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Physics 10 Grade Magnetic field presentation.ppt
Magnetic field. Interaction between the conductor and current, Amper's experiments. Magnetic induction vector.
Lesson objectives
to explain the physical meaning of magnetic induction vector based on problem solving and modern technological advances (magnetic levitation train, etc.)
ALL magnets have two poles
NORTH seeking pole
SOUTH seeking pole
Breaking a magnet produces two magnets!
N
S
N
S
N
N
S
S
Opposites attract!
Opposite poles attract and like poles repel
(Ferro) Magnetic materials
(Ferro) 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
We say it has induced magnetism
N
S
magnet
Hard and Soft Magnetism
Soft Magnetism
Pure iron is a soft magnetic material
It is easy to magnetise but loses its magnetism easily
N
S
before
after
Iron nail
S
N
Not a magnet
N
Hard Magnetism
Steel is a hard magnetic material
It is harder to magnetise, but keeps its magnetism (it is used to make magnets!)
N
S
before
after
Steel paper clip
N
It’s a magnet!
N
S
S
N
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 closer the field lines are, the stronger the magnetic force felt
The arrows show the direction a compass needle would point at that point in the field.
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!)
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 F = BILsinθ
F = B(q/Δt)vΔt = qvBsinθ
v
q
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)
v
q
Note; If the force is perpendicular to the motion, no work is done.
Bubble tracks
5.4 Magnetic force questions
NEED FINISHING!
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