motional EMF

(noun)

An EMF (electromotive force) induced by motion relative to a magnetic field.

Related Terms

Examples of motional EMF in the following topics:

  • A Quantitative Interpretation of Motional EMF

    • A a motional EMF is an electromotive force (EMF) induced by motion relative to a magnetic field B.
    • An electromotive force (EMF) induced by motion relative to a magnetic field B is called a motional EMF.
    • You might have noticed that motional EMF is very similar to the induced EMF caused by a changing magnetic field.
    • From Eq. 1 and Eq. 2 we can confirm that motional and induced EMF yield the same result.
    • (a) Motional EMF.

  • Motional EMF

    • Motion in a magnetic field that is stationary relative to the Earth induces motional EMF (electromotive force).
    • Motion is one of the major causes of induction.
    • For example, a magnet moved toward a coil induces an EMF, and a coil moved toward a magnet produces a similar EMF.
    • In this Atom, we concentrate on motion in a magnetic field that is stationary relative to the Earth, producing what is loosely called motional EMF.
    • (a) A motional emf=Bℓv is induced between the rails when this rod moves to the right in the uniform magnetic field.

  • Mechanical Work and Electrical Energy

    • Mechanical work done by an external force to produce motional EMF is converted to heat energy; energy is conserved in the process.
    • We learned about motional EMF previously (see our Atom on "Motional EMF").
    • For the simple setup shown below, motional EMF $(\varepsilon)$ produced by a moving conductor (in a uniform field) is given as follows:
    • More generally, mechanical work done by an external force to produce motional EMF is converted to heat energy.
    • (a) A motional emf=Bℓv is induced between the rails when this rod moves to the right in the uniform magnetic field.

  • Electric Generators

    • They induce an electromotive force (EMF) by rotating a coil in a magnetic field.
    • We can thus find the induced EMF by considering only the side wires.
    • Motional EMF is given to be EMF=Bℓv, where the velocity v is perpendicular to the magnetic field B (see our Atom on "Motional EMF").
    • Thus in this case the EMF induced on each side is EMF=Bℓvsinθ, and they are in the same direction.
    • The total EMF $\varepsilon$ around the loop is then:

  • Back EMF, Eddy Currents, and Magnetic Damping

    • Back EMF, eddy currents, and magnetic damping are all due to induced EMF and can be explained by Faraday's law of induction.
    • Lenz' law tells us the induced EMF opposes any change, so that the input EMF that powers the motor will be opposed by the motor's self-generated EMF, called the back EMF of the motor.
    • As discussed in "Motional EMF," motional EMF is induced when a conductor moves in a magnetic field or when a magnetic field moves relative to a conductor.
    • If motional EMF can cause a current loop in the conductor, we refer to that current as an eddy current.
    • Magnetic force on the current loop opposes the motion.

  • Charging a Battery: EMFs in Series and Parallel

    • Usually, the cells are in series in order to produce a larger total emf .
    • But, if the cells oppose one another—such as when one is put into an appliance backwards—the total emf is less, since it is the algebraic sum of the individual emfs.
    • When two voltage sources with identical emfs are connected in parallel and also connected to a load resistance, the total emf is the same as the individual emfs.
    • The charger must have a larger emf than the battery to reverse current through it.
    • Two voltage sources with identical emfs (each labeled by script E) connected in parallel produce the same emf but have a smaller total internal resistance than the individual sources.

  • Faraday's Law of Induction and Lenz' Law

    • Faraday's law of induction states that the EMF induced by a change in magnetic flux is $EMF = -N\frac{\Delta \Phi}{\Delta t}$, when flux changes by Δ in a time Δt.
    • First, EMF is directly proportional to the change in flux Δ.
    • Second, EMF is greatest when the change in time Δt is smallest—that is, EMF is inversely proportional to Δt.
    • Finally, if a coil has N turns, an EMF will be produced that is N times greater than for a single coil, so that EMF is directly proportional to N.
    • The units for EMF are volts, as is usual.

  • Sound Systems, Computer Memory, Seismograph, GFCI

    • The microphone works by induction, as the vibrating membrane induces an emf in a coil.
    • Thus, the electrical current variations that pass through the speaker are converted to varying magnetic forces, which move the speaker diaphragm, forcing the driver to produce air motion that is similar to the original signal from the amplifier.
    • If a GFCI detects that there is a leakage of current, it produces an EMF and a current in the opposite direction of the original current.

  • Inductance

    • Induction is the process in which an emf is induced by changing magnetic flux.
    • See , where simple coils induce emfs in one another.
    • These coils can induce emfs in one another like an inefficient transformer.
    • Here a change in current in coil 1 is seen to induce an emf in coil 2.
    • (Note that "E2 induced" represents the induced emf in coil 2. )

  • Sources of EMF

    • The created electrical potential difference drives current flow if a circuit is attached to the source of EMF.
    • Again the EMF is countered by the electrical voltage due to charge separation.
    • The general principle governing the EMF in such electrical machines is Faraday's law of Induction.
    • In nature, EMF is generated whenever magnetic field fluctuations occur through a surface.
    • The voltage output of each depends on its construction and load, and equals emf only if there is no load.