Transcript Notes17

Conceptual Physics
Study Notes & Questions: Electromagnetics (Chap. 17)
1)
Batteries work by chemical reactions that separate electrons from
atoms, creating a difference in electric potential between the battery’s
two terminals. (p354)
2)
The motion of electric charge produces a magnetic field. This field
circles around the electrons’ motion vector—such as in a wire—in a
direction that can be described by the right hand rulethat is, point
your right hand thumb in the direction of the electrons’ motion and the
circulation direction of the generated magnetic field follows the
bending direction of your right hand’s fingers. (p357)
3)
Make a coil of conductive wire and flow an electric current through it.
The generated magnetic field flows in a single direction through the
center of the coil, exits at one end, curves around and re-enters
through the opposite end of the coil. This is an electromagnet. There
are no starting points for magnetic fields—they always form
continuous loops. (p357)
4)
Using the push/pull of interacting magnetic fields, permanent
magnets, electromagnets, and AC (alternating) current, you can
construct an electric motor. How does it work? (p358)
5)
Conductive wires passing through a magnetic field will have an
electric field induced in the wire, causing an electric current to flow.
This is called electromagnetic induction and is the basis of an electric
generator. (p365) An electric motor and generator are essentially the
same device, the former converts electrical energy to work, the latter
converts work to electrical energy.
6)
Electromagnetic induction means that a changing magnetic field
creates an electric field. A transformer is two loops of wire configured
so that the magnetic field generated by one electromagnet will flow
through the other coil (p387). Flowing an AC current through one coil
will create a time-varying magnetic field. This changing magnetic field
will induce an AC current in the second coil. The voltage across the
first coil, compared to that induced in the second coil is equal to the
number of coil windings in the first coil divided by the number of
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windings in the 2nd coil. (p361)
7)
Important note: When a changing magnetic field induces a current in a coil
of wire, the direction of the induced current is generated such that the
magnetic field produced in the coil is in the opposite direction of the
original magnetic field. (p362)
8)
Maxwell’s Equations describe the full interaction of electric and magnetic
phenomena (p367): 1) there are electric point sources (electrons and
protons); 2) there are no magnetic point sources—magnetic force lines
always form continuous loops that curve back on themselves; 3) a
changing electric field (e.g. moving electrons) creates a magnetic field; and
4) a changing magnetic field induces an electric field.
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