Magnetic flux is the amount of magnetic field that penetrates an area perpendicular to it. Gauss’s law states that for a closed surface, the magnetic flux is always zero, and Faraday’s law states that a change in magnetic flux will create a voltage. Materials vary in their reactions to magnetic fields, and the strength of a magnetic field is measured in Teslas. Measuring the magnetic flux at various points on the earth’s surface allows scientists to monitor the planet’s magnetic field.
Magnetic flux is the amount of magnetic field that penetrates an area perpendicular to it. In a simple situation where the field passes at right angles across a flat surface, this quantity is the magnetic field strength times the surface area. In most real-life situations, however, other factors must be taken into consideration. Magnetic flux is an important concept in many areas of science, with applications to electric motors, generators, and the study of the Earth’s magnetic field. In physics it is represented by the Greek letter phi, φ.
Gauss’s law
A bar magnet has two poles, called north and south because of the way they react to the earth’s magnetic field, which is roughly aligned north-south. It is a scientific convention that lines of magnetic force flow from north to south. If a person takes the two-dimensional rectangular surface at the north end of a rod magnet, he has magnetic flux, as does the surface at the south pole. The magnet as a whole, however, has no flux, as the north and south ends are equal in strength and the field “flows” from the north pole to the south pole, forming a closed loop.
Gauss’s law for magnetism states that, for a closed surface, such as a sphere, cube, or bar magnet, the magnetic flux is always zero. It’s another way of saying that an object, however small, with a north pole must always have a south pole of equal strength, and vice versa. Anything that has a magnetic field is a dipole, which means it has two poles. Some scientists have speculated that magnetic monopoles may exist, but none have ever been detected. If they are found, Gauss’ law should be changed.
Faraday’s law
Faraday’s law states that a change in magnetic flux will create a voltage, or electromotive force (EMF), within a coil of wire. Simply moving a magnet near a coil of wire will achieve this, as well as a change in the strength of the magnetic field. The voltage produced can be determined by the rate of change of the magnetic flux and the number of coil turns.
This is the principle behind electricity generators, where motion is created, for example, by flowing water, wind or a fossil fuel-powered engine. Magnets and coils of wire convert this motion into electrical energy, according to Faraday’s law. Electric motors demonstrate the same idea in reverse: an alternating electric current in coils of wire interacts with magnets to produce motion.
Magnetic materials
Materials vary in their reactions to magnetic fields. Ferromagnetic substances produce a stronger magnetic field by themselves, and this field can persist when the external field is removed, leaving a permanent magnet. Iron is the best known element of this type, but other metallic elements, such as cobalt, nickel, gadolinium and dysprosium, also demonstrate this effect. Very strong magnets can be made from alloys of the rare earth metals neodymium and samarium.
Paramagnetic materials produce a magnetic field in response to an external one, producing a relatively weak attraction that is not persistent. Copper and aluminum are examples. Another example is oxygen; in this case, the effect is best demonstrated with the item in liquid form.
Diamagnetic substances create a magnetic field which opposes an external field, producing repulsion. All substances show this effect, but it is normally very weak and always weaker than ferromagnetism or paramagnetism. In some cases, such as a form of carbon called pyrolytic graphite, the effect is strong enough for a small piece of such material to float in the air just above an arrangement of powerful magnets.
Flow calculation and measurement
Calculating flux for a flat surface at right angles to the direction of a magnetic field is straightforward. However, it is often necessary to calculate the quantity for a coil of wire, also called a solenoid. Assuming the field is perpendicular to the wire, the total flux is the strength of the magnetic field times the area through which it passes times the number of turns in the coil. When the field is not perpendicular to the surface, the angle formed by the magnetic field lines with respect to the perpendicular must be taken into account and the product multiplied by the cosine of this angle.
An instrument called a flowmeter is used to measure the amount of the field. It is based on the fact that a magnetic field will create an electric current in a wire if the two move relative to each other. This current can be measured to determine the flow.
Magnetic flux in geology
Measuring the magnetic flux at various points on the earth’s surface allows scientists to monitor the planet’s magnetic field. This field, thought to be generated by electric currents in the Earth’s iron core, is not static, but rather shifts over time. In fact, the magnetic poles have reversed many times in the past and will likely do so again in the future. The effects of a polarity reversal can be severe, as the field strength would be reduced over much of the planet during the shift. Earth’s magnetic field protects life on the planet from the solar wind, a stream of electrically charged particles from the sun that would be harmful.
Unit of measure
The strength of a magnetic field, or magnetic flux density, is measured in Teslas, a unit named after electrical engineer Nikola Tesla. Flow is measured in Webers, named after physicist Wilhelm Eduard Weber. A Weber is 1 Tesla multiplied by 1 square meter and a Tesla is 1 Weber per square meter.
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