Download intro electromagnetism

Do Now (1/23/14):
1. How do magnets work?
2. What are some of their properties?
Welcome back! 
Magnetism and Electro
Magnetism
Timson
January 2014
Atoms themselves have magnetic properties due
to the spin of the atom’s electrons.
Groups of atoms join so that their magnetic fields
are all going in the same direction
These areas of atoms are called “domains”
When an unmagnetized substance is placed in a magnetic
field, the substance can become magnetized.
This happens when the spinning electrons line up in the
same direction.
An unmagnetized
substance looks like
this…
While a magnetized
substance looks
like this…
How to break a magnet:
1. Drop it
2. Heat it
This causes the
domains to become
random again!
Review:
• Field due to current carrying wire:
0 I
B
2r
Ampere’s Law
For any closed loop path, the sum of the length
elements times the magnetic field in the direction of
the length element is equal to the
permeability times the electric current enclosed in
the loop
Ampere’s Law
• The magnetic field in space around an electric
current is proportional to the electric current which
serves as its source, just as the electric field in space
is proportional to the charge which serves as its
source.
• Which law is analogous to Ampere’s Law?
Gauss’s Law:
Example:
A long straight wire carries a current of 5 A. at one
instant, a proton, 4 mm from the wire, travels at
1.5x103 m/s parallel to the wire and in the same
direction as the current. Find the magnitude and
direction of the magnetic force that is acting on
the proton because of the magnetic field
produced by the wire.
-20
6x10
N
Magnetic Force Between Two Parallel
Conductors
Magnetic Field of a Current Loop
• Examining the direction of
the magnetic field
produced by a currentcarrying segment of wire
shows that all parts of the
loop contribute magnetic
field in the same direction
inside the loop.
Magnetic Field of Current Loop
• Electric current in a circular
loop creates a magnetic
field which is more
concentrated in the center of
the loop than outside the
loop.
• Stacking multiple loops
concentrates the field even
more into what is called
a solenoid.
Magnetic Field of a Solenoid
• A long straight coil of wire can be used to
generate a nearly uniform magnetic field similar
to that of a bar magnet.
• The field can be greatly strengthened by the
addition of an iron core.
Example:
• A certain solenoid consists of 100 turns of
wire and has a length of 10 cm. Find the
magnetic field inside the solenoid when it
carries a current of 0.5 A.
-4
6.28x10
T
Inductance, Inductors
Inductance, unit: henry [H] = ability to store magnetic energy
A circuit element that has a large self-inductance is called an
inductor. The circuit symbol is
Potential across inductor: vL(t) = L diL(t) / dt
L = N2 μA / ℓ
UM = ½LI2
Lparallel = (1/L1 + 1/L2 + … + 1/Ln)-1
Lseries = L1 + L2 + …. + Ln