Magnetism Textbook Sections 22-1 – 22-3, 22-8 Physics 1161: Lecture 10.
Chapter 22 Magnetism and its Uses
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Transcript of Chapter 22 Magnetism and its Uses
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Chapter 22 Magnetism and its Uses
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Objects
• 22.1 Describe the properties of magnets • 22.1 Define the region of force around a
magnets • 22.1 Model magnetic behavior using domains
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Objects
• 22.2 Explain the magnetic effects of a current in a wire
• 22.2 Compare and contrast ammeters and voltmeters
• 22.2 Describe the function of an electric motor.
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Objects
• 22.3 Describe how a generator produces an electric current using electromagnetic induction
• 22.3 Distinguish between alternating and direct current
• 22.3 Explain how a transformer can step up or step down the voltage of an alternate current
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Objectives
• 22.4 Describe the characteristics of superconductors
• 22.4 Consider various applications of superconductivity
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Key Words• Magnetism Magnetic Pole Magnetic Field • Magnetic Domain Electromagnet Ammeter
• Voltmeter Electric Motor
• Electromagnetic Induction
• Generator Direct Current Alternating current
• Transformer Solenoid
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Magnetism
• Magnetism is a property of matter in which there is a force of attraction or repulsion between unlike or like poles– One positive end, one negative end
• Most metals area actually NOT magnetic– Iron, Cobalt and Nickel metals which are actually
magnetic– Other metals can be temporary magnets
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Opposites Attract
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The Earth
• Earth is a big magnet. The actual location of the magnetic poles change every year by about 40 miles
• Earth’s magnetic poles change yearly
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Magnetic Declination
• Your compass points to the magnetic pole, not to the true north pole
• As you approach the poles, you have to add/subtract degrees to go the right direction
• Compass is wrong close to poles
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• The north arrow on the compass rose (the large N) is pointed towards the place on the horizon directly beneath the North Star! That is, towards true north!
• And the needle (of course) points towards magnetic north! So the magnetic declination for this locality is 45 degrees west
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Magnetic Domains
• Groups of atoms with aligned magnetic poles are called
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Magnetic Fields• Points towards the South Pole
– So, South must be Negative end – Since magnetic fields point towards the negative charge
• Magnetic Pole: End of magnet, strongest magnetic field• Earth’s North pole
– Is the south magnet
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Electromagnet
• A type of magnet created by current passing through a coil
• Electromagnetic Induction: The process by which moving a wire through a magnetic field produces a current
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Solenoid: Coils wrapped around with a current moving through. Used in MRI’s
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Thumb points towards the North End
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Magnetic Field Force– The current causes the magnetic field– The magnetic field in turns pushes on the wire
• Electric Motor: Turns electrical energy into mechanical energy
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Reading Current and Voltage
• Voltmeter: Determines the voltage drop across a resistor
• Ammeters: Determine the current through a wire– Determined by deflection caused by the magnetic
field
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Generator
• Produces electric current by rotating wires in a magnetic field– See simulator
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Direct Current
• DC = Direct Current. The flow (current) of electrons is in one direction– Like a river runs only down hill, direct current only
flows one way.– Batteries provide this type of current
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Alternating Current
• AC = Alternating Current Electrons flow back and forth. The electrons are pushed one way, then pushed the other way. – The electrons stay in roughly the same area, just
going back and forth. Our outlets provide this, alternating current.
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Alternating Current
– In the USA, there are three wires that transmit the flow of energy. • Two are at + or – 120 Volts (180 degrees out of phase)• One is at 0 Volts
– The US has adopted a frequency of 60 Hz• The polarity (direction of electrons) changes 120 times
per second• Actual voltage of peaks are + and – 170 volts
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Transformers
– A transformer is just a piece of iron with a pair of wires coiled around it - one with many more turns in the coil than the other.
– The coils of wire are not physically connected. The iron core is immersed in an insulating oil bath which does not conduct electricity well.
• Step Up or Step Down the Voltage
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Transformers1
2
1
2
NN
VV
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