Download Magnets and Magnetic Fields

Magnets and Magnetic Fields
Magnets
• Magnets can be used along with circuits to create an
alarm
– A magnet can pull a switch closed to complete the
circuit, but when the magnet is moved out of place,
then it doesn’t pull the switch closed and instead
the switch is open and thus the circuit is now
interrupted and an alarm goes off.
How Magnets work for Alarms
• Magnets get their name from a stone found 3000
years ago in Magnesia, which is now modern day
Greece. This stone is called Iodestone and is
composed of magnetite.
• Some material can be made into permanent
magnets
– You can change any piece of iron, such as a nail,
into a permanent magnet by stroking it several
times with a permanent magnet.
– A slower method is to place the piece of iron near
a strong magnet and will eventually become
magnetic because of the magnetic field and will
remain magnetic even after the strong magnet is
removed
• Even though a material can be considered
permanent, its magnetism can be weakened
or even removed.
– Possible ways to do this is to heat or hammer the
material.
• Magnetically Hard: materials that are difficult
to magnetize but keep their magnetism (like
Nickel or Cobalt)
• Magnetically Soft: materials that are easily
magnetized, however they lose their
magnetism easily as well (like Iron)
Magnetic Force
• A magnetic force can be created
that allows the magnetism to
be carried through material.
– Like when a magnet picks up a
nail, the magnetic force carries
through the nail and attract yet
another nail.
– Nail attached to nail through
that magnetic field, despite not
actually touching the magnet.
– There is a limit to this
• Magnets have poles
– North pole and south pole
– The pole of a magnet exert a force on one another
• Like poles repel and opposite poles attract.
• The north pole of one magnet will attract the
south pole of a different magnet
• The north pole of one magnet will repel the north
pole of a different magnet.
• Poles are always present and cannot be
separated. If you cut a magnet in half, two
opposite poles still exist.
• Magnetic Fields always travel from north
pole towards the south pole
• Magnetic forces are dependant on distance
just like electrical force and gravitational force
• Magnetic force is a filed force and is
dependant on the strength of the magnetic
field.
• A magnetic field gets weaker the farther away
you get.
• Magnetic fields are strongest near its poles
• A compass can track magnetic fields
• Compasses: a compass is a magnet suspended
on top of a pivot so that the magnet can
rotate freely.
– A compass aligns with Earth’s magnetic field just
as iron filing align with the field of a bar magnet.
• The Earth: is like a big bar magnet
– The polarity of the Earth has reversed around 20
times in the last 5 million years.
– So the magnetic field has changed directions
– The Earths magnetic poles are not the same as its
geographical poles
• For historical reasons, the poles of magnets are
named for the graphical pole that they point
towards.
• The end of a magnet that is labeled North is a
north seeking pole, and not actually north
• The end of a magnet that is labeled South is a
south seeking pole, and not actually south
Magnetism from electric currents
• Magnetism is produced by moving electrical
charges.
• Electrical currents produce
magnetic fields
– You can use magnetic material
to show the magnetic field
produced from a current
carrying wire.
• Right hand rules: determines the direction of
the magnetic field.
• Magnetic fields exert forces, but the force is
weak.
• One way to increase the force is to increase
the current in the wire, but large currents can
be fire hazards.
• A safer way to create a strong magnetic field
that will provide a strong force is to wrap the
wire into a coil. This creates a solenoid.
– The coil of a solenoid allows the magnetic field of
one loop to intensify those next to it.
– The coil has a north and south pole, just like a
magnet
• Solenoid: the strength of the magnetic field
depends on the number of loops and the
amount of the current
• Electromagnet: A rod made of iron can be
inserted into the coil to intensify the magnetic
field
Moving charges cause magnetism
• Negatively charged electrons moving around
the nucleus of an atom causes a magnetic
field
• This is because electrons are spinning,
producing tiny magnetic fields around each.
• These magnetic field are balanced by the
protons for most atoms, except those that are
magnetic (like iron..steel)
Electromagnetic Devices
• Modern day devices that use the magnetic
field produced by coils of current-carrying
wire.
– Devices like hair dryers, electric motors, and
stereo speakers
– Electric motors: coil of a wire that spins when a
current is running. This spinning coil is attached
to a shaft and does work (wheel and axel). This
transfers electrical energy into mechanical energy
to make a devices like toys run.
• Magnetic fields can even cause sound wavesas in stereo speakers
– The magnetic field of the coil interacts with the
magnetic field of the permanent magnet and
causes the speaker cone to vibrate. (essentially it
is being pulled in one direction by the coil and
pulled in the other direction by the magnetalternately)
– These vibrations produce sound waves
– In this way magnetic field is converted into sound
waves
Electromagnetic Induction
• Faraday’s Law: an electric current can be
produced in a circuit by a changing magnetic field
• This is an induced current – creating a current in a
circuit by changing magnetic field
• Someone does work to push an electrically
charged loop of wire in between two oppositely
charged magnetic. This loop gets caught in the
magnetic field and is pushed back out. So
mechanical energy is put in but electrical energy
is produced.
• Generator convert mechanical energy to
electrical energy
– AC generator- Alternating current : this is the
current used in your house and the current
changed direction at regular intervals
Electromagnetic energy
• Moving charge produces a magnetic field
• Changing magnetic field causes an electric charge
to move
• The energy that comes from these processes is
called electromagnetic energy, like that of UV
light and X-rays
• They move through space without the need for
matter because the changing electric fields
generate a magnetic field and the changing
magnetic fields generate an electric field.
• Each field generates the other, allowing EM
waves to travel through empty space