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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.
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