will be a voltage drop due to the internal resistance of the battery. The question...
  31. T

    Electromotive Force: AC Voltage, Inductor & Conservative Field

    Hello, In an electrical circuit, let's say we have an AC voltage source In addition, we have an Inductor that stores electrical energy when the current is rising than give back this energy to the voltage source when the current is falling. My question is,at the stage when the energy is given...
  32. A

    Electromotive Force: E=Vab=IR Explained

    The potential difference between the ends of a wire is given by Vab = IR. Combing that with E = Vab we have E = Vab = IR (ideal source of emf). " That is, when a positive charge flows around the circuit, the potential rise as it passes through the ideal source is numerically equal to the...
  33. M

    Electromotive force of automobile battery

    Homework Statement An automobile battery has an emf of 12.6 V and an internal resistance of 0.080 0 Ω. The headlights together present equivalent resistance 5.00 Ω (assumed constant). What is the potential difference across the headlight bulbs (a) when they are the only load on the battery...
  34. J

    Calculating Electromotive Force in a Circuit

    Homework Statement A battery has a current of 3.0A flow through it. It has an internal resistance of 0.039Ω and the circuit has a load of 45Ω. a) What is the emf? b) What is the voltage that flows? Homework Equations V = ε - IR The Attempt at a Solution ε = V + IR ε = (45Ω)(3A) + (3A)(0.039Ω)...
  35. P

    Understanding Electromotive Force (emf)

    Hi, I am learning about emf. I understand that it is the Work done per unit charge to move it around a circuit. However, reading my book, I got a bit confused. It started to explain a bit about how a battery functions. From what I understand, chemical reactions do work in order to force...
  36. R

    Electromotive force and potential difference

    hi everyone, i would like to ask some quick questions that i find confusing. first, electromotive force is the energy supplied by a battery / power source, to ''push'' the charge through a circuit right? its unit is in Volts. then, potential difference is defined as the work done to push 1...
  37. T

    Electromotive Force: Is It Really a Force?

    electromotive force... why is v called electromotive force if it is not a force?? is there a reason for this??
  38. P

    Electromotive force in constant flux

    Homework Statement in book there is written that this turn will make electromotive force, but from faraday law we know that force generates only when flux is changing, what about this? http://img821.imageshack.us/img821/747/schematu.jpg and they say that it generates electromotive force...
  39. P

    Defining Electromotive Force (\epsilon)

    How to define electromotive force? \epsilon=\oint_L\vec{E}\cdot d\vec{l} Can I say electromotive force is work per unit charge which we need to put charge from one place of contoure to the same place?
  40. L

    Understanding Electromotive Force (emf) in a Circuit

    Homework Statement So I'm trying to figure out what exactly is emf in a ciruit and this is what i understand so far: A source of emf converts some form of energy into electrostatic potential energy needed in order to do work to separate positive and negative charges at its terminal against the...
  41. A

    Is electromotive force always constant?

    is electromotive force always constant?? i've been cross-referencing different sources as to the exact definition of the electromotive force, and i keep getting the following: emf = terminal voltage but i know terminal voltage changes due to the voltage drop that occurs when an...
  42. B

    Electricity Basics: Understanding Electromotive Force (EMF)

    A generator is charcterized by its internal resistance and electromotive force E. What is meant by electromotive force..(please i need perfect answer)
  43. I

    Electromotive Force Homework: Vertical Conducting Rods & Uniform Magnetic Field

    Homework Statement Two vertical conducting rods separated by a distance d are connected to a capacitor of capacitance C. Another horizontal conducting rod of mass m is released at position y=y0 at time t=0 always in contact with the two vertical ones. The set is immersed in a uniform magnetic...
  44. X

    Electromotive Force: Calculate Charge, E.M.F & P.D.

    Homework Statement Atorch battery transfers 5200J of chemical energy into electrical energy while supplying a current of 0.25A for 4400 s. The bulb transfers 4800 j of electrical energy into heat and light during this time. Calculate a)the charge that flows b)the e.m.f. of the battery c)...
  45. H

    Internal resitance and electromotive force

    Homework Statement 1) a complete circuit consists of a 12.0 V battery with a 4.50 ohm resistor and switch. the internal resistance of the battery is 0.30 ohms when switch is open. what does the voltmeter read when placed: a. across the terminal of the battery when the switch is open b...
  46. J

    How does change in current induce electromotive force

    Statement: Reviewing the definition of inductance, I found the following definition from Wikipedia: Question Can someone explain to me how change in current will induce an electromotive force (one that will oppose the present current)? And what is meant by a magnetic field is inductive?Thanks,JL
  47. C

    Solving Electromotive Force Homework: 5000V Power Supply & 20kohm Voltmeter

    Homework Statement A power supplyused in a lab has an emf of 5000V. However, when a voltmeter of resistance 20kohm is connected to the terminal of the power supply, a reading of only 24V is obtained. Explain this conservation and calculate any necessary quantity. The Attempt at a Solution...
  48. P

    Electromotive Force problem dealing with spherical shells

    Homework Statement To concentric metal spherical shells , of radius a and b , respectively , are separated by weakly conducting material of conducting sigma a) If they are maintained at a potential difference V, what current flows from one to the other? Homework Equations V=-\intE *dl V=IRThe...