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Electromagnetism
Definition: Magnet- object that exerts a force on a piece of iron
Definition: Magnetic- property where a material is affected by a magnetic field
-like poles repel
-unlike poles attract
-the force of attraction (or repulsion) varies inversely as the square of the distance between the poles
Difference between magnetism & electricity:
-electric charges can be isolated
-magnetic poles cannot be isolated
ex: cut magnet in half --> 2 new magnets
examples of strongly magnetic materials: gadolinium, cobalt, nickel, iron, alnico (Al, Ni, Co)
Definition: Magnetic Domain - groups of aligned magnetic fields in elements
-inducing magnetism on iron  stroke it with a magnet
-magnetic effects can be lost by cooling, heating, or hammering material
-magnitite (loadstone) is a special mineral found on Earth & is magnetized by exposure to Earth's
magnetic field
-soft magnetic materials-easily lose/gain magnetism, i.e. iron
-hard magnetic materials-difficult to lose/gain magnetism, i.e. nickel & cobalt
magnetic field  surrounds a magnet
-can be described with magnetic field lines
-lines can describe magnitude & direction
{direction of MF  direction in which north pole of compass needle points at a given position}
Magnetic Force on a Current-Carrying Conductor
-symbol for MF is "B"
-all magnetic fields are caused by the movement of charged particles
 i.e. electrons moving through a wire
-first discovered by Oersted (1820)
Right-hand Rule #1:
"if you point your thumb of your right hand in direction of
conventional current (+ to - ), the curl of your fingers will indicate
direction of magnetic lines of force around wire"
-symbol for magnetic field going into page
-symbol for magnetic field coming out of page
Magnetic Field of a Current-Carrying Coil (Solenoid)
Definition: Solenoid- long, straight wire bent into several closely-spaced loops
-aka "electromagnet"
-acts like a magnet only when it carries a charge
-MF inside solenoid is nearly uniform and strong
-outside, MF is non-uniform
-strength of solenoid can be increased by adding iron core
-one end acts like North Pole; other acts like South Pole
-
+
Right-hand Rule #2
“if you grasp the coil with the right hand and curl your fingers around the loops in the
direction of the conventional current, your thumb points towards the North pole of the
electromagnet”
-strength of magnetic field of an electromagnet is proportional to current flowing in coil
-# of loops in coil proportional to strength of magnetic field
Forces Caused by Magnetic Fields
-when moving through a magnetic field, charged particles experience a force (a stationary charge
does not!)
-first discovered by Faraday
Right-hand Rule #3:
"if you point the thumb of your right hand in the
direction of the current velocity (v), then your
fingers point in the direction of the magnetic field
(B) and the magnetic force (Fm) on a +ve charge is
directed out of the palm of your hand"
Assume +ve test charge (qo) with velocity (v):
Fm  BIL sin 





Fm is magnetic force (measured in N)
B is strength of magnetic field or magnetic induction (measured in Tesla or T  one N per
ampere-meter is one Tesla)
I is the current (measured in amperes)
L is the length of straight wire (measured in m)
 is the angle between direction of conventional current and the direction of B
Since I 
Q
then
t
Q
Fm  B L sin 
t 
L
Fm  BQ   sin 
t 
Fm  BQv sin 
-max magnetic force occurs when angle between current (or v) and
B is right angle
Using this formula..
B
Fm
.
Qv sin 
Lab magnets are
about 2.5 - 10 T &
Earth's MF is
about 5 x 10-5 T
Electric Motors
-turns electrical energy into mechanical energy
-2 types: DC & AC
-when a current is passed through a wire loop (armature) in a magnetic field  magnetic force on
armature  180o turn
-to make 360o rotation, split-ring commutator (attached to armature) allows current to change
direction in armature (due to contact with brushes)
-change in direction of current means magnetic force in opposite direction  further 180o turn
Direct Current (DC) Motor:
Brushes
-when current supplied to commutator is AC, commutator has two separate parts (slip ring
commutator), each connected to one end of armature
-reversal of current supplied by voltage source (no need for split-ring commutator)
Alternating Current (AC) Motor:
Electromagnetic Induction
-to generate current, either a conductor can move through a magnetic field OR magnetic field can
move past a conductor
Electromagnetic Induction:
"the generation of a current in a wire (circuit) due to the relative motion of the wire and the
magnetic field”
Generators:
-Faraday devised 1st generator
-turns mechanical energy into electrical energy
-2 types: AC & DC
-wire loop rotated in MF by external means
(i.e. crank)
-as loop rotates, motion of wire loop
changes in the magnetic field 
induced current produced
-end of loop connected to slip ring
commutator
-connections to external circuit made
by stationary brushes in contact with
commutator
-when loop is 1/2 way through rotation in MF  current
flows one direction
-when completing other 1/2 rotation  current
flows in opposite direction (AC)
-for DC, split-ring commutator used to produce pulsating (or rectified) direct current
-magnetic force tends to slow down rotation of armature
Lenz's Law:
“When a conductor interacts with a magnetic field, there must be an induced current that
opposes the interaction"
-if a generator produces a small current, opposing force on armature is small and easy to turn
-if it produces larger current, force will be larger and more difficult to turn
-to produce greater electrical energy, more mechanical energy must be supplied to armature rotation
 consistent with law of conservation of energy!
Transformers
-Faraday's experiment: electric current can be produced in 2nd coil by changing a MF
-2nd coil behaves as though a source of emf were connected to it
-a steady MF cannot produce a current; a changing MF can! produces an induced emf
-sometimes necessary to change small AC voltage to a larger one or vice-versa
AC transformer:
-2 coils of wire wound around a core of soft iron
-coil connected to input AC voltage source (primary) has N1 turns
-2nd coil connected to a resistor (secondary) has N2 turns
-if N2 > N1, then V2 > V1 --> step-up transformer
-if N2 < N1, then V2 < V1 --> step-down transformer
Vp
Np
Is


Ip
Vs
Ns