Magnets and magnetic materials
Magnets attract objects made of magnetic materials
Magnetic materials include the elements iron, nickel, cobalt, alloys containing some of these such as steel and some of their compounds.
stainless steel
cobalt
nickel
iron
Hard and soft magnetic materials
Permanent magnets are made of magnetically HARD materials such as steel. These materials retain their magnetisation once magnetised.
Magnetically SOFT materials, such as iron, lose their magnetisation easily. They suitable for temporary magnets such as electromagnets.
Magnetic poles
Magnetic poles are the parts of a magnet that exert the greatest force.
Magnetic poles occur in pairs usually called north (N) and south (S)
Iron filing are attracted mostly to the poles of a magnet
Why poles are called north and south
A magnet suspended so that it can rotate freely horizontally will eventually settle down with one pole facing north and the other south.
This is pole is therefore called the ‘north seeking pole’, usually shortened to just ‘north pole’.
The magnet has been orientated by the Earth’s magnetic field.
A compass is an application of this effect.
north
The law of magnets
Like poles repel unlike poles attract
Magnetic fields
A magnetic field is a volume of space where magnetic force is exerted.
All magnets are surrounded by magnetic fields.
The shape of a magnetic field can be shown by iron filings or plotting compasses.
Magnetic field around a bar magnet
magnetic field line
Arrows on the field lines show the direction of the force on a free to move north pole
The stronger the magnetic field the denser the magnetic field lines.
Magnetic fields between two bar magnets
Producing a uniform magnetic field
A uniform magnetic field exerts a constant force over a region.
Such a field will consist of parallel equally spaced magnetic field lines.
This type of field can almost be found between a north and south magnetic pole.
The Earth’s magnetic field
The earth’s magnetic field is similar in shape to that around a bar magnet.
It is thought to be caused by electric currents flowing through the molten outer core of the Earth.
At the present the field pattern is like that with a magnetic SOUTH pole situated somewhere below northern Greenland
Induced magnetism
Magnetism can be induced in a magnetic material if it is placed within a magnetic field.
If the material is magnetically hard it will retain its magnetism once removed from the field.
Certain rocks in the Earth’s crust such as lodestone have been magnetised in this way by the Earth’s magnetic field.
N S
iron bar
Choose appropriate words to fill in the gaps below:
Magnetic materials are either hard or ______. Hard magnetic materials such as ______ retain their magnetisation once magnetised.
A magnetic _____ is a region where the magnetic force is greatest. Magnetic poles always occur in ______. Like poles _______, unlike attract.
A magnetic ______ is a region where magnetic force is exerted. The ________ of the magnetic field around a bar magnet is from north to south.
soft
repel
field
steel
direction
pole
pairs
WORD SELECTION:
soft
repel
field
steel
direction
pole
pairs
Electromagnetism
In 1820 Hans Ørsted noticed that a wire carrying an electric current caused a compass needle to deflect.
No current, compass points to north
Current, compass deflected
Magnetic field patterns around wires�1. Straight wire
The magnetic field consists of concentric circles centred on the wire.
The magnetic field is strongest near the wire.
This is shown by the field lines being closest together near to the wire.
The strength of the field increases if the electric current is increased.
The right-hand grip rule (for fields)
Grip the wire with the RIGHT hand.
The thumb is placed in the direction of the electric current.
The fingers show the direction of the circular magnetic field.
Complete the diagrams below:
Electric current out of the page
Electric current into the page
Add field arrows
Add field arrows
Add current direction
Add current direction
2. Flat circular coil
Plan view
3. Solenoid
A solenoid is a coil of wire carrying an electric current.
The magnetic field is similar in shape to that around a bar magnet.
The strength of the field increases with:
N
S
The right-hand grip rule (for poles)
Grip the coil with the RIGHT hand.
The fingers are placed in the direction that the eclectic current flows around the coil.
The thumb points towards the north pole end of the coil.
N
S
Complete the diagrams below:
1. Locate north
4. Add coils
3. Add current direction
2. Locate south
N
N
S
N
Electromagnets
An electromagnet consists of a current carrying coil wrapped around an iron core.
Uses of electromagnets
1. Scrap yard crane
The iron core of the electromagnet is a SOFT magnetic material.
When current flows the iron becomes strongly magnetised and so picks up the scrap iron and steel.
When the current is turned off the iron loses its magnetisation and so releases the scrap.
Choose appropriate words to fill in the gaps below:
A wire carrying an electric ______ produces a magnetic field. This field increases in ________ if the current is increased.
A ________ is a coil of wire carrying an electric current. The field produced increases in strength if the number of _____ in the coil is increased or if _____ is placed inside the coil.
An ____________ consists of a coil of a solenoid wrapped around an iron core. Iron is a ______ magnetic material that loses its magnetisation once the current in the coil is switched off.
solenoid
strength
iron
soft
turns
current
electromagnet
WORD SELECTION:
TRIPLE ONLY
solenoid
strength
iron
soft
turns
current
electromagnet
Online Simulations
Bar magnet field - Fendt
Faraday Electromagnetic Lab - PhET - Play with a bar magnet and coils to learn about Faraday's law. Move a bar magnet near one or two coils to make a light bulb glow. View the magnetic field lines. A meter shows the direction and magnitude of the current. View the magnetic field lines or use a meter to show the direction and magnitude of the current. You can also play with electromagnets, generators and transformers!
Field around a straight conductor - Fendt
Magnetic field around a straight wire / coil / solenoid - NTNU
Relay demonstration - Freezeway.com
Electric Bell demonstration - Freezeway.com
BBC KS3 Bitesize Revision:
Using Electromagnets - includes an applet showing how an electric bell works