# Physics :Electromagnetism

Electromagnetism is the study of the effects caused by stationary and moving electric charges.

MAGNETISM

Pieces of some metallic ores, such as lodestone, are magnetic when suspended freely from a thread they point north-south.Such magnetic compasses have been used since 500 BC.

ELECTROMAGNETIC SPECTRUM

At present, science recognises a spectrum of electromagnetic radiation that extends from about 10-15 m to 10° m.

Radio waves have a large range of wavelengths, from a few millimetres up to several kilometres.

Microwaves are radio waves with shorter wavelengths, between 1 mm and 30 cm, and are used in radar and microwave ovens.

Infrared waves of different wavelengths are radiated by bodies at different temperatures. The Earth and its atmosphere, at a mean temperature of 250 K (-23°C or -9.4°F) radiates infrared waves with wavelengths centred at about 10 micrometres.

Visible waves have wavelengths of 400-700 nanometres (nm; 1 nm = 10-6 m).

Ultraviolet waves have wavelengths from about 380 nm down to 60 nm. The radiation from hotter stars, above 25,000°C (45,000°F), shifts towards the violet and ultraviolet  parts of the spectrum

X-rays have wavelengths from about 10 nm to 10-4 nm.

Gamma rays are emitted by certain radioactive nuclei in the course of nuclear reactions. It is now known that the Earth itself has magnetic properties. An important feature of a magnet is that it has two poles, one of which is attracted to the Earth’s magnetic North Pole, while the other is attracted to the South Pole.

STATIC ELECTRIC CHARGES

Static electricity involves electric charges at rest. In 1785, Coulomb formulated the Law of Attraction and Repulsion between electrically charged bodies: F=kQ1Q2/r2 where F is the force, k is a constant, Q1 and Q2 are the sizes of the charges (+ or -), and r is the distance between the charges.

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