Magnetic flux (also called magnetic field flux in some contexts) is a physical quantity that measures the amount of magnetic field that passes through a given surface. In more precise terms, it quantifies the component of the magnetic field that "crosses" that surface as a function of its orientation.
To visualize it, we can imagine a magnetic field in space, like the one that surrounds a magnet. If we place a surface in that region, the magnetic flux will depend on the strength of the field, the surface area and its orientation with respect to the field.
Mathematical definition
In the most general case, magnetic flux is defined by a surface integral:
Where the scalar product indicates that only the component of the magnetic field perpendicular to the surface contributes.
In the particular case of a uniform field and a flat surface, the expression is simplified to:
Where:
- Φ: magnetic flux, in weber (Wb)
- B: magnetic flux density or magnetic induction, in tesla (T)
- A: surface area, in square meters (m²)
- θ: angle between the magnetic field and the normal to the surface
The unit of magnetic flux in the International System is the weber (Wb), where:
Magnetic flux density
The magnetic flux density, represented by BBB, is a vector quantity that describes the strength of the magnetic field at each point in space.
It is defined as the magnetic flux per unit area perpendicular to the field under uniform conditions:
Its unit in the International System is the tesla (T), which is equivalent to:
In the CGS system, the gauss (G) is used, with the ratio:
Instruments for measuring magnetic flux
There are several instruments to measure the magnetic field or flux, depending on the accuracy and application:
Gaussmeters
They measure the magnetic flux density in gauss (G). They are common in industrial applications and in the characterization of magnets.
Teslameters
They measure the magnetic flux density in teslas (T). They are more precise instruments, used in laboratories and scientific applications.
Magnetometers
Devices capable of measuring the strength and direction of the magnetic field. They are used in geophysics, navigation, and space exploration.
Hall Sensors
Devices based on the Hall effect that allow the magnetic field to be measured directly at a point. They are widely used in electronics, electric motors, and control systems.
Search coils
They are coils in which a current proportional to the variation of the magnetic flux is induced. They are used to measure flow changes over time.
Fluxmeters
Instruments specifically designed to measure magnetic flux directly from field variation in measuring coils.
Helmholtz coils
They are not direct measuring instruments, but coil configurations that generate uniform and known magnetic fields, useful for calibration and experiments.
Applications using magnetic flux
Knowledge of magnetic flux is essential in various applications and fields of science and technology. Some of the main applications where it is important to know the magnetic flux are the following:
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Electrical industry and power generation: In the electrical industry it is used in generators and transformers for the production, transmission and distribution of electrical energy.
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Electronics and semiconductor technology: Some devices such as magnetic sensors, microchips, hard drives, and other components use magnetic fields for their operation.
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Medicine: In medicine, magnetic resonance imaging (MRI) is used to obtain detailed information about tissues and organs in the human body. MRI is based on the manipulation and detection of the magnetic flux generated by hydrogen atoms in the body.
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Geophysics and navigation: In geophysics, the study of the Earth's magnetic field helps to understand the internal structure of the planet. Also, in navigation, compasses use magnetic flux to determine direction.
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System control and stabilization: In some applications, such as the stabilization of control systems, it is important to know and compensate for the effects of magnetic fields to avoid interference or unwanted effects.
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Metallurgical and materials industry: Knowledge of magnetic flux is useful in quality testing, inspection of ferromagnetic materials, and control of manufacturing processes.
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Transportation and magnetic levitation: In transportation applications, such as magnetic levitation (Maglev) trains, this physical property is used for propulsion and control of the vehicle without physical contact with the tracks via electromagnets.
Real-life examples of magnetic flux
Magnetic flux is present in many everyday phenomena and devices, although it is often not directly perceived. Its variation is the basis for much of modern electrical technology.
Electric generators
In a generator, a coil or magnet rotates within a magnetic field. This movement causes the magnetic flux passing through the coil to change over time. This variation in flow is what induces an electric current, according to Faraday's law. This is how much of the electricity we consume is produced.
Electricity generation in nuclear power plants
In nuclear power plants, the energy released by the fission of uranium is not directly transformed into electricity. First, that energy is used to heat water and produce high-pressure steam.
This steam moves a turbine, which in turn turns an electric generator. It is precisely in this generator that the magnetic flux appears: when a coil is rotated within a magnetic field (or a magnet around coils), the magnetic flux passing through the coils changes continuously.
This variation in magnetic flux induces an electric current in the generator coils, according to Faraday's law. In this way, nuclear energy ends up being converted into electrical energy through an intermediate process based on electromagnetism.
Transformers
Transformers work thanks to the variable magnetic flux in a ferromagnetic core. When an alternating current circulates through the primary coil, it generates a changing magnetic flux that is transmitted to the secondary coil, inducing a voltage. This makes it possible to raise or reduce the electrical voltage in the distribution network.
Electric motors
In electric motors, the interaction between the magnetic field of the stator and the magnetic flux generated in the rotor produces a torque that causes the movement. Magnetic flux control is key to motor efficiency and performance.
Magnetic resonance imaging (MRI)
In medicine, MRI uses powerful magnetic fields to align the protons in the human body. Although the concept here focuses more on the magnetic field than on the flux itself, the controlled variations of the field allow detailed images of internal tissues to be obtained.
Compasses and Earth's magnetic field
The Earth acts as a great magnet. The magnetic flux associated with the Earth's field allows compasses to orient themselves indicating magnetic north. Although in this case there is no device that "generates" the flow, it is a clear example of how a magnetic field passes through surfaces and affects objects.
Hard Drives and Magnetic Sensors
In magnetic storage devices, information is saved by orienting small magnetic domains. Magnetic flux plays an essential role in writing and reading data. Similarly, sensors such as the Hall effect detect field variations to measure position, speed or proximity.