MAGNETIC FIELD LINES

Magnet: Magnetic field and magnetic field lines, Magnetic field due to a current-carrying conductor, Right-hand thumb rule, Magnetic field due to current through a circular loop. Magnetic field due to current in a solenoid.
A magnet is an object that attracts objects made of iron, cobalt and Nickel. Magnet comes to rest in the North-South direction when suspended freely.  

Use of Magnets: Magnets are used

• in refrigerators.

• in radio and stereo speakers.

• in audio and video cassette players.

• in children’s toys and;

• on hard disks and floppies of computers.

Properties of Magnet

• A free suspended magnet always points towards the north and south directions.

• The pole of a magnet that points toward the north direction is called the north pole or north-seeking.

• The pole of a magnet that points toward the south direction is called the south pole or south seeking.

• Like poles of magnets repel each other while unlike poles of magnets attract each other.

Magnetic field: The area around a magnet where a magnetic force is experienced is called the magnetic field. It is a quantity that has both direction and magnitude, (i.e., Vector quantity).

Magnetic field and field lines: The influence of force surrounding a magnet is called a magnetic field. In the magnetic field, the force exerted by a magnet can be detected using a compass or any other magnet.
The magnetic field is represented by magnetic field lines.

The imaginary lines of the magnetic field around a magnet are called field lines or field lines of a magnet. When iron filings are allowed to settle around a bar magnet, they get arranged in a pattern that mimics the magnetic field lines. The field line of a magnet can also be detected using a compass. A magnetic field is a vector quantity, i.e. it has both direction and magnitude.

The direction of field line: Outside the magnet, the direction of the magnetic field line is taken from the North pole to the South Pole. Inside the magnet, the direction of the magnetic field line is taken from the South pole to the North pole.

Strength of magnetic field: The closeness of field lines shows the relative strength of the magnetic field, i.e. closer lines show a stronger magnetic field and vice – versa. Crowded field lines near the poles of the magnet show more strength.

Properties of magnetic field lines
(i) They do not intersect each other.
(ii) It is taken by convention that magnetic field lines emerge from the North pole and merge at the South pole. Inside the magnet, their direction is from the South pole to the North pole. Therefore magnetic field lines are closed curves.

Magnetic field lines due to current a current-carrying straight conductor
A current-carrying straight conductor has a magnetic field in the form of concentric circles around it. The magnetic field of a current-carrying straight conductor can be shown by magnetic field lines.
The direction of the magnetic field through a current-carrying conductor depends upon the direction of flow electric current.

Let a current-carrying conductor be suspended vertically and the electric current is flowing from south to north. In this case, the direction of a magnetic field will be anticlockwise. If the current is flowing from north to south, the direction of a magnetic field will be clockwise.
The direction of a magnetic field and the direction of electric current through a straight conductor can be depicted by using the Right-Hand Thumb Rule. It can also be depicted by Maxwell’s Corkscrew Rule.

Properties of magnetic field due to current a current-carrying straight conductor

• The magnitude of the magnetic field increases with an increase in electric current and decreases with a decrease in electric current.

• The magnitude of a magnetic field produced by electric current decreases with an increase in distance and vice – versa. The size of concentric circles of magnetic field lines increases with distance from the conductor, which shows that the magnetic field decreases with distance.

• Magnetic field lines are always parallel to each other.

• No two field lines cross each other.

Relation between the magnetic field and number of turns of coil

The magnitude of the magnetic field gets summed up with an increase in the number of turns of the coil. If there are ‘n’ turns of the coil, the magnitude of a magnetic field will be ‘n’ times of the magnetic field in case of a single turn of the coil.

The strength of the magnetic field at the centre of the loop(coil) depends on :
(i) The radius of the coil: The strength of the magnetic field is inversely proportional to the radius of the coil. If the radius increases, the magnetic strength at the Centre decreases
(ii) The number of turns in the coil: As the number of turns in the coil increase, the magnetic strength at the Centre increases, because the current in each circular turn is having the same direction, thus, the field due to each turn adds up.
(iii) The strength of the current flowing in the coil: As the strength of the current increases, the strength of the three magnetic fields also increases.

CONCLUSION

Magnetic fields can be depicted in several ways. Mathematically, it can be represented as a vector field which can be plotted as different sets on a grid. Another way is the use of field lines. The set of vectors are connected with lines. Here the magnetic field lines never cross each other and never stop. The magnetic field has the direction of the lines of force that leave the north pole of the magnet and enter its south pole .  Inside the  magnet, the lines of force are parallel and go from south to north.  This means that the field inside the magnet is homogeneous.