Magnetism: Properties and Magnetic Fields

• A natural magnet (magnetite) chiselled into a needle shape and suspended freely points in a north-south direction
• Freely suspended stone needle called leading stone was called lodestone by sailors
• Sailors and navigators used the directive property of lodestone, as a compass needle

Magnet Properties

• Magnets have two poles: a north and a south pole
• Poles are named based on how a freely suspended magnet aligns with Earth's geographic poles
• Magnetic strength is mostly concentrated near its poles

Magnetic Field

• The space around a magnet where its influence is felt is the magnetic field
• Direction of the magnetic field is the direction along which the north pole of a magnetic compass points

Magnetic Field Lines

• Magnetic field strength is explained by magnetic field lines (magnetic lines of force)
• More lines of force indicate a stronger magnet
• Magnetic lines of force around a magnet start from the north pole and end at the south pole
• Inside a magnet, magnetic lines of force run from the south pole to the north pole
• Magnetic lines of force form continuous closed curves or loops
• Magnetic lines of force are closely concentrated near the poles, indicating maximum magnetic strength
• Magnetic lines of force tend to contract longitudinally
• Field lines bend along the length and are spaced loosely near the middle of the magnet, indicating minimum strength

Attraction and Repulsion

• Like poles of two magnets repel each other
• Unlike poles of two magnets attract each other

Attraction and Repulsion Between Magnets

• If the north pole of a bar magnet is brought near the north pole of a freely suspended magnetic needle, the north end repels
• If the south pole of a bar magnet is brought near the south pole of a freely suspended magnetic needle, the south end repels

Magnet Storage

• In a horse-shoe magnet, a piece of soft iron should be placed at its ends. This piece of iron will stick to its ends and prevent the loss of magnetism.
• Placing a piece of soft iron at the ends will prevent loss of magnetism in horseshoe magnets
• Electronic articles such as computers are very sensitive to powerful magnets and may get demagnetized when in close range

Magnetism Laws:

• A magnet attracts magnetic substances like iron, steel, cobalt, and nickel
• Freely suspended, a magnet points in a north-south direction
• Like poles repel
• Unlike poles attract
• Repulsion confirms magnetism
• Rubbing a bar magnet over unmagnetized iron or steel, it becomes a magnet
• If a magnet is heated or handled roughly, it loses magnetism

Electromagnets

• When an electric current passes through a copper wire around an iron needle, it acts like a magnet
• Coils of insulated copper wire in a cylinder shape create a solenoid
• A solenoid with an iron core is an electromagnet
• Electromagnets are temporary magnets
• Electromagnet magnetism exists as long as electric current flows in the solenoid
• More turns strengthen the electromagnet's magnetic field

Electromagnet Experiment

• A U-shaped soft iron core wrapped with insulated copper wire connected to a battery becomes an electromagnet when the switch is closed
• The iron core attracts iron pins when the switch is closed, but releases them when the wire is disconnected, proving the iron only behaves like a magnet as long as current flows
• More current and coil turns result in a more powerful electromagnet

Permanent Magnets

• Steel or ALNICO can be used for permanent magnets in a closed coil
• These are used in magnetic stickers, magnetic toys, and magnetic compasses
• A magnet that operates only with electric current flowing through a coil around iron is an electromagnet

Electromagnet Uses

• Used in electrical applicances like electric bells, fans, and motors
• Used in electric generators where strong magnetic field is required
• Used to deflect the electron beam in televisions
• Used in magnetic separation to remove iron ores from non-magnetic substances
• Used to prepare more strong permanent magnets
• Used by doctors to cure certain diseases

Temporary and Permanent Magnets

• Temporary magnets lose magnetism when the causing factor is removed
• Electromagnets and magnets made from soft iron are temporary
• Permanent magnets do not lose magnetism when the cause is removed
• Magnets of steel or ALNICO are permanent

Electromagnetism and its Effects

• Electricity and Magnetism were separate according to pre 18th century thought
• Discovery of electromagnets made the physics community look at the relationship between the two
• Electromagnetism is the branch of physics that studies the relationship between electricity and magnetism

Oersted's experiment

• Used to show the magnetic effect of current by seeing how a magnetic needle pivots according to electrical charge
• Connect a dry cell/switch to a thick wired AB
• Observe the magnetic needle move when current flows south to north
• North-South is always deflected towards the west
• Continue to increase electricity = stage where needle deflects to geographic west
• Furthermore, find out that if the conductor is beloe the needle- current flowing south to north will result in the north pole of magnetic needle point towards the east
• Conversely, when the conductor is held above the magnetic needle, but current flows from north to south, then the north pole of needle points east
• If the conductor is held below the magnetic needle, the north pole of magnetic needle points west

Oersted experiment conclusions

• Current through a straight conductor behaves like a magnet
• Magnetic effect magnitude rises with current strength
• The conductor's magnetic field is at right angles to current flow

Electric Bell Function

• Electromagnet activates, pulling the armature
• Armature spring tenses
• Hammer sounds the gong
• The circuit breaks but the armature continues moving due to inertia

Electric Bell Parts

• Electromagnet is made by winding insulated copper wire around wooden reels on a U-shaped soft iron rod
• An armature, soft iron bar suspended by flat spring faces the electromagnet's poles
• A flat steel spring contact spring is fixed to the armature
• Silver-cadmium contact point prevents oxidation, for the contact point
• Brass pillar contact screw allows for frequency adjustments
• The sound from steel hammer is attached to the armature, striking the brass gong
• The silver-cadmium tip of a brass screw also prevents oxidation