Instruct: Given a question, retrieve documents that can help answer the question.
Query: What is a magnetic field?
A very common source of magnetic field found in nature is a dipole, with a "South pole" and a "North pole", terms dating back to the use of magnets as compasses, interacting with the Earth's magnetic field to indicate North and South on the globe. Since opposite ends of magnets are attracted, the north pole of a magnet is attracted to the south pole of another magnet. The Earth's North Magnetic Pole (currently in the Arctic Ocean, north of Canada) is physically a south pole, as it attracts the north pole of a compass.
A magnetic field contains energy, and physical systems move toward configurations with lower energy. When diamagnetic material is placed in a magnetic field, a "magnetic dipole" tends to align itself in opposed polarity to that field, thereby lowering the net field strength. When ferromagnetic material is placed within a magnetic field, the magnetic dipoles align to the applied field, thus expanding the domain walls of the magnetic domains.
The magnetostatic energy is the energy stored in the field generated by a material's magnetic moments. The magnet's field reaches its maximum intensity if all the magnetic moments orient in one direction; this is what occurs in a hard magnet. In order to prevent building up the magnetic field, sometimes magnetic moments tend to form loops. That way, the energy stored in the magnetic field can be constrained; this is what occurs in a soft magnet. What determines whether a magnet is hard or soft is the dominant term of its magnetic energy. For hard magnets, the anisotropy constant is relatively large, making the magnetic moments align with easy axis. The opposite case applies to soft magnets, in which the magnetostatic energy is dominant.
where the integral can be over all space or, equivalently, where is nonzero.
Magnetic potential energy is the form of energy related not only to the distance between magnetic materials, but also to the orientation, or alignment, of those materials within the field. For example, the needle of a compass has the lowest magnetic potential energy when it is aligned with the north and south poles of the Earth's magnetic field. If the needle is moved by an outside force, torque is exerted on the magnetic dipole of the needle by the Earth's magnetic field, causing it to move back into alignment. The magnetic potential energy of the needle is highest when its field is in the same direction as the Earth's magnetic field. Two magnets will have potential energy in relation to each other and the distance between them, but this also depends on their orientation. If the opposite poles are held apart, the potential energy will be higher the further they are apart and lower the closer they are. Conversely, like poles will have the highest potential energy when forced together, and the lowest when they spring apart.
Human magnetism is a popular name for an alleged ability of some people to attract objects to their skin. People alleged to have such an ability are often called human magnets. Although metal objects are the most popular, some are also alleged to be able to stick other types of materials, such as glass, porcelain, wood or plastic as well as metals with no ferromagnetic properties such as brass and aluminum. In fact none of the recorded claims of human magnetism corresponds with the actual physics of magnetism, indicating that this "ability" is in fact nothing more than a misunderstanding of the physics and meaning of the term and a misapplication of it to what has been shown to be nothing more than unusually sticky skin.According to scientists, if people can stick objects to their body, not only metal but also other materials, it has actually nothing to do with magnetism. Skeptic Benjamin Radford has used a compass to check the magnetic field of a person that claimed to be a human magnet. He claims that person didn't actually produce magnetic fields. It shows that human magnetism uses different kinds of physical effects. Many scientists and proponents of science, including James Randi, claim that this ability is caused by sticky skin.
An example of a magnetobiological effect is the magnetic navigation by migrant animals by means of magnetoreception. 
Many animal orders, such as certain birds, marine turtles, reptiles, amphibians and salmonoid fishes are able to detect small variations of the geomagnetic field and its magnetic inclination to find their seasonal habitats. They are said to use an "inclination compass". Certain crustaceans, spiny lobsters, bony fish, insects and mammals have been found to use a "polarity compass", whereas in snails and cartilageous fish the type of compass is as yet unknown. Little is known about other vertebrates and arthropods. Their perception can be on the order of tens of nanoteslas.
Magnetoception (or magnetoreception) is the ability to detect the direction one is facing based on the Earth's magnetic field. Directional awareness is most commonly observed in birds, which rely on their magnetic sense to navigate during migration. It has also been observed in insects such as bees. Cattle make use of magnetoception to align themselves in a north-south direction. Magnetotactic bacteria build miniature magnets inside themselves and use them to determine their orientation relative to the Earth's magnetic field.
In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol F.
Discovered by Michael Faraday in 1845, the Faraday effect was the first experimental evidence that light and electromagnetism are related. The theoretical basis of electromagnetic radiation (which includes visible light) was completed by James Clerk Maxwell in the 1860s and 1870s. This effect occurs in most optically transparent dielectric materials (including liquids) under the influence of magnetic fields.
Many materials have unpaired electron spins, and the majority of these materials are paramagnetic. When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed as magnetic). Because of the way their regular crystalline atomic structure causes their spins to interact, some metals are ferromagnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt and nickel, as well as the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring ferromagnets were used in the first experiments with magnetism. Technology has since expanded the availability of magnetic materials to include various man-made products, all based, however, on naturally magnetic elements.
