Simple phenomena of magnetism

Describe the forces between magnets, and between magnets and magnetic materials
Magnetic poles are the parts of a magnet that exert the greatest force.
Magnetic poles occur in pairs usually called North (N) pole and South (S) pole.
A magnet attracts magnetic materials such as iron, steel, cobalt and nickel.

Distinguish between magnetic and non-magnetic materials
Magnetic and non-magnetic materials
Magnetic materials can be attracted by the magnet. Iron, cobalt, nickel, steel are magnetic materials.
Strongly magnetic metals are called ferromagnetic materials.
Non magnetic materials such as cork, glass, plastic, rubber, wood can not be attracted by the magnet.

Describe methods of magnetisation, to include stroking with a magnet, use of d.c.in a coil and hammering in a magnetic field.
Describe methods of demagnetisation, to include hammering, heating and use of a.c. in a coil

Magnetisation is to give magnetic property by making all the magnetic domains pointing in the same direction.

Methods of magnetising a magnet;
1. Stroking a steel bar with a permanent magnet
2. Placing a steel bar in a solenoid carrying a large direct current

Demagnetisation is the process of removing magnetism from a magnet. Heating and hammering cause the atoms of the magnet to vibrate vigorously, mixing up the directions of the magnetic domains.
The magnetic domains point in random directions. The magnetic effects of the atomic magnets cancel out so there is no resultant magnetic effect.

Methods of demagnetising a magnet;
1. Heating
2. Hammering
3. Withdraw the magnet that is placed in a solenoid with an alternating current.

Draw the pattern of magnetic field lines around a bar magnet
Describe an experiment to identify the pattern of magnetic field lines, including the direction
1. The more densely packed the field lines, the stronger the field at a point.
2. Magnetic field lines never intersect.
3. The field lines point in the same direction as a compass (from N toward S).
4. Magnetic field lines have no ends they continue through the interior of the magnet.
magnetic field lines.jpg
Magnetic field line by a bar magnet.jpg
Image from cyberphysics
The shape of a magnetic field can be shown by iron filings or plotting compasses.
Magnetic field lines shown by compasses.jpgField lines around a bar magnet.jpg
Image from BBC bitesize
Image from PMT wikispaces

Supplement
• Explain that magnetic forces are due to interactions between magnetic fields
A magnetic field is a volume of space where magnetic force is exerted.

Unlike poles attract , and like poles repel each other.
Like poles repel.jpg
Images from gcsescience.com
Unlike poles attract.jpg
Notice that the lines of magnetic force between
the north and south poles are parallel.

Give an account of induced magnetism
Magnetism can be induced in a magnetic material such as iron if it is placed within a magnetic field. Iron becomes magnetised when they are near or in contact with a permanent magnet.
In an experiment with a bar magnet, a piece of wood is held between the N pole of a magnet and two iron nails as shown in the diagram. It is observed that the pointed tips of the iron nails point away from each other.

The iron nails become induced magnets. The pointed tips of the iron nails are like poles (N pole) and thus they will repel each other.
Magnetic induction.jpg

Distinguish between ferrous and non-ferrous materials

Ferromagnetism: The property of being strongly attracted to either pole of a magnet.
Magnetic forces tend to push all magnetic moments in the same direction.

A group of atomic magnets pointing in the same direction is called a magnetic domains.
Characteristic of substances such as iron, nickel, or cobalt and various alloys that exhibit extremely high magnetic permeability. These are magnetic, ferrous materials.

Non ferrous, which is non-magnetic materials do not attract to magnets. Copper, wood and plastics are non-ferrous materials.

Distinguish between the magnetic properties of iron and steel
Iron
Steel
  1. Easily magnetised
  2. Does not retain its magnetism
  3. Soft magnetic material
  4. Used in electromagnets and in cores of a transformer
  1. Harder to magnetise
  2. Retains its magnetism
  3. Hard magnetic material
  4. Used to make permanent magnets
Permanent magnets are made of magnetically HARD materials such as steel. These materials retain their magnetisation once magnetised.
SOFT materials, such as iron, lose their magnetisation easily. They are suitable for temporary magnets such as electromagnets.

Distinguish between the design and use of permanent magnets and electromagnets
.
Uses of permanent magnets
Moving coil ammeter
Magnetic door catch
Loud speaker
moving coil ammeter.jpg

It consists of a coil suspended in the magnetic field of a permanent magnet. When a current flows into and out of the coil, a turning effect is produced on the coil and the pointer attached to it will move.
magnetic door.jpg

Magnetic strips are fitted to the doors of freezers and refrigerators to keep the doors closed.
loudspeakers.jpg


Magnets are used in moving-coil loudspeakers. At the core of the loudspeaker, there is a very strong permanent magnet.
Uses of electromagnets
A magnetic field is also created if an electric current moves through a coil of wire. This is how electromagnets work. (The force between them is at a maximum when the velocity of the charge is perpendicular to the magnetic field.)
1) A moving charge or current creates a magnetic field in the surrounding space.
2) The magnetic field exerts a force on any other moving charge or current present in that field
1. Scrap yard crane
2. Electric bell
3. Relay switch
4. Circuit breaker
Scrap yard crane.jpg

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.
Electric bell.jpg

When the push switch is closed current flows around the circuit turning on the electromagnet.
The soft iron armature is pulled towards the electromagnet and the hammer hits the gong.
This causes the contact switch to open cutting off the electric current.
The spring now pulls the armature back again closing the contact switch.
Current now flows again and the hammer hits the gong again.
Relay switch.jpg

A relay switch is a way of using a low voltage circuit to switch remotely a high voltage (and possibly dangerous) circuit.
When switch A is closed, the small current provided by the cell causes the electromagnet to become magnetised..
The iron armature is then attracted to the electromagnet causing the springy contact switch B to close in the high voltage circuit.
Circuit breaker.jpg

Current normally flows between terminals A and B through the contact and the electromagnet.
When the current in a circuit increases, the strength of the electromagnet will also increase. This will pull the soft iron armature towards the electromagnet.
As a result, spring 1 pulls apart the contact and disconnecting the circuit immediately, and stopping current flow.
The reset button can be pushed to bring the contact back to its original position to reconnect the circuit

Magnet quiz