Post on 15-Oct-2020
5.4 Magnetic effects of electric currents
ALL magnets have two polesNORTH seeking pole
SOUTH seeking pole
Breaking a magnet produces two magnets!
N S
N S
NN SS
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
NSNS
magnet
Hard and Soft Magnetism
Soft Magnetism
Pure iron is a soft magnetic material
It is easy to magnetise but loses its magnetism easily
NS
before after
Iron nail
SN
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!)
NS
before after
Steel paper clip
NNS
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
N
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
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.
Centre finger = Conventional Current
First finger = Field direction
Thumb = Motion
Remember, conventional current is opposite the flow of electrons!!
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?
NS
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 B
In other words , F α BIL, or F = kBIL
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.
vq
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
vq
The force on a moving charge in a magnetic field
The current is given by I = q/Δt
vq
The force on a moving charge in a magnetic field
Given that F = BILsinθ F = B(q/Δt)vΔt = qvBsinθ
vq
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.
Bubble tracks
5.4 Magnetic force questions