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Magnetism
Can you read
through last
lessons slide
show?
Magnetism
ALL magnets have two poles
NORTH seeking pole
SOUTH seeking pole
Breaking a magnet produces two
magnets!
N
N
S
N
S
S
N
S
Opposites attract!
Opposite poles attract and like poles repel
Magnetic materials
Magnetic materials
Iron (steel), Cobalt
and Nickel
Magnetic induction
Magnetic induction
When a magnetic material is close to a
magnet, it becomes a magnet itself
magnet
S
N
S
We say it has induced magnetism
N
Hard and Soft Magnetism
Soft Magnetism
Purebefore
iron is a soft magnetic material
after
S
N
S
N
S
NN
Not a
magnet
Iron nail
It is easy to magnetise but loses its
magnetism easily
Hard Magnetism
Steelbefore
is a hard magnetic material
after
S
N
S
S
NN
S
N
Steel paper
clip
It is harder to magnetise, but keeps its
magnetism (it is used to make magnets!)
N
It’s a
magnet!
Magnetic field
Magnets and electric currents produce magnetic
fields around them.
In a magnetic field, another magnet, a magnetic
material or a moving charge will experience a
magnetic force.
www.physchem.co.za
Magnetic field lines
We can represent the magnetic field around
a wire or magnet using field lines.
Magnetic field lines
The arrows show the direction a compass
needle would point at that point in the field.
Magnetic field lines
The arrows show the direction a compass
needle would point at that point in the field.
The closer the field lines
are, the stronger the
magnetic force felt
Note that
magnetic field is
a vector quantity
Earth’s Magnetic Field
Remember the
North of a compass
needle points to the
geographic north
pole (i.e. the
geographic North
pole is a magnetic
south pole!)
Moving charges (currents)
Moving charges (electric currents) also
produce a magnetic field
Conventional current –
electrons flow in the
opposite direction
http://www.sciencebuddies.org
Magnetic field around a straight wire
Stronger field closer to
wire
Magnetic field around a flat circular coil
http://physicsed.buffalostate.edu
Magnetic field around a solenoid
The Motor Effect
When a current is placed in a magnetic field
it will experience a force. This is called the
motor effect.
The Motor Effect
The direction of the force on a current in a
magnetic field is given by Flemming’s left
hand rule.
Thumb = Motion
First finger = Field direction
Centre finger = Conventional Current
Sample question
In this example, which way will the wire be
pushed? (red is north on the magnets)
Another sample question!
An electron approaches a bar magnet as
shown. What is the direction of the force on
the electron?
S
N
D.C.Motor
Commutator ensures that every
half rotaion the current direction
reverses in the coil
D.C.Motor
Defining Magnetic Field B
The size of the force on a wire in a field
depends on the size of the field (B), the
length of wire in the field (L) and the
current in the wire (I)
Defining Magnetic Field B
In other words , F α BIL, or F = kBIL
Defining Magnetic Field B
F = kBIL
We can make k = 1 by defining the Tesla as
the magnetic field when the force on 1 m of
wire carrying a current of 1 A is 1 N.
Force on a current in a field
Thus the force on a length L of wire
carrying a current I in a magnetic field B is
given by F = BILsinθ where θ is the angle
between the current and the magnetic field.
The force on a moving charge in
a magnetic field
Since a current
experiences a force in
a magnetic field, and a
current is just made of
moving charges,
moving charges
themselves must
experience a force in a
magnetic field.
www.nearingzero.net
The force on a moving charge in
a magnetic field
Consider a positive charge q moving with
speed v.
v
q
Magnetic
field B out
of the slide
The force on a moving charge in
a magnetic field
In time Δt the charge will have moved a
distance L = vΔt
v
q
The force on a moving charge in
a magnetic field
The current is given by I = q/Δt
v
q
The force on a moving charge in
a magnetic field
Given that
v
q
F = BILsinθ
F = B(q/Δt)vΔt = Bvqsinθ
The force on a moving charge in
a magnetic field
The fact that this force is always at right angles to the
velocity means that the charge will move in a circle (if the
speed is constant)
q
v
Note; If the
force is
perpendicular
to the motion,
no work is
done.
Homework – Due Monday
Let’s do some
questions!
Page 344
onwards
Questions 2, 6,
7, 9, 13, 15, 20.