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# magnetic field

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## Ampère's law

A law of the formdB = (μo I sin θ dl)/4πr2,where dB is the infinitesimal element of the magnitude of the magnetic flux density (see magnetic field) at a distance r at a point P from the element ...

## biomagnetism

The influence of the magnetic field on living organisms, which is strong in some species. Certain Spirillum bacteria orient themselves along the geomagnetic lines of force. Higher organisms, e.g. ...

## compass

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A small magnet pivoted at its central point to revolve in a horizontal plane. In the earth's magnetic field the magnet (called the compass needle) aligns itself so that its north-seeking end points ...

## constitutive equations

The equations D = εE and B = μH, where D is the electric displacement, ε is the permittivity of the medium, E is the electric field intensity, B is the magnetic flux density, μ is the permeability of ...

## electric field

A region in which an electric charge experiences a force usually because of a distribution of other charges. The electric field strength or electric intensity (E) at any point in an electric field is ...

## electric motor

A machine for converting electrical energy into mechanical energy. They are quiet, clean, and have a high efficiency (75–95%). They work on the principle that a current passing through a coil within ...

## electricity

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Any effect resulting from the existence of stationary or moving electric charges.

## electromagnet

A magnet consisting of a soft ferromagnetic core with a coil of insulated wire wound round it. When a current flows through the wire the core becomes magnetized; when the current ceases to flow the ...

Two laws describing electrolysis: (1) The amount of chemical change during electrolysis is proportional to the charge passed.(2) The charge required to deposit or liberate a mass m is given by Q = ...

## field

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A central concept of physical theory. A field is defined by the distribution of a physical quantity, such as temperature, mass density, or potential energy, at different points in space. In the ...

## geomagnetic storm

A worldwide disturbance of the Earth's magnetic field which is created when particles sent from the Sun in solar flares are drawn towards the Earth by its magnetic field. It often coexists with an ...

## induction

The creation of a voltage by changing the magnetic flux such that the amount of voltage induced is directly proportional to the rate of change of the magnetic flux according to Faraday's or Neumann's ...

## intensity

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1. The rate at which radiant energy is transferred per unit area. See radiant intensity.2. The rate at which sound energy is transferred as measured relative to some reference value. See decibel.3. ...

## Kramers theorem

The energy levels of a system, such as an atom that contains an odd number of spin-1/2 particles (e.g. electrons), are at least double degenerate in the absence of an external magnetic field. This ...

## loudspeaker

A transducer for converting an electrical signal into an acoustic signal. Usually it is important to preserve as many characteristics of the electrical waveform as possible. The device must be ...

## magnet

A piece of magnetic material (see magnetism) that has been magnetized and is therefore surrounded by a magnetic field. A magnet, often in the shape of a bar or horseshoe, that retains appreciable ...

## magnetic anomaly

Any magnetic field remaining after allowance for some particular model. Normally this is the field remaining after allowance for the International Geomagnetic Reference Field, but sometimes it is the ...

## magnetic bottle

A nonuniform magnetic field used to contain the plasma in a thermonuclear experimental device. At the temperature of a thermonuclear reaction (108 K) any known substance would vaporize and the plasma ...

## magnetic flux

The magnetic induction, perpendicular to the surface area of a nearby body, multiplied by the area of the body.

## magnetic force

The attractive or repulsive force exerted on a magnetic pole or a moving electric charge in a magnetic field.

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