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Magnetic Field Lines

What are Magnetic Field Lines

A magnetic field can be visually represented by imaginary lines drawn around a magnet or a magnetized object. These lines are known as magnetic field lines or magnetic lines of force. The number of field lines passing through a given cross-sectional area is called the magnetic flux. As the magnetic field is a vector quantity it will have both magnitude and direction. The flux per unit cross-sectional area perpendicular to the lines is called magnetic flux density, which gives the magnitude of the magnetic field. A line drawn tangentially at any point on the field lines gives the magnetic field’s direction. The magnetic field lines are analogous to electric field lines.

Properties of Magnetic Field Lines

Here are some general facts and characteristics of the magnetic field lines.

  • Closed and continuous curve
  • Never cross each other – if they do, the tangent at the point of intersecting will show different directions, which is not possible
  • Density determines the magnetic field strength – crowding of field lines indicates a strong magnetic field
  • Density decreases with increasing distance from the object
  • The magnetic field and magnetic force are tangential to the lines

Examples of Magnetic Field Lines

1. Bar Magnet

A bar magnet is a permanent magnet whose magnetism remains forever. The magnetic field lines in a bar magnet form closed lines. A bar magnet has two poles – the north pole and the south pole. The magnetic field lines come out of the north pole and terminate into the south pole. Inside the magnet, they travel from south to north pole. The magnetic field lines can be sketched by bringing a compass near to the magnet. The compass, a magnet itself, aligns along these lines such that its needle points in the magnetic force direction.

Properties of Magnetic Field Lines of a Bar Magnet

Apart from the properties discussed in the previous section, the magnetic field lines of a bar magnet have the following additional properties.

  • Flow from north to south pole outside the magnet and south to north pole inside the magnet
  • Parallel and uniform inside the magnet and diverges and non-uniform outside
  • Closer together at the poles
  • Increases the magnetic field strength at the poles
Magnetic Field Lines

Two Magnets Next to Each Other

When two bar magnets are placed next to each other, their magnetic field lines are distorted due to the forces of attraction and repulsion between magnets. The rule is that like poles repel and opposite poles attract. When the north pole of one bar magnet faces the south pole of another, the magnetic field lines will join together. The lines will come out from the north pole and terminate in the south pole. The lines will become denser in the region between the two poles. When two north poles or two south poles are close to one another, the magnetic field lines will bend away from each other due to repulsion.

2. Current-carrying Wire

Ampere’s law has shown that current flowing through a wire produces a magnetic field. The magnetic field lines around any current-carrying wire are concentric circles with the center lying on the wire. The direction of magnetic field lines can be determined using the right-hand rule. Suppose the thumb points in the current direction. The fingers curling around the wire give the direction of the magnetic field. The strength of the magnetic field is inversely proportional to the distance from the wire.

Magnetic Field Lines About a Current-carrying Wire

3. Solenoid

A solenoid is a coil of many circular turns of insulated copper wire wrapped tightly like a helix, often around a metallic core. The shape of a solenoid is typically cylindrical with a straight axis. When current passes through the wires, a magnetic field is produced, and thus, the solenoid behaves like an electromagnet. The magnetic field pattern is similar to that of a bar magnet. One end of the solenoid behaves as a north pole, while the other behaves as the south pole. Inside, the field lines are parallel and uniform, which indicates that the magnetic field is the same at all points inside. The magnetic field outside the solenoid is zero.

Article was last reviewed on Wednesday, September 15, 2021

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