James Clerk Maxwell began working on Michael Faraday's lines of force. In his 1861 paper "" he modelled these magnetic lines of force using a sea of molecular vortices that he considered to be partly made of aether and partly made of ordinary matter. He derived expressions for the dielectric constant and the magnetic permeability in terms of the transverse elasticity and the density of this elastic medium. He then equated the ratio of the dielectric constant to the magnetic permeability with a suitably adapted version of Weber and Kohlrausch's result of 1856, and he substituted this result into Newton's equation for the speed of sound. On obtaining a value that was close to the speed of light as measured by Hippolyte Fizeau, Maxwell concluded that light consists in undulations of the same medium that is the cause of electric and magnetic phenomena.
Lorentz force flowmeters are usually classified in several main conceptual setups. Some of them designed as static flowmeters where the magnet system is at rest and one measures the force acting on it. Alternatively, they can be designed as rotary flowmeters where the magnets are arranged on a rotating wheel and the spinning velocity is a measure of the flow velocity. Obviously, the force acting on a Lorentz force flowmeter depends both on the velocity distribution and on the shape of the magnet system. This classification depends on the relative direction of the magnetic field that is being applied respect to the direction of the flow. In Figure 3 one can distinguish diagrams of the longitudinal and the transverse Lorentz force flowmeters.
Rare earth (lanthanoid) elements have a partially occupied "f" electron shell (which can accommodate up to 14 electrons). The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore, these elements are used in compact high-strength magnets where their higher price is not a concern. The most common types of rare-earth magnets are samarium-cobalt and neodymium-iron-boron (NIB) magnets.
MRI uses strong magnetic fields to align atomic nuclei (usually hydrogen protons) within body tissues, then uses a radio signal to disturb the axis of rotation of these nuclei and observes the radio frequency signal generated as the nuclei return to their baseline states. The radio signals are collected by small antennae, called coils, placed near the area of interest. An advantage of MRI is its ability to produce images in axial, coronal, sagittal and multiple oblique planes with equal ease. MRI scans give the best soft tissue contrast of all the imaging modalities. With advances in scanning speed and spatial resolution, and improvements in computer 3D algorithms and hardware, MRI has become an important tool in musculoskeletal radiology and neuroradiology.
The primary measurement in magnetochemistry is magnetic susceptibility. This measures the strength of interaction on placing the substance in a magnetic field. The volume magnetic susceptibility, represented by the symbol formula_1 is defined by the relationship
where, formula_3 is the magnetization of the material (the magnetic dipole moment per unit volume), measured in amperes per meter ( SI units), and formula_4 is the magnetic field strength, also measured in amperes per meter. Susceptibility is a dimensionless quantity. For chemical applications the molar magnetic susceptibility (χ) is the preferred quantity. It is measured in m·mol (SI) or cm·mol (CGS) and is defined as
where ρ is the density in kg·m (SI) or g·cm (CGS) and "M" is molar mass in kg·mol (SI) or g·mol (CGS). 
A variety of methods are available for the measurement of magnetic susceptibility.When an isolated atom is placed in a magnetic field there is an interaction because each electron in the atom behaves like a magnet, that is, the electron has a magnetic moment. There are two types of interaction.
When the atom is present in a chemical compound its magnetic behaviour is modified by its chemical environment. Measurement of the magnetic moment can give useful chemical information.
The most promising leads — cryptochromes, iron-based systems, electromagnetic induction — each have their own pros and cons. Cryptochromes have been observed in various organisms including birds and humans, but does not answer the question of night-time navigation. Iron-based systems have also been observed in birds, and if proven, could form a magnetoreceptive basis for many species including turtles. Electromagnetic induction has not been observed nor tested in non-aquatic animals. Additionally, it remains likely that two or more complementary mechanisms play a role in magnetic field detection in animals. Of course, this potential dual mechanism theory raises the question, to what degree is each method responsible for the stimulus, and how do they produce a signal in response to the low magnetic field of the Earth?
Microbats make use of magnetoreception, in that they have a high sensitivity to the Earth's magnetic field, as birds do. Microbats use a polarity-based compass, meaning that they differentiate north from south, unlike birds, which use the strength of the magnetic field to differentiate latitudes, which may be used in long-distance travel. The mechanism is unknown but may involve magnetite particles.
The value for the neutron's magnetic moment was first directly measured by Luis Alvarez and Felix Bloch at Berkeley, California in 1940. Using an extension of the magnetic resonance methods developed by Rabi, Alvarez and Bloch determined the magnetic moment of the neutron to be . By directly measuring the magnetic moment of free neutrons, or individual neutrons free of the nucleus, Alvarez and Bloch resolved all doubts and ambiguities about this anomalous property of neutrons.
