Magnetism

 

Field and Field Lines

Magnet

A magnet is a material that produces a field that attracts or repels other such materials of magnetic nature.
Lodestone is a naturally occurring magnet. It attracts materials like Iron, Nickel, Cobalt, etc.

North and South Poles

A magnet is always bipolar with poles named north and south poles. These two poles always exist together and can not be separated. North pole of a magnet is the side which points to Earth’s geographic north when it is freely suspended.

Like poles repel and unlike poles attract

Similar to charges, poles attract and repel. Like poles repel while unlike poles attract each other.

Bar magnet

A bar magnet is a rectangular object, composed of iron, steel or any form of a ferromagnetic substance, that shows permanent magnetic properties. It has two different poles, a north and a south pole such that when suspended freely, the north pole aligns itself towards the geographic north pole of the Earth.

Magnetic field

The region around a magnet where its magnetic influence can be experienced is called a magnetic field. The direction and strength of a magnetic field are represented by magnetic lines of force.

Iron filings test around a bar magnet

Iron filings around a bar magnet exhibit the magnetic field lines that en-girdle the bar magnet. The magnetic field lines can be explained as imaginary lines that graphically represents the magnetic field that is acting around any magnetic substance.

Magnetic field lines

• Magnet’s magnetic field lines result in the formation of continuous/running closed loops.
• The tangent to the field line at any given point indicates the direction of the total magnetic field at that point.
• The greater the number of field lines crossing per unit area, the higher the intensity, the stronger the magnitude of the magnetic field.
• There is no intersection between the magnetic field lines.

Magnetic field lines for a closed loop

Since magnets have dipoles, magnetic field lines must originate and end. Therefore by convention, it starts at the north pole and moves towards the south pole outside the bar magnet and from south $\to$ north inside the magnet. Hence, it forms closed loops.

No two magnetic field lines intersect

Magnetic field lines do not intersect as there will be two tangential magnetic field directions associated with the same point, which does not occur. If a compass needle is placed at that point, it will show two different directions of the magnetic field which is absurd.

Relative strength of magnetic field inferred from magnetic field lines

The closer or denser the magnetic field lines, greater is the magnetic field’s strength.

Magnetic Field Due to a Current Carrying Conductor

Oersted’s experiment

When electric current flows through a current carrying conductor, it produces a magnetic field around it. This can be seen with the help of a magnetic needle which shows deflection. The more the current, the higher the deflection. If the direction of current is reversed, the direction of deflection is also reversed.

Electromagnetism and electromagnet

An electromagnet is an artificial magnet which produces a magnetic field on the passage of electric current through a conductor. This field disappears when the current is turned off. The phenomenon of producing or inducing a magnetic field due to the passage of electric current is called electromagnetism.