Magnetic Fields: Properties, Behavior, and Laws

Magnetic Fields

A magnetic field is the disturbance produced by a magnet in an area of space. It is manifested by the interactions that occur in that area with other magnets or moving electric currents.

Oersted's Experience and the Lorentz Force

Oersted's experience shows that electric charges at rest do not create magnetic fields, but if they are moving, then they do.

The Lorentz Force: If we introduce a moving charge into a magnetic field, it will experience a force given by the expression...

Similarities and Differences Between Electric and Magnetic Fields: An Analogy

• Both fields are generated by electric charges.
• Electric field strength and magnetic field strength depend inversely on the square of the distance to the point where we make the measurement.
• Both fields depend on the material.

Differences:

• The charges that generate the magnetic field must be in motion.
• Electric field lines are open, while magnetic field lines are closed.
• There are separate electric charges but not separate magnetic poles.
• Electric fields are fields of central conservative forces, while magnetic fields are not. Therefore, potential and potential energy are not defined for magnetic fields.

Behavior of Matter in Magnetic Fields

In an electric field, the field strength decreases. In a magnetic field, there are substances that can decrease the intensity and others that can increase it.

Substances with respect to the magnetic field are classified as:

• Diamagnetic: Examples include gold, silver, copper, and water. The field strength is lower than in a vacuum because they have a smaller magnetic permeability.
• Paramagnetic: Examples include platinum, chromium, manganese, aluminum, and oxygen gas. The field strength is greater than in a vacuum because they have a larger magnetic permeability.
• Ferromagnetic: Examples include iron, cobalt, and nickel. The field strength is much greater than in a vacuum because they have a very high magnetic permeability.

Magnetic Flux

Magnetic flux is defined as the number of field lines that pass through a certain area, or the number of field lines that cross a given surface.

Faraday's Law

If the magnetic flux through the coils of a circuit varies with time, an induced electromotive force is generated in the circuit. This force is directly proportional to the number of turns and the rate of change of the flux.

Lenz's Law

The induced current generated by a change in magnetic flux will flow in such a way that it opposes the change in flux.

Electromagnetic Waves

Electromagnetic waves have the following characteristics:

• They are transverse waves that do not need a material medium for propagation.
• They are caused by accelerated electric charges. These accelerated charges produce a variable electric field, which in turn creates a magnetic field perpendicular to the variable electric field.