Transcript Magnetism

Magnetism
19 & 20
Properties of Magnets
 Magnets have 2 ends (polarized)
 North-seeking (north pole)
 South-seeking (south pole)
 Magnets always have two opposite
magnetic poles. If you break a magnetic in
half you get 2 smaller magnets with 2
poles. You do not get separate poles.
 Like poles repel
 Unlike poles attract
Orientation
 Magnets always orient themselves in a
north-south direction
 Earth is a giant magnet
 The north pole of a compass points north
so the south magnetic pole of the Earth is
near the Earth’s geographic north pole
Temporary Magnets
 Either end of a magnet will attract either
end of a piece of metal
 If you touch a magnet to a piece of metal
the piece of metal will become a magnet
 If you pull away the magnet, the piece of
metal’s magnetism disappears
Permanent Magnets
 Many permanent magnets are made of
ALNICO V, an iron alloy
 Contains 8% Aluminum, 14% Nickel, and 3%
Cobalt
Magnetic Field Lines
 Imaginary
 Field lines are closed loops that leave the
north pole and enter the south pole
 Magnetic field around a magnetic does
exist
Electromagnetism
 1820- Hans Christian Oersted was
experimenting with electric current in
wires.
 By placing a wire on top of a compass he
demonstrated a connection between
magnetism and electricity
 Magnetic field and electric field are
perpendicular to each other
Magnetic Field near Wire
 When there is an electric current in a coil
of wire, the coil has a field like that of a
permanent magnet
 The current carrying coil has a north and
south pole and is itself a magnet
 Called an electromagnet
 Strength of the field is proportional to the
current in the coil
Solenoids
 An electric current in a single circular
loop of wire forms a magnetic field all
around the loop
 A long coil of wire consisting of many
loops is a solenoid. The field from each
loop in a solenoid adds to the electric
field of the other loops.
Solenoids
 When a wire is looped several times to
form a coil and a current is allowed to
flow through the coil, the field around all
the loops is always in the same direction.
Electromagnets
 The strength of the field of an electromagnet is
proportional to the current in the coil.
 Increasing the number of loops in an
electromagnet increases the strength of the
magnetic field.
 Placing an iron rod inside the coil of an
electromagnet will also increase the strength of
the field
Electrons
 Each electron in an atom acts like a tiny
electromagnet
 The magnetic fields of electrons in a group of
neighboring atoms can combine together
 The direction of the magnetic field between two
magnets is from the N-pole of one magnet to
the S-pole of a second magnet
Force on a Current in a
Magnetic Field
 By placing a wire between magnets you
have an effect on the current. The
strength of some typical magnetic fields
is in the book.
 F = BIL
 F = force, B = magnetic field measured in
teslas (T), I = current (amps), L = length of
the wire that lies in the magnetic field
Uses of a wire in a
magnetic field
 Loudspeakers
 Galvanometers
 A device used to measure small currents
 Electric motors
 Converts electrical energy to kinetic energy
Force on a single charged
particle
 Charged particles do not have to be
confined to a wire, but can move as long
as there is a vacuum.
 F = Bqv
Electromagnetic Induction
 Superconductors and permanent
magnets can cause objects to float
 If a current passes through the coil of
wire around a central rod, it produces a
continually changing magnetic field that
can affect magnetic substances
Producing electric current
 Michael Faraday and Joseph Henry
showed that a changing magnetic filed
could produce electric current
Generating Current
 Either the conductor can move through a
magnetic field or the magnetic field can
move past a conductor
 It is the relative motion between the wire
and the magnetic field that produces the
current
EMF
 The potential difference, or voltage, given
to charges by a battery is called
electromagnetic force (EMF)
 This is not a true force and is measured
in volts
 EMF = BLv
Self- inductance
 A microphone depends on induced EMF
 Self-inductance produces an EMF when
current changes in a single coil
 A transformer has two coils wound
around the same iron core. One coil is
the primary and the other is the
secondary coil
The secondary coil
 The EMF induced in the secondary coil,
called the secondary voltage, is
proportional to the primary voltage
 Secondary voltage = # of turns on secondary coil
 Primary voltage
# of turns on primary coil
Electric Generator
 When a generator is connected in a
closed circuit, the induced EMF produces
an electric current
 The current is greatest when the motion
of the loop is perpendicular to the
magnetic field (horizontal)
Generators / Motors
 Almost identical in construction, but they
convert energy in opposite directions
 A generator converts mechanical energy
to electric energy, while a motor converts
electrical energy to mechanical energy
Alternating-Current
 The power produced by a generator is
the product of the current and the voltage
 Average power: Pac = ½ Pac max
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P = I2R
Effective current: Ieff = 0.707 Imax
Average AC power: Pac = Ieff2R
Effective Voltage: Veff = 0.707 Vmax
Lenz’s Law
 The direction of the induced current is such
that the magnetic field resulting from the
induced current is moved toward the left end of
a coil
 Also applies to motors. When a currentcarrying wire moves in a magnetic field, an
EMF is generated. This is in a direction that
opposes the current and is called the backEMF
Transformer
 Induction between coils is the basis for
the operation of a transformer.
 Transformers are widely used because
they change voltages with little loss of
energy
Electromagnetism
 The source of most radio and TV waves
is accelerating electrons
 It is the electron’s charge that results in
electric fields, and the electron’s motion
that produces magnetic fields
Electric & Magnetic Force
 The force exerted by a magnetic field is
perpendicular to the field and to the
direction of motion of the electrons
 The forces due to the electric and
magnetic field are equal and opposite in
direction
Electric & Magnetic Force
 The forces are balanced only for
electrons that have a specific velocity
 The magnetic force is perpendicular to
the direction of motion of the electrons
causing a centripetal acceleration of the
electrons
Elements and ions
 Thomson showed that atoms of the same
element could have the same chemical
properties but different masses, called
isotopes
 The masses of positive ions can be
measured precisely by using a mass
spectrometer
Electric & Magnetic Fields
 A changing magnetic field produces a
changing electric field and a changing
electric field produces a changing
magnetic field
 Either accelerating charges or changing
magnetic fields produce electric and
magnetic fields that move through space
 These combined fields are called an
electromagnetic wave
Antenna
 A wire called an antenna is used to form
an electromagnetic wave
 EM waves can be generated over a wide
range of frequencies.
Antenna
 An AC generator is one method of
creating the oscillating fields in the
antenna
 To generate waves at higher frequencies
is to use a coil and capacitor connected
in a series circuit
Oscillation
 To increase the oscillation frequency the
size of the coil and capacitor must be
made smaller
 Quartz crystals have a property called
piezoelectricity which also allows them to
generate electromagnetic waves
Reception of EM waves
 A simple wire antenna can detect EM waves,
several wires can be used to increase the
detected EMF
 To select waves of a particular frequency and
reject others, a coil and capacitor circuit is
connected to the antenna
 The combination of antenna, coil & capacitor circuit,
and amplifier is a receiver
X rays
 High-frequency electromagnetic waves
 They are produced when electrons are
accelerated to high speeds by means of
potential differences of 20000+ volts
 When the electrons crash into matter, their
kinetic energies are converted into the very
high-frequency EM waves called X rays