23.3 Motional Emf

23.3 Motional Emf

  • Any change in magnetic flux causes an emf opposing that change to occur.
    • Motion is one of the main causes.
    • A coil moved toward a magnet produces a similar emf.
    • Motion in a magnetic field that is stationary relative to the Earth is called motional emf.
  • The Hall effect is a situation where motional emf occurs.
    • The magnetic force that moves opposite charges in opposite directions creates an.
    • The Hall effect has applications.
    • We will see that motional emf can be used as a power source, and that the Hall effect is one aspect of the broader phenomenon of induction.
  • A rod is moved along a pair of rails separated by a magnetic field.
    • The rails are connected to a Resistor.
    • A light bulb, for example, could have a Resistor.
    • Consider the area surrounding the moving rod.
    • The rails, rod, and Resistor are enclosing a magnetic flux.
    • An emf is created when the flux changes.
  • The moving rod and rails have a magnetic field that goes into the page and into the area enclosed by them.
    • The current direction shown by RHR-2 is the direction the rod will drive.
    • The rod's polarity is also indicated by RHR-1.
  • The magnitude of the emf is implied here and below.
  • The area was swept out by the rod.
  • The expression is the same as before.
  • The electric force and magnetic force have many connections.
    • Electric and magnetic forces are different manifestations of the same force because of the fact that a moving electric field produces a magnetic field.
    • The inspiration for contemporary efforts to unify other basic forces comes from this classic unification of electric and magnetic forces.
  • To find the direction of the field, the direction of the current, and the direction of the emf, we apply the law of Lenz's Law.
    • The area enclosed is increasing.
    • The existing field must be out of the page.
    • The RHR-2 requires that I becounterclockwise, which means that the top of the rod is positive.
  • If the magnetic field moves and the rod is stationary relative to the Earth, motional emf can also occur.
    • An example of this is when a moving magnet causes an emf in a coil.
    • The relative motion is important.
    • There is a connection between magnetic and electric fields.
  • An electric field is created by a moving magnetic field.
    • We already know that a moving electric field produces a magnetic field.
  • Motional emfs in the Earth's weak magnetic field are not usually very large, or we would notice voltage along metal rods, such as a screwdriver, during ordinary motions.
    • A simple calculation of the motional emf of a 1 m rod moving at 3.0 m/s is enough to give.
    • The small value is consistent with experience.
    • There is an exception.
    • The space shuttle tried to create large motional emfs.
    • The tethered satellite was to be let out on a 20 km length of wire to create a 5 kV emf by moving through the Earth's field.
    • If a complete circuit could be made, this emf could be used to convert some of the shuttle's potential energy into electrical energy.
    • The return path for the current to flow was supplied by the stationary ionosphere.
    • It conducts because of the sound it makes.
    • The shuttle is able to be converted to electrical energy by Drag on the current in the cable due to the magnetic force.
    • Both tests were unsuccessful.
    • In the first, the cable hung up and could only be extended a few hundred meters; in the second, the cable broke when almost fully extended.