Download Magnetism - Physics: 1(AE) 2(B,D)

Magnetism
What are magnets?
• A magnetic field is created around any
moving charged object.
What is charged that is moving
within an atom?
• Electrons (e-)
• The atoms within most
materials have paired up
electrons spinning in
opposite directions so
the magnetic field that is
created by one is
cancelled out by the
other.
• Some materials like
iron, nickel, and
colbalt have a
single electron or
pair of electrons
that spin in the
same direction
creating a magnetic
field or a small
atomic magnet.
The atomic sized magnets line up to create
domains within the material.
The individual domains line up to form a magnet.
A magnet has two ends called poles
(dipoles), where the magnetic force is the
strongest.
• No matter how
many times a
magnet is broken,
each piece always
has a north pole
and a south pole.
• There is no such
thing as a
monopole.
A magnetic field exists around a magnet or
any moving charged object.
Magnetic fields are like electric fields or
gravitational fields in that they allow
magnets to interact without touching.
Imaginary lines that map out the magnetic
field around a magnet are known as
magnetic field lines or magnetic flux lines.
The direction of the magnetic field is
determined by using a compass. The
needle of the compass points in the direction
of the magnetic field.
Rules for drawing magnetic field lines:
1. Magnetic field lines always go from the north pole to the
south pole (outside a magnet).
Bar Magnet
Horseshoe Magnet
Rules for drawing magnetic field lines:
2. Magnetic field lines are closed loops and never cross
or intersect.
Rules for drawing magnetic field lines:
3. Where the magnetic field lines are closer the magnetic
field is stronger.
Draw magnetic flux lines around
the magnets below:
S
S
N
N
S
N
N
S
Magnetism
• Magnetism is
the force of
attraction or
repulsion
between
magnetic poles.
Magnetic Field around the Earth
What we do know…
The north magnetic pole and the geographic North
Pole do not coincide. The magnetic pole is about 1500 km
(930 mi) south of the geographic North Pole and it wanders.
A compass actually indicates the direction of magnetic north,
not true north. Therefore a navigator must need to know the
magnetic declination for a specific area. This is the angular
difference between magnetic and true north.
The details and mechanisms of
how and why the Earth has a
magnetic field are unclear.
They do believe that it may
have something to do with
motions in the liquid outer core.
…Electromagnet
An electromagnet is a type of magnet whose
magnetic field is produced by the flow of electric
current. The magnetic field disappears when the
current ceases.
An electromagnet is most commonly made by coiling wire
around a piece of iron. This electromagnet is called a
solenoid. The shape of the magnetic field is the same as
a bar magnet.
As electrons move through the coil of wire, the magnetic field of one electron
adds to the field of any others moving in the same direction.
Factors affecting the magnetic field of a solenoid:
1. The magnetic field around a solenoid is directly
related to the current through the coil.
Factors affecting the magnetic field of a solenoid:
2. The magnetic field around a solenoid is directly
related to the number of turns or coils around
the solenoid.
Factors affecting the magnetic field of a solenoid:
3. The magnetic field around a solenoid is directly
related to the material around which the wire is
coiled. The soft iron is more permeable to the
magnetic field than the air is.
Electromagnetic Induction
This is the process of generating a potential difference
(voltage) in a conductor (wire) due to the motion
of the conductor in a magnetic field.
Generating a voltage in a wire would mean generating
areas of uneven charge within a wire.
How can this be done?
What can be forced to move within a wire that will create a potential difference?
Moving a wire through a magnetic field will cause a magnetic force
which will move electrons towards one end of the wire
creating a potential difference.
Let’s take a closer look…
Motion of wire
B
- -
- wire
If this wire is moved up or down (perpendicular to the field), a voltage
will be generated and if the wire is part of a complete
circuit then current is induced.
If this wire is moved left or right (parallel to the field),
no current will be generated because no force is generated to
move electrons and create a voltage.
Other ways to induce voltage…
This is how Niagara Falls was used to
produce electricity…
Water flow is used to turn a
turbine through a magnetic field
inducing a potential difference and
producing electrical current.
This is how an electric motor works…
An electric motor utilizes the
property of electromagnetic
induction to convert electricity
into mechanical energy to make
things move. The conductor itself,
a coiled wire, will move to oppose
the magnetic field. Just when it
gets into position the current is
reversed, and the coil spins round
and round and round full of
mechanical energy.
A Generator…
A generator is simply the same
process in reverse, converting
mechanical energy into electricity.
Almost all of the electrical energy
we use in our daily lives is
supplied by electric generators.