Download Magnetic Field

Magnetism
Magnets have 2 poles
(north and south)
Unlike poles attract
Like poles repel
Magnets create a
MAGNETIC FIELD
around them
Magnetic Field
A bar magnet has a magnetic field
around it. This field is 3D in nature
and often represented by lines
LEAVING north and ENTERING south
To define a magnetic field you need to
understand the MAGNITUDE and
DIRECTION
We sometimes call the magnetic field
a B-Field as the letter “B” is the
SYMBOL for a magnetic field with the
TESLA (T) as the unit.
Magnetic Dipoles
Electric Dipole & ε field
Electric dipole consists of two equal but
opposite charges separated by some
distance such as in a polar molecule.
Every magnet is a magnetic dipole. A bar
magnet is a simple example.
Note how the ε field due an electric dipole
is just like the magnetic field (B field) of a
bar magnet.
Field lines emanate from the + or N pole
and reenter the - or S pole.
Although they look the same, they are
different kinds of fields.
ε fields affect any charge in the vicinity
B field only affects moving charges.
Magnetic Dipole & B field
Magnetic Monopoles?
Unlike electricity, there are no magnetic monopoles. If a bar
magnet is split in two, it will create a new N&S pole at the split.
Magnetic Dipole
Electric monopole & ε field
Lodestones were the first magnets discovered(?) by the Greeks.
Lodestones consisted of iron oxide and was able to exert forces of
attraction on small iron objects.
Magnets are now mainly made of iron, nickel, colbalt, and
gadolinium which are part of the Ferromagnetic materials.
Ferromagnetism actually means the ability for certain materials to
be magnetic or magnetized.
Four of the most common permanent magnets are:
Neodymium (NdFeB:neodymium-iron-boron)
Samarium-Cobalt (SmCo)
Alnicos (aluminum-nickel-cobalt)
Ceramic/Ferrite
unmagnetized state
If a ferromagnetic object is placed in a magnetic field, some of the
domains rotate to align themselves with the external magnetic
field.
Earth’s Magnetic Field
Force Due to Magnetic Field
The force exerted on a charged particle by a magnetic field is given by
the vector cross product:
F = force (N)
q = charge on the particle (C)
v = velocity of the particle relative to field (m/s)
B = magnetic field strength measured in Teslas (T)
We'll get to Telsas soon
What is a Vector Cross Product?
Let v1 = x1 i+y1 j+z1 k and v2 = x2 i+y2 j+z2 k.
By definition, the cross product of these vectors is given by the
following determinant.
= (y1 z2 - y2 z1) i + (z1 x2 - x1 z2) j + (x1 y2 - x2 y1) k
The cross product of two vectors is a vector itself that is to each of
the original vectors. i, j, and k are the unit vectors pointing, along the
positive x, y, and z axes, respectively.
Right Hand Rule
For vector cross products
A quick way to determine the direction of a vector cross
product is to use the right hand rule. To find vector a b,
place the knife edge of your right hand (pinky side) along
vector a and curl your hand toward vector b, making a fist.
Your thumb then points in the direction of vector a b
It can be proven that the magnitude of vector a
is given by:
|a
b
b| = ab·sinθ
Where θ is the angle between vector a and vector b.
θ
Force Due to Magnetic Field
You will use
Units: 1 N
= 1C(m/s)·(T)
A magnetic field of one tesla is very powerful magnetic field.
Sometimes it may be convenient to use the gauss, which is equal to
1/10,000 of a tesla. Earth’s magnetic field, at the surface, varies but
has the strength of about one gauss.
Magnetic Force on a moving charge
If a MOVING CHARGE moves
into a magnetic field it will
experience a MAGNETIC
FORCE. This deflection is 3D in
nature.
The conditions for the force are:
Must have a magnetic field present
Charge must be moving
Charge must be positive or negative
Charge must be moving PERPENDICULAR to the field.
Magnetic Force on a moving charge
perpendicular vectors
This force is always perpendicular to
both the magnetic field and velocity
Direction of the magnetic force?
Right Hand Rule #1 (RHR#1)
RHR#1 determines the directions of
magnetic force, velocity of the charge,
and the magnetic field. Given any two
of theses, the third can be found.
Using your right-hand:
point your index finger in the direction
of the charge's velocity, v; point your
middle finger in the direction of the
magnetic field, B.
Your thumb now points in the direction
of the magnetic force, Fmagnetic.
Use your left-hand if you have an electron
Determine the direction of the unknown variable for a proton moving in the
field using the coordinate axis given
B = -x
v = +y
F=
Determine the direction of the unknown variable for a proton moving in the
field using the coordinate axis given
B = +z
v = +x
F=
Determine the direction of the unknown variable for a proton moving in the
field using the coordinate axis given
B = -z
v = +y
F=
Determine the direction of the unknown variable for an electron moving in the
field using the coordinate axis given
B = +x
v = +y
F=
Determine the direction of the unknown variable for an electron moving in the
field using the coordinate axis given
B=
v = -x
F = +y
Determine the direction of the unknown variable for an electron moving in the
field using the coordinate axis given
B = +z
v=
F = +y
Electromagnetism
While demonstrating to students that
the current passing through a wire
produces heat, Danish professor
Hans Christian Ørsted (1777–1851)
noticed that the needle of a nearby
compass deflected each time the
circuit was switched on.
Right hand rule #2
If your right thumb is pointing in the direction of conventional current, and you curl your fingers forward, your curled fingers point in the direction of the magnetic field lines.
Magnetic Field of a Loop
If you make a circular loop from a
straight wire and run a current through
the wire, the magnetic field will circle
around each segment of the loop.
The field lines inside the loop create a
stronger magnetic field than those on
the outside because they are closer
together.
Right-Hand Rule #2 for a Solenoid
If you coil the fingers of your right hand around a solenoid in the direction of the conventional current, your thumb points in the direction of the magnetic field lines in the centre of the coil.
Applying a current through a solenoid as
described above causes the solenoid to
become an electromagnet. Stronger
electromagnets can be made using a
solenoid with a magnetic material, such
as iron, nickel, or cobalt, within the coil.
Junk yard Scrap metal
Speakers
Door bell
Do the questions on page 80, #1-9