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MAGNETISM
Section 1: Magnets and Magnetic Fields
Section 2: Magnetism from Electric Currents
Section 3: Electric Currents from Magnetism
Magnets and Magnetic Fields
Key Terms
Magnetic Poles
Magnetic Fields
Magnets and Magnetic Fields
Magnets
The name magnet comes from the region of
Magnesia which now modern day Greece
The first magnetic rocks called Lodestones were
found in this region almost 3000 yrs ago.
A lodestone is composed of an iron-based material
called magnetite
Magnets and Magnetic Fields
Some materials can be made into permanent
magnets
Some materials like lodestones are always
magnetic. These materials are called
permanent magnets.
However, some materials like iron can become
permanent magnets
Magnets and Magnetic Fields
By rubbing a permanent magnet against a piece of
iron, the iron will become permanent magnetic.
A slower process in producing a permanent magnet
from a piece of iron would be placing the iron
near a strong magnet.
Creating a permanent magnet from a piece of iron
does not mean the iron will always be magnetic
Magnets and Magnetic Fields
The magnetism of the iron can be weakened or
completely removed. This can be accomplished
by heating or hammering the piece of iron.
Magnetically Soft – a material that is easily
magnetized but also loses its magnetism easily.
(Iron)
Magnetically Hard – a material that is hard to
magnetize and does not lose their magnetism
easily. ( Cobalt and Nickel)
Magnets and Magnetic Fields
Magnets exert magnetic forces on each other
Like poles repel, and opposite poles attract
Magnets have a pair of poles, the north pole and
south pole.
It is impossible to isolate a magnetic pole. If you cut
a magnet in half it will still have a north and south
pole. NO MATTER HOW SMALL
Magnets and Magnetic Fields
Magnetic Fields
If two like poles are brought close together and
one magnet is free to move, the closer you
bring the poles the free moving magnet will
start to move away.
Magnets are sources of magnetic fields
Magnets and Magnetic Fields
Magnetic force is a field force. When magnets
repel or attract each other, it is due to the
interaction of their magnetic fields.
The strength of a magnetic field depends on
what the magnet is made of and the degree to
which it has been magnetized
Magnetic field lines are used to show magnetic
fields
Magnets and Magnetic Fields
The magnetic field gets weaker with distance.
The further apart magnetic field lines are
represents a weak field. The closer together,
the stronger the field
Magnetic fields are strongest at the poles
Compasses can track magnetic fields. A compass
is a magnet suspended on top of a pivot so
that the magnet can rotate freely
Magnets and Magnetic Fields
A compass aligns with Earth’s magnetic field.
The first compasses were made from
lodestones.
Earth’s magnetic field is like that of a bar magnet
Magnets and Magnetic Fields
Earth’s magnetic poles are not the same as its
geographic poles
The magnetic pole in Antarctica is actually a
magnetic N pole, and the magnetic pole in
northern Canada is actually a magnetic S pole.
Magnetism from Electric Currents
Key Terms
Solenoid
Electromagnet
Galvanometer
Electric Motor
Magnetism from Electric Currents
Magnetism from Electric Currents
In 1820 a Danish science teacher named Hans Christian
Oersted first experimented with the effects of an
electric current on the needle of a compass
Electric currents produce magnetic fields
Magnetism from Electric Currents
Use the right-hand rule to finde the direction of the
magnetic field produced by a current
If you imagine holding the wire in your right hand with your
thumb
pointing in the direction of the positive current, the
direction your fingers
would curl is in the direction of the magnetic field.
The magnetic field of a coil of wire resembles that of a
bar magnet
Magnetism from Electric Currents
By wrapping a wire in to a coil you can increase
the magnetic field without increasing a fire
hazard.
A coil of wire with an electric current is called a
solenoid
A solenoid will have a north and a south pole
just like a magnet
Magnetism from Electric Currents
The strength of the magnetic field in a solenoid
depends on the number of coils or the
amount of current in the wire. By increasing
the number of coils or the amount of current
you can increase the strength of the magnet.
Another way to increase the magnetic field of a
solenoid is to place a magnetic material in the
center of the coil
Magnetism from Electric Currents
By placing a magnetic material in the center of a
solenoid, an electromagnet is created.
