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Magnetic fields and electric currents
A magnetic field circulates around a current-carrying wire.
Moving charges create a magnetic field
Right hand rule
to get direction
of field around
an electric
current
Magnetic fields and electric currents
Tiny iron filings align with the magnetic field to show the field lines
around these current-carrying wires.
Moving charges create a magnetic field
Solenoid – use right hand
rule to get direction of field
around electric current in
wire loops
Magnetic fields and electric currents
A coil of current carrying wire is called a solenoid. The magnetic field lines circle the
wire and cancel in the spaces between the wires. Out side and inside the coil, a
strong magnetic field is produced where the field lines get closer together.
http://www.bugman123.com/Physics/index.html
Electromagnets
By placing ferromagnetic material inside a coil of wire, we can make the
magnetic field inside the solenoid even stronger. When the current is turned
on, this configuration becomes an electromagnet.
Force on a
moving charge
(Lorentz force)

 
F  qv  B
Direction of
magnetic field (B)
F
The force that a magnetic field
exerts on a moving charge is
perpendicular to the direction of
the charge, and also
perpendicular to the direction of
the field. So if a proton were
moving in the direction shown,
which way would the magnetic
field deflect it?
The Lorentz force is zero if the magnetic field is
parallel or anti-parallel to the velocity
Force on a current carrying wire
N
N
Electrons moving through a wire that is in a magnetic field also feel a force (recall
that the direction of the current is the direction that positive charges move, or
opposite to the direction that electrons move). In that case, the current-carrying
wire feels a force exerted on it.
Electric motors
A magnetic field exerts a force on a current loop, at right angels to
the direction of the field and to the direction of the current.
Electric motors
As the current loop turns, the magnetic field continues to exert a
force on it.
Electric motors
When the loop turns far enough, the force would work to stop turning
it, so the polarity of the current is switched to run the other way. This
makes the force on the loop such that it continues to turn. This is the
basic mechanism of an electric motor.
Electric motors
A simple electric motor operates using switching magnetic fields to
push and pull a rotator with magnets affixed to it.
http://www.animations.physics.unsw.edu.au/jw/electricmotors.html#DCmotors
Motor demos
Homopolar motor - demo
http://www.youtube.com/watch?v=3aPQqNt15-o
Battery and generator as a motor
Magnetic field of the Sun
The magnetic field is evident in this photo from the sun's surface. Remember that
magnetic fields exert a force on moving charged particles that deflects them
perpendicular to the field. This causes them to spiral along the strong magnetic field
lines of the sun, mapping out the field as they release light.
Magnetic field of the Sun
The solar wind contains charged particles which distort Earth's magnetic field.
The magnetic field of Earth deflects charged particles in the solar wind. Huge
magnetic storms on the sun can hurl gigantic clouds of plasma into space. If
they happen to come our way, they can cause problems like surges in power
grids and breakdown of satellite communication.
http://www.flickr.com/photos/gsfc/4541101546/
Aurorae
Charged particles in the solar wind can
collide with particles in Earth's atmosphere,
especially near the north and south magnetic
poles. When they do, they excite atoms
which then return to ground state, emitting
light. We see the eerie streaming flows of
color that result. They are called the aurora
borealis and aurora australis for the northern
and southern phenomena respectively.
Aurorae
The colors of the aurorae change with height in the atmosphere,
depending on the type of gas found at that height.
Magnetism history
People used to think electricity and magnetism were the same, and sometimes
students conflate or confuse positive/negative & north/south.
Some distinctions:
A non-neutral net charge produces an electric field. Moving charges
produce a magnetic field.
An electric field always exerts a force on a charge in a direction parallel
to the field. A magnetic field only exerts a force on a moving charge, and
the force is perpendicular to the direction of the field.
Single electric (positive or negative) charges exist. Single magnetic poles
do not seem to exist.
The distinction between electricity and
magnetism depends on your frame of reference
v
current
above wire, B is into page
e
v = drift velocity of electrons in wire
Lab frame of reference: Consider a neutral, current-carrying wire containing
moving electrons (producing a magnetic field and no electric field), with a
single electron moving parallel to the wire at the same speed as the drift
velocity (v) of the electrons in the wire. There is no electrical force on the
electron (because the wire is neutral), but there is a magnetic force pushing
the the electron towards the wire.
Electron (moving) frame of reference: In the frame of reference of the electron
outside the wire, it feels no magnetic field. But according to special relativity,
in the frame of reference of the electrons drifting, the spacing between the
protons in the wire is less (length contraction), and so the positive charge
density is greater than the negative charge density. There is an overall
positive charge in the wire, so an electric field attracts the electron to the wire.