magnetism can you read through last lessons slide show?
TRANSCRIPT
Magnetic induction
When a magnetic material is close to a magnet, it becomes a magnet itself
We say it has induced magnetism
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NSmagnet
Soft Magnetism
Pure iron is a soft magnetic material
It is easy to magnetise but loses its magnetism easily
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before after
Iron nail
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NS
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!)
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before after
Steel paper clip
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It’s a magnet!
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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
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
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S
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
http://www.sciencebuddies.org
Conventional current – electrons flow in the opposite direction
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.
Centre finger = Conventional Current
First finger = Field direction
Thumb = Motion
Another sample question!
An electron approaches a bar magnet as shown. What is the direction of the force on the electron?
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Defining Magnetic Field BThe 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 BF = 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 fieldThus 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
Given that F = BILsinθ
F = B(q/Δt)vΔt = Bvqsinθ
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)
vq
Note; If the force is
perpendicular to the motion,
no work is done.