Transcript Magnetism, Electromagnetism, & Electromagnetic Induction

Magnetism,
Electromagnetism, &
Electromagnetic Induction
Chapters 24-25
Magnetic Fields
• The source of all magnetism is moving
electric charges.
• Iron is the element with the most magnetic
properties due to its net electron spin of 4.
• Magnetic field lines are vectors with a
direction from North to South.
• Magnetic field lines must not cross each
other.
• Magnetic fields are measured in Teslas.
Earth’s Magnetic Field
The angle between the magnetic and
geographic poles is called the magnetic
variation.
Compasses
• Compass needles are magnetized and line up
along magnetic field lines.
• The North magnetic pole of a compass points
to the geographic north.
• Since opposites attract, the magnetic pole in
the Northern Hemisphere is actually a South
magnetic pole.
• The North pole of a compass points in the
direction of the field lines.
Magnetic Field around a
current-carrying wires
• A current moving through
a wire creates a magnetic
field around that wire.
• The magnetic field forms
concentric circles around
the wire.
• Use the right hand rule
shown to predict the
direction of the field.
Electromagnets
• Electromagnets are
temporary magnets
formed by wrapping
wire around an iron
core.
• The iron becomes
magnetized when the
current is flowing due
to the magnetic field
being concentrated
inside the coil of wire.
To find the North pole of
an electromagnet, wrap the
fingers around the coil in
the direction that the
current is flowing and the
thumb points to the North
pole.
Force of a magnetic field on a
charged particle
• A charged particle moving
through a magnetic field will
experience a force that will
cause it to move in a circular
path.
• The force is  to both the
velocity and the magnetic
field direction.
F  qvB sin
F = force(N), q = charge(C), v = velocity(m/s),
B = mag. field strength(T), =angle between v & B
Force of a magnetic field on a
current-carrying wire
• A conductor with
a current flowing
through it in a
magnetic field
will experience a
force.
F  IlB sin
F = force(N), I = current(A), l = length of wire(m),
B = mag. field strength(T), =angle between l & B
Force between 2 currentcarrying wires
• When a current flows
through a wire a magnetic
field is produced around it.
• When 2 wires carry
current near each other
there will be an interaction
(force) between the
magnetic fields produced
by each individual wire.
Induced EMF (Voltage)
• A conductor in a
changing magnetic
field will have an
EMF (voltage)
induced .
• Either the conductor
can be moving across
field lines or the
magnetic field can EMF = electromotive force,
itself be changing. voltage(V), B = magnetic field strength
EMF
EMF
EMF  Blv
(T), v = velocity  to l(m/s),
Induced Current
• When a EMF (voltage, or potential
difference) is present in a closed loop of
conducting material current will flow.
EMF
V
I

R
R
I=current(A), EMF = V = Voltage(V), R = resistance
Lenz’s Law
• The motion of a
conductor through a
magnetic field will
induce a current in
that conductor.
• That current will
cause the wire to
experience a force
that opposes the
motion of the wire.
Motors vs. Generators
• Motors
– Electric current is
changed to motion.
– A coil of wire with
a current through it
will be forced to
turn in a magnetic
field.
• Generators
– Motion is changed
to electric current.
– Turning a coil in a
magnetic field will
induce an EMF
(voltage), thus
current is
produced.
• As the loop of wire is turned
in the magnetic field, one side
is moving up while the other
is moving down, therefore a
current is induced in opposite
directions in the different
sections of the loop.
• As the loop continues to turn,
the sections of wire change
places and so the current
switches direction.
• This causes the current to
change constantly as shown
in the graph.
AC Generator
AC/DC
• Alternating Current (AC) • Direct Current (DC)
– current that switches
– current that flows in
direction of flow on
only one direction
regular time intervals
through a circuit
– 60 Hz in US
– supplied by batteries or
electrochemical cells
– created by EMF
induced in a coil of wire
– created by a chemical
turning in a magnetic
reaction that produces
field
a potential difference
(voltage) between the
two electrodes
(terminals)
Effective vs. Maximum with AC
Current
• DC values are comparable to Effective AC values.
• AC circuits do not get the effect of the maximum
current and voltage produced
• The power equivalent of AC to DC voltage is half.
Peff 
1
PDC 
1
P
max
2
P
AC
2
Ieff  .707Imax
Veff  .707Vmax
Transformers
• An alternating current flows through the
primary coil creating an alternating magnetic
field.
• This changing magnetic field induces an EMF
(Voltage) in the secondary coil and thus
current flows.
• In an ideal transformer, Power in = Power out
To solve Transformer Problems
The ratio of
voltages on the
two coils is
equal to the
ratio of the
number of turns
in the coils.
Vs  Ns
Vp Np
Pin  Pout
VpIp  VsIs