<text><span class="style10">lectro-magnetism</span><span class="style7"> </span><span class="style10">(4 of 5)Electromagnetic fields</span><span class="style7">In 1820 the Danish physicist Hans Christiaan Oersted (1777-1851) discovered that a copper wire bearing an electric current caused a pivoted magnetic needle to be deflected until it was tangential to a circle drawn around the wire (see diagram 4). This was the first connection to be established between the electrical and magnetic forces. Oersted's work was developed by the Frenchmen Jean-Baptiste Biot (1774-1862) and Félix Savart (1791-1841), who showed that the field strength of a current flowing in a straight wire varied with the distance from the wire. Biot and Savart were able to find a law relating the current in a small part of the conductor to the magnetic field. Ampère, at about the same time, found a more fundamental relationship between the current in a wire and the magnetic field about it.We now believe that the Earth's magnetic field is generated by the motion of charged particles in the liquid iron part of the core. This is known as the </span><span class="style26">dynamo theory</span><span class="style7">.From Newton's third law and Oersted's observation it might be expected that a magnetic field can exert a force on a moving charge. This is observed if a magnet is brought up close to a cathode-ray tube in a TV set. The beam of electrons moving from the cathode to the screen is deflected. The force acts in a direction perpendicular to both the magnetic field and the direction of electron flow. If the magnetic field is perpendicular to the direction of the electrons, then the force has its maximum value. This is the second way in which the electric and magnetic properties are linked.</span><span class="style10">Electromagnetic induction</span><span class="style7">The next advance came in 1831, when the English physicist Michael Faraday (1791-1867) found that an electric current could be induced in a wire by another, changing current in a second wire. Faraday published his findings before the US physicist Joseph Henry (1797-1878), who had first made the same discovery. Faraday showed that the magnetic field at the wire had to be changing for an electric current to be produced. This may be done by changing the current in a second wire, by moving a magnet relative to the wire, or by moving the wire relative to a magnet. This last technique is that employed in a dynamo generator, which maintains an electric current when it is driven mechanically. An electric motor uses the reverse process, being driven by electricity to provide a mechanical result.</span><span class="style10">Maxwell's theory</span><span class="style7">The work of the Scottish physicist James Clerk Maxwell (1831-79) on electromagnetism is of immense importance for physics. It united the separate concepts of electricity and magnetism in terms of a new </span><span class="style26">electromagnetic force</span><span class="style7">. Maxwell ex tended the ideas of Ampère, then in 1864 he proposed that a magnetic field could also be caused by a changing electric field. Thus, when either an electric or magnetic field is changing, a field of the other type is induced. Maxwell predicted that electrical oscillations would generate electromagnetic waves, and he derived a formula giving the speed in terms of electric and magnetic quantities. When these quantities were measured he calculated the speed and found that it was equal to the speed of light in a vacuum. This suggested that light might be electromagnetic in nature - a theory that was later confirmed in various ways. Thus, when an electric current in a wire changes, electromagnetic waves are generated, which will be propagated with a velocity equal to that of light.The electric and magnetic field components in electromagnetic waves are perpendicular to each other and to the direction of propagation. The existence of electromagnetic waves was demonstrated experimentally in 1887 by the German physicist Heinrich Rudolf Hertz (1857-94) - who also gave his name to the unit of frequency. In his laboratory, Hertz transmitted and detected electromagnetic waves, and he was able to verify that their velocity was close to the speed of light.AS</span></text>
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<text><span class="style10">. Oersted's discovery.</span><span class="style7"> When a current flows through a wire, magnetic compasses on a plane at right angles to the wire will be deflected until they are tangential to a circle drawn around the wire.</span></text>
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<text>ΓÇó STARS AND GALAXIESΓÇó QUANTUM THEORY AND RELATIVITYΓÇó WAVE THEORYΓÇó OPTICSΓÇó ELECTRICITY IN ACTIONΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó THE EARTH'S STRUCTUREΓÇó MEDICAL TECHNOLOGYΓÇó RADIO, TELEVISION AND VIDEOΓÇó SEEING THE INVISIBLE</text>
