physical science chapter 18

Magnetism

Physical ScienceChapter 18

2

Magnets

• Pull iron (and some other metals) towards them.

• When small and allowed to rotate freely, always point north (compasses).

physical science chapter 18

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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


5

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.


Discuss

• Section 1 Review question #4 on page 624

9physical science chapter 18

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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.


http://upload.wikimedia.org/wikipedia/commons/6/6a/Galvanometer_diagram.png

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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.


22

Transformer

• Increases or decreases the voltage of AC current

• Uses two coils with different numbers of turns


http://upload.wikimedia.org/wikipedia/en/b/bf/Transformer3d.svg

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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

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