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Ch 36 and 37 Magnetism & Electromagnetic Induction Characteristics of Magnets • Each magnet has a north-seeking and south-seeking pole • Exert a magnetic force on the space around them = magnetic field – Lines closer together – stronger field – Where is the field strength the greatest on a magnet? The Earth Nature of a Magnetic Field • Produced by the motion of an electric charge (current) • Every spinning electron is a tiny magnet. • If two electrons are spinning the same way, they make a stronger magnet • If two electrons are spinning in opposite directions, their magnetic field cancels out (why most substances are not magnets) A current carrying wire can produce a magnetic field More about magnets • Common magnets are made of iron, nickel, and cobalt. – The spin of their e- do not cancel • When atoms are aligned, they have magnetic domains. • How can a magnet become weaker? • How does a magnet attract a piece of metal that is not magnetized? What is electromagnetic induction? • We learned about induction when we studied how objects become charged. • Inducing voltage by changing the magnetic field around a conductor How is voltage induced? • Moving a magnet in a coil of wire will induce voltage and therefore produce a current • Either the magnet must move through coil of wire or a coil of wire must move back and forth across a magnet to induce voltage Clarification • Magnets do not produce voltage!!! • Voltage is produced by the work done when a magnet and closed loop of wire are moved relative to each other. Amount of voltage • Depends on how quickly the magnetic field lines are traversed by the wire – Quick motion = more voltage – Slow motion = less voltage • Depends on how many coils – 1 coil of wire = some amount of voltage (V) – 2 coils of wire = 2 x V – 4 coils of wire = 4 x V Faraday’s Law • Induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops Amount of Current • Depends on the amount of voltage induced • Also depends on the resistance of the coil and the circuit to which it is connected • What are some examples? Practical applications of electromagnets • Maglev trains – page 569, http://www.okeating.com/hsr/maglev.htm • Stereo speaker, http://electronics.howstuffworks.com/speaker3.htm • Alarm systems • Junkyard crane • MRI, http://electronics.howstuffworks.com/mri2.htm Motor vs. Generators • Motor- turns electrical energy into mechanical energy • Generator- turns mechanical energy into electrical energy • Simply opposites of one another Motor Generator MultiQuip Gas Generators MULGA25H $1,320.00 $1,056.00 "2.5KW Portable Generator-120V, 5.5hp Honda GX160, Recoil Start ****FREE FREIGHT Anywhere in The Continental US ** NOTE: Any Residential/Tailgate or Re-Delivery Charges NOT INCLUDED****" I’m on SALE!!! ©2000-2005 Southern Tool Generators • When the wire is rotated in a magnetic field, the # of magnetic field lines change, this causes induced voltage to change. – Causes alternating current current changes magnitude and direction periodically Generating electricity • The faster you turn the coil the greater the frequency of magnetic field change. • Greater frequency of AC (N. America = 60 Hz) • Greater induced voltage Generators used in power plants • Chemical or nuclear energy mechanical energy electrical energy Look familiar? Fuel: natural gas, oil Motor vs. Generator Effect • Battery involved • No battery • Initial moving • Wire moved charges = current mechanically • Wire forced upwards • Interaction between due to magnetic field electric field and magnetic field • Battery provides causes charges to voltage = move = induce mechanical energy voltage = induce results current Transformers Coils • Primary – connected to a power source (input), opening or closing the primary switch can induce voltage in the secondary circuit • Secondary – output, reacts to changes in the magnetic field intensity of the primary coil and voltage can be induced Transformers • What is the point in using a transformer? – Voltages can be stepped up or down • See figure 37.12 – Instead of opening and closing the switch on the primary circuit – we just use an alternating current to produce the same effects. Primary voltage #primary turns = secondary voltage # secondary turns Do transformers disobey the law of conservation of energy? • No….. – POWERIN = POWEROUT – (VOLTAGE X CURRENT)PRIMARY = (VOLTAGE X CURRENT)SECONDARY • Power is delivered to your home at high voltages but low currents – (120 V and 1/10 amp) Transformers • Step-up transformers: the secondary has more loops than the primary • Step-down transformers: the secondary has less loops than the primary • When would you want a step-up transformer? • When would you want a step-down transformer? Faraday • An electric field will be created when you change a magnetic field over time Maxwell • A magnetic field will be created when you change an electric field over time • Magnitude of the resulting field depends on the rate at which the initial field is changed • The resulting field is at right angles to the initial field. Electromagnetic Waves • Composed of vibrating electric and magnetic fields that regenerate each other, therefore they are self-sustaining – Conservation of energy • No medium is required • Electric field is perpendicular to the magnetic field and both are perpendicular to the direction of motion of the wave • EM waves move at the speed of light