Download - Physical Science Chapter 18
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Magnetism
Physical ScienceChapter 18
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Magnets
• Pull iron (and some other metals) towards them.
• When small and allowed to rotate freely, always point north (compasses).
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Magnetism
• A property of matter in which there is a force of repulsion or attraction between like or unlike poles.
• Related to electricity.
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Magnetic poles
• The ends of magnets– Where the magnetic forces are strongest
• Like poles repel– North and north or south and south
• Opposite poles attract
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Permanent magnets
• Naturally magnetic• Retain magnetic properties for long periods
of time.• Made of materials like iron, cobalt, and
nickel
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Temporary magnets
• Formed by objects being near or rubbing against a permanent magnet
• Lose their magnetic properties soon after being separated from permanent magnet.
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Magnetic field
• The area around a magnet where magnetic forces act.
• Strongest near the poles.
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Compass
• Magnetic needle that rotates freely• Lines up with the Earth’s magnetic field.
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Discuss
• Section 1 Review question #4 on page 624
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Creating magnetic fields
• An electric current causes a magnetic field.• The direction of the current affects the
direction of the magnetic field.
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Electromagnets
• Magnets created by electric current flowing through wires
• When a wire is coiled into a solenoid, a strong magnetic field is created in the coil.
• Inserting an iron core inside the solenoid increases the magnetic field.
• See page 627
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Galvanometer• Instrument used to detect
currents.• A coil of wire around an
iron core with a needle.• Between permanent
magnets.• When there is current in the
wire, the magnetic forces cause it to rotate, moving the needle.
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Ammeters
• Galvanometers calibrated to measure current.
• Connected in series with the circuit.
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Voltmeters
• Galvanometers calibrated to measure potential difference.
• Connected in parallel with the circuit
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Electric motor
• Contains an electromagnet that can rotate.• Rotates between opposite poles of a
permanent magnet.• The current produces a magnetic field that
makes the coil turn.
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Electric motor
• To keep the coil turning, the direction of the current in the coil must be reversed after every half turn.
• Done by a reversing switch called a commutator
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Discuss
• Describe an electromagnet.• What is an instrument that can be used to
measure either current or potential difference?
• What does an ammeter measure?• What does a voltmeter measure?• What is a commutator?
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Electromagnetic induction
• Moving a wire through a magnetic field produces a current in the wire
• You can also move a magnet through a coil of wire to produce a current in the wire
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Generators
• Use electromagnetic induction to make current (electricity)
• A loop of wire is rotated in a magnetic field.• It needs mechanical energy to make it rotate
– Usually comes from burning fuel• Similar to an electric motor, but with the
opposite function
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Direct current
• DC• From batteries• Current only flows in one direction in the
wire
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Alternating current
• AC• Wall outlets• Current reverses direction in a regular
pattern• In US, reverses 120 times per second for
60 Hz alternating current• Most appliances have built in converters
that change AC to DC.
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Transformer
• Increases or decreases the voltage of AC current
• Uses two coils with different numbers of turns
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Step-down transformer
• Reduces voltage• Used between power lines and home
electrical circuits• High voltage electricity travels more
efficiently, but is not good for household appliances
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Discuss
• Describe electromagnetic induction.• A generator operates by changing
_____________ energy into ____________ energy.
• Which kind of current reverses its direction in a regular pattern?
• Describe a step-down transformer.
physical science chapter 18