Magnetism and Earth’s Magnetic Field: A Comprehensive Guide
Magnetic Field
We can say that a magnetic field exists at a point near a magnetic source (current or magnetized bodies) if a force, due to the magnetic field, acts on a charged particle moving through this point.
Oersted argued that magnetic effects may occur due to the motion of electric charges, while Faraday and Henry argued that current can be obtained by moving magnets.
History of Electromagnetism
Part of the history of electromagnetism traces back to the Chinese, suggesting that it was known as early as 2000 BC. Another part of the story goes back to the ancient Greeks, who observed electrical and magnetic phenomena possibly as early as 700 BC. They found that a piece of rubbed amber could be electrified to attract pieces of straw or feathers. The existence of the magnetic force was noted through the observation that pieces of natural rock called magnetite attract iron. (The word electricity comes from the Greek word for amber, elektron. The word magnet comes from the name of a central district in northern Greece where it was discovered, Magnesia.)
Historical Background
Faraday’s and Henry’s discovery of electromagnetic induction introduced a certain symmetry in the world of electromagnetism. Maxwell brought together basic knowledge about electricity and magnetism into one theory. His electromagnetic theory predicted, before being observed experimentally, the existence of electromagnetic waves. Hertz proved their existence and began humanity’s era of telecommunications.
In 1819, Hans Christian Oersted, a professor of physics at the University of Copenhagen, found that by bringing a compass close to a conductor with an electric current circulating, the compass needle moved perpendicular to the direction of the conductor. The discovery that an electric current produces a magnetic field stimulated the imagination of physicists of the time and increased the number of experiments in search of new relations between electricity and magnetism.
Through studies by Ampere, it was also known that some materials, like magnetite, magnets, and compasses, have magnetic properties due to the existence of microscopic currents. In this scientific environment, the reverse idea of producing electric currents by magnetic fields would soon emerge. Some famous and lesser-known physicists were about to demonstrate experimentally that nature also supported this attractive idea. In fact, Joseph Henry, an American physicist, discovered electromagnetic induction a year before Faraday but made his discovery public a few months later. This is why the discovery is attributed to Faraday. Faraday called the electrical currents produced by magnetic fields “eddy currents.” Since then, the phenomenon that consists of generating electric fields from magnetic fields is called electromagnetic induction.
Magnet
It is a body that has the property of attracting iron filings, which are oriented in space.
Poles of a Magnet
They are the areas of a magnet where the magnetization is concentrated more intensely.
Earth’s Magnetic FieldAs we all know, Earth is a huge natural magnet. If magnetite or any other type of magnet or magnetic element is allowed to rotate freely on a plane parallel to its surface, as a compass does, it will always point to the magnetic north pole. For clarification, we must distinguish the Earth’s magnetic north pole from the geographic North Pole. The geographic North Pole is where all meridians converge, as is the case with the South Pole. However, the magnetic north pole is located at 1,200 km from the geographic north at coordinates 78° 50′ N (latitude North) and 104° 40′ W (west longitude), approximately on Amund Ringness Island. This is the place to which the compass needle always points and not to true north, as some people mistakenly believe. |
The Earth is one giant magnet with its corresponding poles. |
The Earth has a magnetic field with north and south poles. The Earth’s magnetic field reaches up to 36,000 miles in space. The Earth’s magnetic field is surrounded by a region called the magnetosphere.
The magnetosphere prevents most particles from the Sun, which move with the solar wind, from colliding with Earth.
How is Earth’s Magnetic Field Generated?
The Earth’s core is liquid. This is a very hot magma, a conductive material. As the planet rotates, so does the magma, although not uniformly. A non-uniform rotation of a conductive material creates a dynamo, and this is what gives rise to Earth’s magnetic field, which has a North Pole and a South Pole. At some points, they have been exchanged: the North Pole has become the South Pole and vice versa.
Some Solar Wind Particles Can Penetrate the Magnetosphere
These particles give rise to the light shows of the Aurora.
The Sun and other planets have magnetospheres, but the Earth has the strongest of all the rocky planets. The north and south magnetic poles of Earth are reversed at irregular intervals of hundreds of thousands of years.
Yet There Are Many Unanswered Questions About Earth’s Magnetic Field
For example, how it grew and fell over thousands of years by changing polarity (the north magnetic pole is transformed into the south magnetic pole and vice versa), and how it persists.
The Most Accepted Theory is the Dynamo Effect
(like a car) in the liquid outer core of Earth. I will explain it to you. Above all, please note that the Earth behaves like a giant magnet located in its center, whose axis is tilted about 11° to the axis of rotation, which creates lines of force entering the magnetic north pole (near the North Pole) to penetrate into the Earth and out the south magnetic pole.
