Transcript Magnetism and its uses

Chapter 22
Magnetism and its uses
22-1 Characteristics of Magnets
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Greeks experimented more than 2000 years ago
with a mineral that pulled iron objects toward it.
They described this mineral as being magnetic.
Together, magnetic and electric forces can
generate electricity and operate electric motors.
Magnetism is a property of matter in which
opposite magnetic poles attract and like poles
repel each other. Magnetic forces are strongest
near the poles (north and south).
Only a few materials are naturally magnetic.
22-1
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The area around a magnet where the attractive or
repulsive forces are felt is called the magnetic
field. The field can be seen if iron filings are
placed around a magnet.
A compass is a device that uses the magnetic field
of magnets and the magnetic field of Earth to help
individuals navigate. Magnets always point north.
What makes some materials magnets, other
materials attracted to magnets and many materials
that are neither?
22-1
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Each spinning electron causes a magnetic field to
form around it. Most materials have electrons that
exist in pairs that spin in opposite directions thus
canceling out the magnetic field. The atoms in
materials such as iron, cobalt and nickel have
unpaired electrons, so the electrons' magnetic fields
do not cancel. As a result, each atom of these
elements act like a very small magnet.
The magnetic fields of each atom cause groups of
atoms to align into poles. These groups of atoms are
called magnetic domains.
If a permanent magnet is rubbed against a nail it will
become a temporary magnet until the nail's atoms
return to their random arrangement.
22-2 Uses of Magnetic Fields
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In 1820, a Danish physics teacher, Hans Christian
Oersted, observed that a current moving through a
wire moved the needle on a nearby compass.
When the current was reversed, the compass
needle moved in the opposite direction. Oersted
hypothesized, the electric current must produce a
magnetic field around the wire.
A magnetic field forms around any wire that has
current flowing through it. If the wire is coiled the
magnetic field lines overlap making the magnetic
field stronger. When a current passes through a
coiled wire the temporary magnet that is formed is
called an electromagnet.
22-2
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Electromagnets can be strengthened in two ways
- by adding more coils to the wire
- by adding an iron core inside the coil
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By adding the iron core, the iron becomes a
magnet and its magnetic field aligns with the
electromagnet.
Electromagnets operate doorbells, loudspeakers
and to lift large metal objects in construction
machines. They can be turned on and off by
controlling the flow of current through the coil.
They change electrical energy to mechanical
energy to do work.
22-2
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Because electromagnets are sensitive to electrical
currents, they can detect electric current. An
instrument used to do so is called a galvanometer.
A galvanometer consists of a coil of wire
connected to a circuit and suspended so it can
rotate in the magnetic field of a permanent
magnet.
Galvanometers can be calibrated to measure
current or electrical potential depending on
whether it will be used as an ammeter or a
voltmeter.
Ammeters measure current passing though a
series circuit, whereas, voltmeters measure
potential different in parallel circuits in volts.
22-2 Electric motors
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Electric motors like galvanometers contain an
electromagnetic that is free to rotate. To make the
coil in an electric motor spin steadily the direction
of the current through the coil must be reversed
after each half revolution.
The device used to change the direction of the
current is called the commutator. The commutator
is a reversing switch that rotates with the
electromagnet.
Electric Motors
22-3 Producing Electric Current
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Oersted found that magnetism could be produced
from electrical current. It was later discovered
that moving a wire through a magnetic field
induced a current in the wire. This process is
called electromagnetic induction and can be done
in two ways
1. Surrounding a moving wire with a magnetic field
2. Moving a magnet in and out of a coil of wire.
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A generator works using the first method. The
wire loop is connected to a source of mechanical
energy and placed between the poles of a
magnet.
Generator
22-3
• The design of a generator is very similar to that of
an electric motor. The difference is that in an
electric motor the wire loop is made to rotate by
external forces (battery) and in the generator the
wire crosses through the magnetic forces causing it
to rotate. Motors consume electricity to make
mechanical power, whereas, generators consume
mechanical power to produce electricity.
• If a motor is using a battery it is using direct current
(DC). DC flows only in one direction through a wire.
Electrons always move from the negative to the
positive terminal.
• When plugging an appliance in to the wall it is using
alternating current (AC). AC reverses its direction
in a regular pattern.
22-3
• Current flowing from power plants to homes through
power lines are AC and have extremely high
voltage. Before the AC can enter your home its
voltage must be decreased. The device that can
raise or lower the voltage of an AC is a transformer.
• A transformer reduces voltage to 120 volts before it
enters your home. A transformer that reduces
voltage is called a step-down transformer.
• In step-up transformers the output voltage is greater
than the input voltage. This helps transmit AC
current from power plants long distances.
22-4 Superconductivity
• Remember that all conducting materials have some
resistance to electron flow and thus electricity
moving through the conductor is lost as heat. Also
as the temperature rises, the resistance rises and
more current is lost. A material that has no electrical
resistance is called a superconductor.
• In 1911 scientists discovered that some materials
when cooled to temperatures near absolute zero
(0K) -273oC lost all electrical resistance. One way
to cool a material to superconducting temperatures
is to submerge it in liquid helium. Helium is
normally a gas but liquefies at low temperatures.
• The use of superconductors could eliminate
electrical energy waste and improve the function of
electric motors, generators and computer parts.