Transcript magnetCh. 8 Magnetism

Attraction and Repulsion
Magnetism and Its Uses
Magnetism
• Discovered over 2000 years ago in Magnesia, Turkey
• Magnetic Force
– You can feel the magnetic ability before magnets touch
• Magnetic Field
– Surrounds the magnet
– North and South pole is strongest point
• Round magnet has poles on the top and bottom
– Field has direction north to south
• Unlike poles attract
– Produce new magnetic field
• Like poles that are the same
– Repel
• Magnetic Field Direction
– Goes from north to south
Earth’s Magnetic Field
South Seeking Pole North Geographic Pole
North Seeking Pole South Geographic Pole
Reason for earth’s
magnetism is unknown
Magnetic Materials
• Iron, cobalt and nickel are naturally magnetic
– Electron’s magnetic properties not cancelled in these
metals
– Each atom magnetic
– Not always magnetic
– Groups of atoms must be aligned
• Called Magnetic Domain
Magnetism
8.1
Lining Up Domains
•
Even though each domain behaves like a magnet, the poles of the domains are
arranged randomly and point in different directions.
•
As a result the magnetic fields from
all the domains cancel each other
out.
Magnetism
8.1
Lining Up Domains
•
If you place a magnet against the same nail, the atoms in the domains orient
themselves in the direction of the nearby magnetic field.
•
The like poles of the domains point in
the same direction and no longer cancel
each other out.
• Domain alignment can be temporary or
permanent
– Magnet held next to metal will align domain
– Vibrations/Heat will knock domain out of alignment
• Permanent magnets have strong magnetic
domain not influenced by atomic motion
Electricity and Magnetism
Electricity and Magnetism
• Electric currents creates a magnetic field
• Magnetic field reverses when the current
reverses
• Electromagnet
–
–
–
–
Temporary
Solenoid-cylindrical wire coil
Strength determined by coil
Speakers and motors (Electric to Mechanical)
Electricity and Magnetism
8.2
Moving Charges and Magnetic
Fields
•
It is now known that moving charges, like those in an electric current, produce
magnetic fields.
•
Around a current-carrying
wire the magnetic field
lines form circles.
Increasing Coils
Increasing Current Larger Magnetic Field
Faraday’s Law
• Flux in magnetic force=flux in electric voltage
Electricity and Magnetism
8.2
Using Electromagnets to Make
Sound
• The
electromagnet
changes
electrical energy
to mechanical
energy that
vibrates the
speaker cone to
produce sound.
Electricity and Magnetism
8.2
Using Galvanometers
• In a galvanometer, the electromagnet is
connected to a small spring.
• Then the
electromagnet
rotates until the
force exerted by
the spring is
balanced by the
magnetic forces on
the electromagnet.
Electricity and Magnetism
8.2
A Simple Electric Motor
• A simple electric motor also includes
components called brushes and a commutator.
• The brushes are conducting pads connected
to the battery.
• The brushes make contact with the
commutator, which is a conducting metal ring
that is split.
• The brushes and the commutator form a
closed electric circuit between the battery
and the coil.
Electricity and Magnetism
8.2
Making the Motor Spin
• Step 1. When a current flows in the coil, the
magnetic forces between the permanent
magnet and the coil cause the coil to rotate.
Electricity and Magnetism
8.2
Making the Motor Spin
• Step 2. In this position, the brushes are not in
contact with the commutator and no current
flows in the coil.
• The inertia of
the coil keeps it
rotating.
Electricity and Magnetism
8.2
Making the Motor Spin
• Step 3. The commutator reverses the
direction of the current in the coil.
• This flips the
north and south
poles of the
magnetic field
around the coil.
Electricity and Magnetism
8.2
Making the Motor Spin
• Step 4. The coil rotates until its poles are
opposite the poles of the permanent magnet.
• The
commutator
reverses the
current, and
the coil keeps
rotating.
• Superconductors-mercury, lead and tin
• Conductors-copper and aluminum
• Semiconductors-silicon
• Credit Card Magnetic Strip
Producing Electricity
• From Mechanical to Electrical
• Electromagnetic induction
• Generator
– Coil rotates between the poles of a permanent
magnet
– Rotations change the direction of the current
– Rotate 60 times a second-How many times per
minute?
Types of Electrical Currents
• Direct Current
– Battery
– Current in one direction
• Alternating current
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Household current
Current changes direction 2x per second
Produce at 60 Hz per second
How many times does direction change per minute?
• Transformers
– 150,000 volts generated by power plants
– Too much for house
– Voltage needs to be lowered