Magnetic Lines & Their Characteristics | Physics Class 12 - NEET PDF Download

What is a Magnetic Field?

The magnetic field is the region around a magnet (or a magnetic material or a current-carrying conductor) where magnetic forces can be detected.

• It is a vector quantity, symbolized by B.

• Magnetic fields are detected using magnetic needles or iron filings.

Visualizing the Magnetic Field Lines

Michael Faraday introduced the concept of magnetic field lines to represent magnetic fields graphically.

• A magnetic field line is an imaginary line such that the tangent at any point gives the direction of the magnetic field at that point.

• These lines help visualize the strength and direction of the magnetic field.

Key Characteristics of Magnetic Field Lines

1. Direction of Lines:

• Outside a magnet: North → South

• Inside a magnet: South → North

• Hence, magnetic field lines form closed loops.

2. Magnetic Flux:

• Magnetic flux (Φ) represents the total number of field lines.

• The number of lines per unit area perpendicular to the field represents the strength (B) of the field.

• The net magnetic flux through any closed surface is zero.

  This is due to the absence of magnetic monopoles.

3. No Intersection:

• Magnetic field lines never intersect.

• If they did, it would imply two directions of the magnetic field at the same point, which is not possible.

4. Elastic Nature:

• Magnetic field lines behave like stretched elastic strings:

  • They tend to contract longitudinally (attracting opposite poles).

  • They repel laterally (repelling like poles).

5. Field Strength and Density:

• Denser lines → stronger field

• If field lines are parallel and equally spaced, the field is uniform.

• If field lines are diverging or converging, the field is non-uniform.

6. Lines in Magnetic Materials:

• Magnetic field lines can enter or leave a magnetic material at any angle.

• Magnetic field lines also exist within all magnetised materials.

7. Neutral Point:

• A point where the net magnetic field is zero due to the cancellation of two or more magnetic fields.

• At a neutral point, no magnetic field lines exist.

8. Gauss’s Law for Magnetism:

• States that the net magnetic flux through a closed surface is always zero.

• Mathematically:

• This confirms the non-existence of magnetic monopoles.

Different Magnetic Field Lines

1. Straight Current-Carrying Wire

• Right-hand thumb rule:
  If the thumb points in the direction of current, the fingers curl in the direction of magnetic field lines.

• Field lines: Concentric circles around the wire.

• Field strength: where I = current, r = radial distance from the wire.

2. Circular Loop of Wire

• Magnetic field lines form concentric circles around the wire.

• At the center, field lines are straight and perpendicular to the plane of the loop.

• The field at the center:

  

  where R is the radius of the loop and $x$x is the distance from the center along the axis.

3. Solenoid

• A solenoid is a coil of many circular loops of wire stacked together.

• Magnetic field inside: Strong, uniform, and parallel lines

• Magnetic field outside: Very weak

• Resembles the field of a bar magnet.

• Field inside a long solenoid:

  $B\; =\; \backslash mu\_0\; n\; I$B=μ₀nI

  where $n$n = number of turns per unit length, I = current