Magnetism Flashcards
How to determine the north pole of a magnet?
The end of a freely suspended bar magnet that points to the northern end of the Earth is called the north-seeking pole (North pole or N pole)
How to determine the south pole of a magnet?
The other end of a magnet that points to the southern end of the Earth is called the south-seeking pole (South pole or S pole)
Law of magnetic poles
Like poles repel, unlike poles attract
How to confirm if an object is a magnet?
Repulsion test
What is magnetic induction?
Magnetic induction is the process of inducing magnetism in ferromagnetic materials.
It can also occur without any contact with the magnet
What is the theory of magnetism?
When a bar magnet is cut into many small pieces, every piece becomes a small magnet itself.
The bar magnet is made up of many such ‘tiny magnets’, or magnetic domains
What are magnetic domains?
The orbiting motion of electrons in a magnetic material makes each atom an atomic magnet.
A group of atomic magnets pointing in the same direction is called a magnetic domain
Magnetic domains: Unmagnetised Bar
In an unmagnetised bar, the magetic domains point in random directions.
The magnetic effects of the atomic magnets cancel out so there is no resultant magnetic effect
Magnetic domains: Permanent Magnet
In a permanent magnet, magnetic domains point in the same direction
The atomic magnets at the ends of the bar magnet fan out due to repulsion between like poles
Magnetic saturation
When all the magnetic domains point in the same direction, the magnet is magnetically saturated and cannot be any stronger
What is demagnetisation
Demagnetisation is the process of removing magnetism from a magnet
Cause the atoms of the magnet to vibrate vigorously, mixing up the directions of the magnetic domains
Why do magnets get weaker over time when placed side by side
Over time, magnets placed side by side will become weaker
This is because the magnetic domains will be altered due to the repulsion between the ‘free’ poles
How to prevent magnet weakening
To prevent this, bar magnets are stored in pairs with soft iron keepers across the ends of the bar magnets
The poles of the bar magnets are in closed loops with no ‘free’ poles
How to make magnets: Stroking method
-An unmagnetised steel bar is stroked several times with the same pole of a permanent magnet from one end to the other in one direction
-The steel bar will become a permanent magnet due to magnetic induction
-The pole produced at the end of the steel bar where the strokes finish is opposite to the stroking pole used
How to make magnets: Electrical method using a direct current
-The steel bar to be magnetised is placed in a solenoid (a cylindrical coil of insulated copper wires)
-When a large d.c. is passed through the solenoid, the unmagnetised steel bar will become magnetised due to the strong magnetic field produced
-The poles of the magnet can be determined by the right-hand grip rule
(Grip with right hand, fingers pointing in direction of current flow. End of magnetised steel bar where thumb points is the N pole)
How to demagnetise magnets: Heating
When heated, the atoms of the magnet vibrate vigorously, causing the magnetic domains to lose their alignment
How to demagnetise magnets: Hammering
This method also alters the alignment of the magnetic domains
How to demagnetise magnets: Electrical method using an alternating current
-Place a magnet inside a solenoid connected to an alternating current (a.c.) supply
-The magnet is withdrawn in the East-West direction
Q. State the difference between an unmagnetised bar and a magnetised bar
An unmagnetised bar has no magnetic effects. The magnetic domains in an unmagnetised bar are in random directions
A magnetised bar has magnetic effects of N and S poles at the ends. This is because the magnetic domains tend to be aligned in the same direction
Q. Explain why every magnet has a maximum strength
When all the magnetic domains in the magnet are pointing in the same direction, it is magnetically saturated and cannot be any stronger
Q. What will happen to the magnetic domains of a magnet placed in a solenoid carrying an a.c., as it is slowly withdrawn?
The direction of the magnetic domains of the magnets will be mixed up, causing the magnet to lose its magnetism
Magnetic field pattern between N pole and S pole
N pole (arrows move outward)
S pole (arrows move inward)
Magnetic field pattern between two N poles
N poles (both have arrows moving outward)
Point X: In the middle of the N poles
-Point X is the neutral point because the fields from both magnets cancel out each other
-If a compass is placed at X, it will point in any arbitrary direction
The Earth’s magnetic field
The imaginary magnet within the Earth has the imaginary ‘S’ pole at the magnetic north and the ‘N’ pole at the magnetic south.
In this way, a free suspended magnet will have its N pole seeking the ‘S’ pole of the imaginary magnet and its S pole seeking the ‘N’ pole of the imaginary magnet