Magnetism can be caused by moving charges
Negatively charged electrons moving around the
nuclei of all atoms make magnetic fields.
Nuclei also have magnetism because of proton
Magnetism from Electric Currents
Each electron with in an atom has a property
called electron spin, which also produce
magnetic fields
When a potentially magnetic substance is not
magnetized, its domains are randomly
oriented
When the domains become more uniform the
substance becomes magnetized
Magnetism from Electric Currents
Electromagnetic Devices
Galvanometer detect current
Galvanometers are devices used to measure current
in ammeters and voltage in voltmeters.
A galvanometer consists of a coil of insulated wire
wrapped around an iron core that can rotate
between the poles of a permanent magnet
Magnetism from Electric Currents
Electric motors convert electrical energy to
mechanical energy
A device called a commutator is used to make
the current change direction every time the
flat coil makes a half revolution.
Devices called brushes connect the wires to the
commutator.
Magnetism from Electric Currents
Stereo speakers use magnetic force to produce
sound
In a speaker, when the direction of the current
in the coil of wire changes, the paper cone
attached to the coil moves, producing sound
waves.
Electric Currents from Magnetism
Key Terms
Electromagnetic Induction
Generator
Alternating Current
Transformer
Electric Currents from Magnetism
In 1831, Michael Faraday discovered that a
current can be produced by pushing a magnet
through a coil or wire
Electromagnetic induction is the process of
creating a current in a circuit by changing a
magnetic field
Electric Currents from Magnetism
Electromagnetic induction and Faraday’s Law
Faraday’s Law states that an electric current can
be produced in a circuit by a changing
magnetic field.
It may seem that electromagnetic induction is
created from nothing, but it does not violate
the law of conservation of energy.
Electric Currents from Magnetism
An outside source is needed to move a loop
through the magnetic field.
Moving electric charges experience a magnetic
force when in a magnetic field
The force is at its maximum value when the
charge moves perpendicular to the magnetic
field.
Electric Currents from Magnetism
When the wire in a circuit moves perpendicular
to a magnetic field, the current induced in the
wire is at a maximum
When the wire moves parallel to a magnetic
field, there is zero current induced in the wire
Generators convert mechanical energy to
electrical energy
Electric Currents from Magnetism
Alternating currents are electric currents that
change direction at regular intervals (AC)
In an alternating current generator, the
mechanical energy of the loop’s rotation is
converted to electrical energy when a current
is induced in the wire.
Table 1 pg 579
Electric Currents from Magnetism
Generators produce the electrical energy you
use in your home
The mechanical energy used in a commercial
power plant comes from a variety of sources.
Dams
Coal
Nuclear Fission
Hot Water
Solar Power
Wind
Electric Currents from Magnetism
Electricity and magnetism are two aspects of a
single electromagnetic force
Light is a from of electromagnetic energy.
Visible light travels as electromagnetic waves or
EM waves, as to other forms of radiation
(X rays and radio signals) called EMF
electromagnetic frequency waves
Electric Currents from Magnetism
EM waves are made up of oscillating electric and
magnetic fields that are perpendicular to each
other
EM waves are transverse waves
Electric Currents from Magnetism
Transformers
Transformers are devices that increase or
decrease the voltage of alternating current
Two wires are coiled around opposite sides of a
closed iron loop. One wire is attached to a
source of alternating current, such as a power
outlet. The other wire is attached to an
appliance.
Electric Currents from Magnetism
Transformers can increase or decrease voltage
The voltage induced in the secondary coil of a
transformer depends on the number of loops,
or turns in the coil
When the primary and secondary circuits in a
transformer each have one turn, the voltage
across each is about equal.
Electric Currents from Magnetism
When an additional secondary circuit is added,
the voltage across each is again about equal
When the two secondary circuits are combined,
the secondary circuit has about twice the
voltage of the primary circuit. Actual
transformers may have thousands of turns.
Electric Currents from Magnetism
When the secondary coil produces a higher
voltage than the primary coil this is a step-up
transformer.
If the voltage on the primary coil is greater than
the voltage on the secondary coil this is a
step-down transformer. (Transformers of
power lines)
Electric Currents from Magnetism
A step-up transformer is used at or near the
power plant to increase the voltage of the
current to about 120,000 V.
A step-down transformer is use near homes to
reduce the voltage to around 120 V.