Homing pigeons can use magnetic fields as part of their complex navigation system. William Keeton showed that time-shifted homing pigeons are unable to orient themselves correctly on a clear, sunny day which is attributed to time-shifted pigeons being unable to compensate accurately for the movement of the sun during the day. Conversely, time-shifted pigeons released on overcast days navigate correctly. This led to the hypothesis that under particular conditions, homing pigeons rely on magnetic fields to orient themselves. Further experiments with magnets attached to the backs of homing pigeons demonstrated that disruption of the bird's ability to sense the Earth's magnetic field leads to a loss of proper orientation behavior under overcast conditions. There have been two mechanisms implicated in homing pigeon magnetoreception: the visually mediated free-radical pair mechanism and a magnetite based directional compass or inclination compass. More recent behavioral tests have shown that pigeons are able to detect magnetic anomalies of 186 microtesla (1.86 Gauss).
Magnetoreception refers to the ability of animals to navigate using the inclination of the magnetic field of the earth. A possible explanation for magnetoreception is the entangled radical pair mechanism. The radical-pair mechanism is well-established in spin chemistry, and was speculated to apply to magnetoreception in 1978 by Schulten et al.. The ratio between singlet and triplet pairs is changed by the interaction of entangled electron pairs with the magnetic field of the earth. In 2000, cryptochrome was proposed as the "magnetic molecule" that could harbor magnetically sensitive radical-pairs. Cryptochrome, a flavoprotein found in the eyes of European robins and other animal species, is the only protein known to form photoinduced radical-pairs in animals. When it interacts with light particles, cryptochrome goes through a redox reaction, which yields radical pairs both during the photo-reduction and the oxidation. The function of cryptochrome is diverse across species, however, the photoinduction of radical-pairs occurs by exposure to blue light, which excites an electron in a chromophore. Magnetoreception is also possible in the dark, so the mechanism must rely more on the radical pairs generated during light-independent oxidation.
Inertial electrostatic confinement is a branch of fusion research that uses an electric field to elevate a plasma to fusion conditions. Electric fields can do work on charged particles (either ions or electrons), heating/confining them to fusion conditions. This is typically done in a sphere, with material moving radially inward, but can also be done in a cylindrical or beam geometry. The electric field can be generated using a wire grid or a non-neutral plasma cloud.
Scientists are still studying whether or not animals are affected by this, some suggesting this is why whales beach themselves. Some have stated the possibility that other migrating animals including birds and honey bees, might be affected since they also use magnetoreception to navigate, and geomagnetic storms alter the Earth's magnetic fields temporarily.Links related to power grids:
The magnetic field of permanent magnets can be quite complicated, especially near the magnet. The magnetic field of a small straight magnet is proportional to the magnet's "strength" (called its magnetic dipole moment ). The equations are non-trivial and also depend on the distance from the magnet and the orientation of the magnet. For simple magnets, points in the direction of a line drawn from the south to the north pole of the magnet. Flipping a bar magnet is equivalent to rotating its by 180 degrees.
Magnetic resonance is a phenomenon that affects a Magnetic dipole when placed in a uniform static magnetic field. Its energy is split into a finite number of energy levels, depending on the value of quantum number of angular momentum. This is similar to energy quantization for atoms, say in H atom; in this case the atom, in interaction to an external electric field, transitions between different energy levels by absorbing or emitting photons. Similarly if a magnetic dipole is perturbed with electromagnetic field of proper frequency(formula_1), it can transit between its energy eigenstates, but as the separation between energy eigenvalues is small, the frequency of the photon will be the microwave or radio frequency range. If the dipole is tickled with a field of another frequency, it is unlikely to transition. This phenomenon is similar to what occurs when a system is acted on by a periodic force of frequency equal to its natural frequency.
In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. The origin of the magnetic moments responsible for magnetization can be either microscopic electric currents resulting from the motion of electrons in atoms, or the spin of the electrons or the nuclei. Net magnetization results from the response of a material to an external magnetic field, together with any unbalanced magnetic dipole moments that may be inherent in the material itself; for example, in ferromagnets. Magnetization is not always uniform within a body, but rather varies between different points. Magnetization also describes how a material responds to an applied magnetic field as well as the way the material changes the magnetic field, and can be used to calculate the forces that result from those interactions. It can be compared to electric polarization, which is the measure of the corresponding response of a material to an electric field in electrostatics. Physicists and engineers usually define magnetization as the quantity of magnetic moment per unit volume.
It is represented by a pseudovector M.
The interplanetary magnetic field (IMF), now more commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field which is dragged out from the solar corona by the solar wind flow to fill the Solar System.
The coronal and solar wind plasmas are highly electrically conductive, meaning the magnetic field lines and the plasma flows are effectively "frozen" together and the magnetic field cannot diffuse through the plasma on time scales of interest. In the solar corona, the magnetic pressure greatly exceeds the plasma pressure and thus the plasma is primarily structured and confined by the magnetic field. With increasing altitude through the corona, solar wind acceleration results in the flow momentum exceeding the restraining magnetic tension force and the coronal magnetic field is dragged out by the solar wind to form the HMF.