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L 28 Electricity and Magnetism [6] Basic Facts of Magnetism Induced

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L 28 Electricity and Magnetism [6] Basic Facts of Magnetism Induced L 28 Electricity and Magnetism [6] Basic facts of Magnetism • Oersted discovered that a compass needle • magnetism responded to the a current in a loop of wire • Ampere deduced the law describing how a • Faraday’s Law of Electromagnetic magnetic field is produced by the current in a wire Induction • magnetic field lines are always closed loops – no – induced currents isolated magnetic poles; magnets always have a north and south pole – electric generator • permanent magnets: the currents are atomic – eddy currents currents – due to electrons spinning in atoms - • Electromagnetic Waves (Maxwell & Hertz) these currents are always there • electromagnets: currents in wires produce magnetic fields 1 2 Faraday’s Law of Electromagnetic Induction Induced currents (a) • Faraday thought that if A B currents could produce magnetic field lines magnetic fields, (Oersted, Ampere) magnetic fields might produce currents battery • He was correct, with one current important qualification: indicator the magnetic field must be switch changing in some way to • When a current is turned on or off in coil A, a magnetic produce a current field is produced which also passes through coil B. • the phenomenon that a • A current then briefly appears in coil B changing magnetic field can produce a current is called • The current in coil B is called an induced current. Michael Faraday • The current in B is only present when the current in A is electromagnetic induction (1791-1867) 3 turned on or off, that is, when the current in A is changing 4 Induced currents (b) Induced currents (c) • If an AC (time varying) (a) (b) (c) current is used in the primary circuit, a current is induced in the secondary windings. • If the current in the primary windings were a) No current is induced if the magnet is stationary. DC, there would be NO b) When the magnet is pushed toward the coil or pulled induced current in the away from it, an induced current appears in the coil. secondary circuit. c) The induced current only appears when the magnet is being moved • Levitated coil demo 5 6 1 electric generators The transformer The voltage on the secondary depends on the number When a coil is rotated in a magnetic field, an of turns on the primary and secondary. induced current appears in it. This is how electricity Step-up the secondary has more turns than the primary is generated. Some external source of energy is Step-down the secondary has less turns than the primary needed to rotate the turbine which turns the coil. 7 8 Eddy currents Eddy currents application • Eddy currents are induced in conductors if time- varying magnetic fields are present An induction stove uses • As the magnet falls the magnetic field strength at eddy currents to cook food the plate increases Falling Copper magnet plate Eddy currents Induced magnetic Only the metal pot gets hot, not the 10 9 field glass pot or the stove. Floating magnet – induced currents The Laws of Electricity and Magnetism • As the magnet falls, it • Laws of electricity bar induces currents in the – electric charges produce electric fields magnet copper pipe known as eddy (Coulomb) currents. – electric fields begin and end on charges • These eddy currents • Laws of magnetism produce a magnetic field – currents produce magnetic fields (Ampere) that opposes the field of the – magnetic field lines are closed loops slotted falling magnet, so the – a changing magnetic field can produce a copper pipe magnet does not accelerate current (induced currents) (Faraday) but descends slowly. – A changing electric field can produce a magnetic field (Maxwell) 11 12 2 ELECTROMAGNETIC (EM) WAVES Electromagnetic (EM) waves Faraday laid the groundwork with his discovery of electromagnetic induction • Mechanical wave: a disturbance that propagates in a medium (eg, water, strings, air) EM • An electromagnetic wave is a combination WAVES of electric and magnetic fields that oscillate together in space (no medium) and time in a synchronous manner, and propagate at the LIGHT speed of light 3 ×108 m/s or 186,000 miles/s. • EM waves include radio, microwaves, x-rays, James Clerk Maxwell in 1865 Heinrich Hertz showed light waves, thermal waves gamma rays predicted theoretically that EM experimentally in 1886 waves should exist that EM waves exist 13 14 the generation of an electromagnetic wave EM waves: transverse • the electromagnetic wave is a transverse electric field wave, the electric and magnetic fields oscillate in the direction perpendicular to wave emitter the direction of propagation e.g. antenna E field magnetic field direction of propagation The time varying electric field generated the time varying magnetic field which generates the time Bfield varying electric field and so on and so on . 15 16 Electromagnetic waves How radio waves are produced • the EM wave propagates because the electric field recreates the magnetic field and the magnetic field recreates the electric field transmission line • an oscillating voltage applied to the antenna makes the charges in the antenna vibrate up and down sending out a synchronized pattern of electric and magnetic fields • an electromagnetic wave must have both an High Frequency electric and magnetic field component Oscillator Dipole 17 Antenna 18 3 Electromagnetic Waves Radio antenna the EM wave causes the electrons in the receiving antenna Sound waves are transformed to an to oscillate at the electrical signal same frequency which is amplified and sent to the EM WAVE: time and space transmitter Antenna: varying electric and magnetic emits waves fields moving through space the amplifier at the speed of light, c = converts the 3 x 108 m/s = 186,000 miles/sec electrical signal 19 to sound waves20 The periodic wave relation applies to Electromagnetic spectrum electromagnetic waves = c 21 Visible light 22 Common frequency bands Microwaves • 1 vibration per second = 1 Hertz (Hz) • 1 KHz (kilohertz) = 103 Hz • are in the frequency range of a few billion Hz or • 1 MHz (megahertz) = 106 Hz wavelengths of about several cm (about the • 1 GHz (gigahertz) = 109 Hz same range as radar the “Radarange” • How do microwaves heat water? • Remember that the water molecule has a positive end and a negative end. • The electric field of the microwave grabs onto these charges and shakes them violently a few • AM radio: 535 KHz – 1.7 MHz billion times each second • FM radio: 88 – 108 MHz • all this shaking energizes the molecules making • GPS: 1.227 and 1.575 GHz the water hotter and hotter. • Cell phones: 824 MHz – 2 GHz 23 24 4.
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