Dynamo Effect
The dynamo effect is a geophysical theory that explains the origin of the Earth’s main magnetic field as a self-excited (or self-sustaining) dynamo. In this dynamo mechanism, fluid movement in the Earth’s outer core moves conducting material (liquid iron) across a weak magnetic field, which already exists, and generates an electric current (the heat of radioactive decay in the core induces convective motion). The electric current produces a magnetic field that interacts with the fluid motion to create a secondary magnetic field. Together, both fields are more intense than the original and essentially lie along the axis of rotation of the Earth.
The dynamo theory was proposed by the German-American physical oceanographer Walter M. Elsasser and the British geophysicist
Edward Bullard in the mid-1900s. Although other mechanisms were proposed to generate the magnetic field, only the dynamo concept is seriously considered today.
But What is Magnetism?
Until 1821, only one form of magnetism was known, produced by iron magnets. Subsequently, a Danish scientist, Hans Christian Oersted, while demonstrating to friends the flow of electric current in a wire, noticed that the current caused a nearby compass needle to move. The new phenomenon was studied in France by Andre-Marie Ampere, who concluded that the nature of magnetism was very different from what was previously thought. It was basically a force between electric currents: two parallel currents in the same direction attract, in opposite directions they repel. Iron magnets are a very special case, which Ampere was also able to explain. In nature, magnetic fields are produced in the rarefied gas of space, in the glowing heat of sunspots, and the Earth’s molten core. Such magnetism must be produced by electric currents, but it remains a great challenge to find how to produce these flows.
Magnetic Charge (Q*)
It is a scalar quantity associated with a magnetic pole that directly measures the amount of magnetism it has (Ampere-meter).
Qualitative Law
“Two poles of the same nature or name repel, and those of a different kind attract.”
Quantitative Law
“Two magnetic charges attract or repel with equal force, but from opposite directions, and whose values are directly proportional to the product of their charges but inversely proportional to the square of the distance that separates them.”
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Magnetic Field or Magnetic Field Strength
It is a vector quantity that, at a point in the magnetic field, gives the force acting on each unit of magnetic charge placed at that point north.
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The basic vector of the magnetic field is represented by 
, called magnetic induction, and can be represented by lines of induction. The magnetic field vector is related to induction lines as follows:
- The tangent to a line of induction at a given point indicates the direction of
at that point.
- Induction lines are drawn so that the number of lines per unit area in cross-section (perpendicular to the lines) is proportional to the magnitude of
. Where the lines are highly concentrated,
is large, and where they are widely separated,
is very small.
Plotting the intensity of the magnetic field
at a point P is a vector that satisfies the following relationship:
Where: q = positive test charge
V = velocity of the charge
Where: 
where 
Unit of
in SI is the Tesla (T)
1 Tesla = 1 WB/m2 = 1 N/Am
Lorentz Equation
If a particle of charge q moving in an electric field 
and a magnetic field
experiences a force given by:
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Magnetic Force on a Current
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= is a vector of displacement that points along the wire (right) in the direction of flow.
Torque on a Current Loop
The figure shows a coil carrying a current i placed in an external uniform magnetic field.
The torque on a loop of the coil is: 
The torque on the coil of N turns is:
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Where A = ab (loop area).
In vector form, the torque on a loop can be expressed as:
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Where: u = ma, is called the Magnetic Dipole Moment. It is along an axis perpendicular to the plane of the loop; its direction is determined by applying the right-hand rule.
Magnetic Potential Energy in a Position
is defined as the work to be performed by an external agent to rotate the dipole from the zero-energy position (
) to a given position
. Therefore:
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In vector form:
Charge Movement
The figure shows a negatively charged particle introduced at a rate 
in a uniform magnetic field 
.
If
and
are perpendicular, Newton’s second law is satisfied:
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r = radius of path
The angular velocity 
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The frequency f is determined by:
Af is sometimes called the cyclotron frequency of the particle in the field because particles circulate in a cyclotron with this frequency.
The Cyclotron
It is a device used to accelerate charged particles, such as hydrogen nuclei (protons) or the nuclei of heavy hydrogen (deuterons), to high energies so that they can be used in atomic collision experiments.
The essential elements of a cyclotron are: an electromagnet, two metal chambers called “dees,” which are part of an electrical oscillator that provides the accelerating potential difference in the space between the “dees,” and an ion source S.
The dees are immersed in a magnetic field whose direction is such that it is out of the plane of the figure.
The key to the operation of the cyclotron is that the characteristic frequency f with which the ion travels in the field must be equal to the fixed frequency for of the oscillator power supply, i.e.,
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For the resonance condition:
The energy of the particles produced in a cyclotron depends on the radius “R” of the “dees.” The speed of the particles circulating with this radius is determined by:
Therefore, its kinetic energy is: 
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