Download magnetism - WordPress.com

MAGNETISM
Magnets and Electromagnets
MAGNETS
• A special stone first discovered more than 2000 years ago in Greece, in a
region called Magnesia; it attracted iron; they called it magnetite hence the
magnet name.
• About 1000 years ago they noticed that a hanging magnet always pointed to
the North Star or Lodestar. So another name for naturally occurring magnets
is lodestone.
MAGNETISM
• Magnetism is the force of attraction or repulsion due to electron
motion.
MAGNETISM
• Magnetic Poles – the ends of the magnet, area where the magnetic effect is
the strongest.
• If a bar magnet is suspended by a thread or string, it will align itself so that
one strong end points north and the other points south, hence the names for
the “North” and “South” poles of the magnet.
MAGNETISM
• Like poles of separate magnets repel
– push away from – each other
• Unlike poles attract each other.
MAGNETISM
• The moving electron acts as a mini electrical charge and therefore
has a magnetic field associated with it.
• Groups of atoms with magnetic poles aligned are called magnetic
domains.
• Materials with most of the domains lined up are considered
magnetized.
• The more domains that are aligned, the stronger the magnetic field
in the material.
MAGNETISM
MAGNETISM
• When domains are randomly
arranged, their forces cancel each
other out and there is no net
magnetic effect.
• When domains have their magnetic
effect in alignment, their forces are
additive and create a strong
magnetic effect.
MAGNETS
• If you snap a magnet in half, the inside pieces become the opposite
poles:
DRAW THE DOMAINS
MAGNETISM AND ELECTRICITY
• A connection between electricity and magnetism was discovered
(accidentally) over 100 years ago.
• Hans Christian Oersted discovered in 1820 that an electric current near
a compass causes the compass needle to be deflected.
• Oersted's experiment showed that every electric current has a
magnetic field surrounding it.
• A magnetic field causes a magnet to align in the direction of the
opposite pole.
Hans Christian
Oersted
(1777-1851)
MAGNETISM AND ELECTRICITY
• See how a magnetic field compares to an electric field.
ELECTRIC FIELD
MAGNETIC
FIELD
MAGNETIC FIELD
• Field lines were introduced by Michael Faraday , who named them “lines of
force.’’ . All magnetic fields originate from moving electric charges.
• A magnetic field is the magnetic effect of electric currents and magnetic
materials. A magnetic field appears only when relative motion is present
between an electric charge and an observer. Electric and magnetic fields are
different aspects of a single electromagnetic field.
MAGNETIC FIELD
• A magnetic field is that region around a
magnet that is affected by the magnet.
• Strongest at the poles, the Force forms
lines that go out of the North Pole and
wrap back around to enter in at the
South Pole.
THE GIANT MAGNET
• The Earth’s core is Iron so Earth is a giant magnet.
DRAW THE MAGNETIC FIELD LINES
MAGNETIC FIELDS LAB
INTRODUCTION: A magnetic material is made of small regions
called magnetic domains. These magnetic domains can be pictured as
small bar magnets. When the domains are aligned, as shown in Figure 1,
the magnetic fields of the domain add together. This causes the material
to be surrounded by a magnetic field. The magnetic field surrounding a
magnet exerts a magnetic force on other magnets and magnetic
materials. The direction of the magnetic field around a magnet can be
represented by magnetic field lines. Magnetic field lines always begin
on the north pole of a magnet and end on the south pole. Magnetic field
lines are closer together where the magnetic field is stronger, and farther
apart where the field is weaker.
ELECTROMAGNETISM
• An electromagnet consists of an iron
core placed inside a wire coil.
• The magnetic field strength of a wire coil
carrying an electric current increases in
direct proportion to the number of turns
of the coil.
• Current gives rise to magnetic fields,
just as electric charge gave rise to
electric fields.
ELECTROMAGNETISM
• The basic idea behind an
electromagnet is extremely simple: By
running electric current through a
wire, you can create a magnetic field.
• Electricity through the coiled wire
creates a magnetic field which induces
magnetism in the iron core. The
domains in the core become aligned
with the magnetic field of the coil.
ELECTROMAGNETISM
ELECTROMAGNETISM
• An electromagnet can be used to
move large quantities of metal.
• When the current is on, the
magnet will pick up the metal.
• When you want to drop it, you
turn off the power and the
electromagnet is disabled and the
metal drops.
ANIMATION
ELECTROMAGNETISM
• A rotating electromagnet is used in an electric motor to convert
electrical energy to mechanical energy.
ELECTRIC MOTOR
• Electric motors convert electrical energy to mechanical energy.
• An electromagnet turns inside of a permanent magnet.
Electromagnet
ELECTRIC MOTOR
• By changing the direction of the current, the poles will keep switching
between N and S, and therefore cause the electromagnet to continually
rotate.
• Electric motors are used in most household appliances which convert
electricity into motion.
ELECTROMAGNETIC INDUCTION
• When a magnet is moved near a wire, an electric current can be
created.
• This process is called electromagnetic induction.
• The direction of the induced current can be reversed by reversing the
motion of the wire or reversing the field direction.
• The strength of the current depends on the strength of the magnetic
field and the speed of the wire's motion.
• This is the basis for electric generators.
ELECTRIC GENERATOR
• Electromagnetic induction is used in the operation of a generator, a
device that converts mechanical energy to electrical energy.
• In a generator, an electric current is produced when a coil of wire is
mechanically rotated in a magnetic field.
ELECTRIC GENERATOR VS
ELECTRIC MOTOR
Electric Generator
Electric Motor
Animation
mechanical to electrical
electrical to mechanical.
USES OF ELECTROMAGNETISM
• The use of magnetic waves
to make pictures of the
inside of the body.
• Using a large magnet, radio
waves, and a computer, a
Magnetic Resonance
Image, or MRI, produces
two-dimensional and threedimensional pictures.
USES OF ELECTROMAGNETISM
• A transformer is a device composed of two unconnected coils, usually
wrapped around a soft iron core, that can increase or decrease the
voltage of AC current.
USES OF
ELECTROMAGNETISM
• A transformer is used to step the voltage
down and the current up (P=IV) so that
we can use it.
• Low power is desired for the transport of
electricity over long distances to avoid
loss of energy to heat loss.
USES OF ELECTROMAGNETISM
• A moving coil activated by voice
vibrations is used as a microphone.
• The coil induces a current in the magnet
that can be amplified or recorded.
USES OF ELECTROMAGNETISM
• The experimental Japanese Maglev train uses
magnetic forces for both support and
propulsion.
• It is a magnetic levitation (maglev) railway
system based on the principle of magnetic
repulsion between the track and the cars.
• In April 2015, a manned seven-car Maglev
train reached a speed of 603 km/h (375 mph).
USES OF ELECTROMAGNETISM
• An Aurora is an electric discharge
through the atmosphere connected with
magnetic disturbance.
• Most aurora are green and red emission
from atomic oxygen.
• Molecular nitrogen and nitrogen ions
produce some low level red and very
high blue/violet aurora.
• Northern Hemisphere: aurora borealis
• Southern Hemisphere: aurora australis
EOC PRACTICE
1. Which of these correctly identifies both devices
illustrated?
a. Picture A: electric generator; Picture
B:electromagnet
b. Picture A: electromagnet; Picture B: electric
motor
c. Picture A: electric motor; Picture B:
electromagnet
d. Picture A: electric generator; Picture B: electric
motor
EOC PRACTICE
2. A magnetic field can be found in which of the following?
a.
b.
c.
d.
electric motor
sound wave
plastic cord
glass rod