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Britannica Money. Discover all you need to know about retirement, investing, and household finance, without the jargon or agenda. Get reliable guidance, insight, and easy-to-understand explanations, written, edited, and verified to Britannica’s exacting standards. Advocacy for Animals.
Large numbers are numbers above one million that are usually represented either with the use of an exponent such as 10 9 or by terms such as billion or thousand millions that frequently differ from system to system. The American system of numeration for denominations above one million was modeled on a French system, but in 1948 the French ...
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In the United States, commonwealth has continued to be the official description of four states (Kentucky, Massachusetts, Pennsylvania, and Virginia). It confers no distinction, other than in name, from the other states. The same term also was applied to Puerto Rico after an act of Congress of 1950 and adoption of the constitution of 1952.
2024年5月3日 · sedimentary rock, rock formed at or near Earth’s surface by the accumulation and lithification of sediment (detrital rock) or by the precipitation from solution at normal surface temperatures (chemical rock).
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geomagnetic field, magnetic field associated with Earth. It is primarily dipolar (i.e., it has two poles, the geomagnetic North and South poles) on Earth’s surface. Away from the surface the dipole becomes distorted.
In the 1830s the German mathematician and astronomer Carl Friedrich Gauss studied Earth’s magnetic field and concluded that the principal dipolar component had its origin inside Earth instead of outside. He demonstrated that the dipolar component was a decreasing function inversely proportional to the square of Earth’s radius, a conclusion that led scientists to speculate on the origin of Earth’s magnetic field in terms of ferromagnetism (as in a gigantic bar magnet), various rotation theories, and various dynamo theories. Ferromagnetism and rotation theories generally are discredited—ferromagnetism because the Curie point (the temperature at which ferromagnetism is destroyed) is reached only 20 or so kilometres (about 12 miles) beneath the surface, and rotation theories because apparently no fundamental relation exists between mass in motion and an associated magnetic field. Most geomagneticians concern themselves with various dynamo theories, whereby a source of energy in the core of Earth causes a self-sustaining magnetic field.
Earth’s steady magnetic field is produced by many sources, both above and below the planet’s surface. From the core outward, these include the geomagnetic dynamo, crustal magnetization, the ionospheric dynamo, the ring current, the magnetopause current, the tail current, field-aligned currents, and auroral, or convective, electrojets. The geomagnetic dynamo is the most important source because, without the field it creates, the other sources would not exist. Not far above Earth’s surface the effect of other sources becomes as strong as or stronger than that of the geomagnetic dynamo. In the discussion that follows, each of these sources is considered and the respective causes explained.
Earth’s magnetic field is subject to variation on all timescales. Each of the major sources of the so-called steady field undergoes changes that produce transient variations, or disturbances. The main field has two major disturbances: quasiperiodic reversals and secular variation. The ionospheric dynamo is perturbed by seasonal and solar cycle changes as well as by solar and lunar tidal effects. The ring current responds to the solar wind (the ionized atmosphere of the Sun that expands outward into space and carries with it the solar magnetic field), growing in strength when appropriate solar wind conditions exist. Associated with the growth of the ring current is a second phenomenon, the magnetospheric substorm, which is most clearly seen in the aurora borealis. An entirely different type of magnetic variation is caused by magnetohydrodynamic (MHD) waves. These waves are sinusoidal variations in the electric and magnetic fields that are coupled to changes in particle density. They are the means by which information about changes in electric currents is transmitted, both within Earth’s core and in its surrounding environment of charged particles. Each of these sources of variation is also discussed separately below.
Electric and magnetic fields are produced by a fundamental property of matter, electric charge. Electric fields are created by charges at rest relative to an observer, whereas magnetic fields are produced by moving charges. The two fields are different aspects of the electromagnetic field, which is the force that causes electric charges to interact. The electric field, E, at any point around a distribution of charge is defined as the force per unit charge when a positive test charge is placed at that point. For point charges the electric field points radially away from a positive charge and toward a negative charge.
A magnetic field is generated by moving charges—i.e., an electric current. The magnetic induction, B, can be defined in a manner similar to E as proportional to the force per unit pole strength when a test magnetic pole is brought close to a source of magnetization. It is more common, however, to define it by the Lorentz-force equation. This equation states that the force felt by a charge q, moving with velocity v, is given by F = q(vxB).
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In this equation bold characters indicate vectors (quantities that have both magnitude and direction) and nonbold characters denote scalar quantities such as B, the length of the vector B. The x indicates a cross product (i.e., a vector at right angles to both v and B, with length vB sin θ). Theta is the angle between the vectors v and B. (B is usually called the magnetic field in spite of the fact that this name is reserved for the quantity H, which is also used in studies of magnetic fields.) For a simple line current the field is cylindrical around the current. The sense of the field depends on the direction of the current, which is defined as the direction of motion of positive charges. The right-hand rule defines the direction of B by stating that it points in the direction of the fingers of the right hand when the thumb points in the direction of the current.
In the International System of Units (SI) the electric field is measured in terms of the rate of change of potential, volts per metre (V/m). Magnetic fields are measured in units of tesla (T). The tesla is a large unit for geophysical observations, and a smaller unit, the nanotesla (nT; one nanotesla equals 10−9 tesla), is normally used. A nanotesla is equivalent to one gamma, a unit originally defined as 10−5 gauss, which is the unit of magnetic field in the centimetre-gram-second system. Both the gauss and the gamma are still frequently used in the literature on geomagnetism even though they are no longer standard units.
2024年5月21日 · The Starry Night, a moderately abstract landscape painting (1889) of an expressive night sky over a small hillside village, one of Dutch artist Vincent van Gogh’s most celebrated works. Description The oil-on-canvas painting is dominated by a night sky roiling with chromatic blue swirls, a glowing yellow crescent moon, and stars rendered as radiating orbs.
2024年5月6日 · In chemical terms, photosynthesis is a light-energized oxidation–reduction process. (Oxidation refers to the removal of electrons from a molecule; reduction refers to the gain of electrons by a molecule.) In plant photosynthesis, the energy of light is used to drive the oxidation of water (H 2 O), producing oxygen gas (O 2 ), hydrogen ions (H ...
